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diff --git a/src/main/java/jsr166e/ForkJoinPool.java b/src/main/java/jsr166e/ForkJoinPool.java new file mode 100644 index 0000000..8a04973 --- /dev/null +++ b/src/main/java/jsr166e/ForkJoinPool.java @@ -0,0 +1,3344 @@ +/* + * Written by Doug Lea with assistance from members of JCP JSR-166 + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/publicdomain/zero/1.0/ + */ + +package jsr166e; + +import jsr166y.ThreadLocalRandom; + +import java.lang.Thread.UncaughtExceptionHandler; +import java.util.ArrayList; +import java.util.Arrays; +import java.util.Collection; +import java.util.Collections; +import java.util.List; +import java.util.concurrent.AbstractExecutorService; +import java.util.concurrent.Callable; +import java.util.concurrent.ExecutorService; +import java.util.concurrent.Future; +import java.util.concurrent.RejectedExecutionException; +import java.util.concurrent.RunnableFuture; +import java.util.concurrent.TimeUnit; + +/** + * An {@link ExecutorService} for running {@link ForkJoinTask}s. + * A {@code ForkJoinPool} provides the entry point for submissions + * from non-{@code ForkJoinTask} clients, as well as management and + * monitoring operations. + * + * <p>A {@code ForkJoinPool} differs from other kinds of {@link + * ExecutorService} mainly by virtue of employing + * <em>work-stealing</em>: all threads in the pool attempt to find and + * execute tasks submitted to the pool and/or created by other active + * tasks (eventually blocking waiting for work if none exist). This + * enables efficient processing when most tasks spawn other subtasks + * (as do most {@code ForkJoinTask}s), as well as when many small + * tasks are submitted to the pool from external clients. Especially + * when setting <em>asyncMode</em> to true in constructors, {@code + * ForkJoinPool}s may also be appropriate for use with event-style + * tasks that are never joined. + * + * <p>A static {@link #commonPool()} is available and appropriate for + * most applications. The common pool is used by any ForkJoinTask that + * is not explicitly submitted to a specified pool. Using the common + * pool normally reduces resource usage (its threads are slowly + * reclaimed during periods of non-use, and reinstated upon subsequent + * use). + * + * <p>For applications that require separate or custom pools, a {@code + * ForkJoinPool} may be constructed with a given target parallelism + * level; by default, equal to the number of available processors. The + * pool attempts to maintain enough active (or available) threads by + * dynamically adding, suspending, or resuming internal worker + * threads, even if some tasks are stalled waiting to join others. + * However, no such adjustments are guaranteed in the face of blocked + * I/O or other unmanaged synchronization. The nested {@link + * ManagedBlocker} interface enables extension of the kinds of + * synchronization accommodated. + * + * <p>In addition to execution and lifecycle control methods, this + * class provides status check methods (for example + * {@link #getStealCount}) that are intended to aid in developing, + * tuning, and monitoring fork/join applications. Also, method + * {@link #toString} returns indications of pool state in a + * convenient form for informal monitoring. + * + * <p>As is the case with other ExecutorServices, there are three + * main task execution methods summarized in the following table. + * These are designed to be used primarily by clients not already + * engaged in fork/join computations in the current pool. The main + * forms of these methods accept instances of {@code ForkJoinTask}, + * but overloaded forms also allow mixed execution of plain {@code + * Runnable}- or {@code Callable}- based activities as well. However, + * tasks that are already executing in a pool should normally instead + * use the within-computation forms listed in the table unless using + * async event-style tasks that are not usually joined, in which case + * there is little difference among choice of methods. + * + * <table BORDER CELLPADDING=3 CELLSPACING=1> + * <caption>Summary of task execution methods</caption> + * <tr> + * <td></td> + * <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td> + * <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td> + * </tr> + * <tr> + * <td> <b>Arrange async execution</b></td> + * <td> {@link #execute(ForkJoinTask)}</td> + * <td> {@link ForkJoinTask#fork}</td> + * </tr> + * <tr> + * <td> <b>Await and obtain result</b></td> + * <td> {@link #invoke(ForkJoinTask)}</td> + * <td> {@link ForkJoinTask#invoke}</td> + * </tr> + * <tr> + * <td> <b>Arrange exec and obtain Future</b></td> + * <td> {@link #submit(ForkJoinTask)}</td> + * <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td> + * </tr> + * </table> + * + * <p>The common pool is by default constructed with default + * parameters, but these may be controlled by setting three + * {@linkplain System#getProperty system properties}: + * <ul> + * <li>{@code java.util.concurrent.ForkJoinPool.common.parallelism} + * - the parallelism level, a non-negative integer + * <li>{@code java.util.concurrent.ForkJoinPool.common.threadFactory} + * - the class name of a {@link ForkJoinWorkerThreadFactory} + * <li>{@code java.util.concurrent.ForkJoinPool.common.exceptionHandler} + * - the class name of a {@link UncaughtExceptionHandler} + * </ul> + * The system class loader is used to load these classes. + * Upon any error in establishing these settings, default parameters + * are used. It is possible to disable or limit the use of threads in + * the common pool by setting the parallelism property to zero, and/or + * using a factory that may return {@code null}. + * + * <p><b>Implementation notes</b>: This implementation restricts the + * maximum number of running threads to 32767. Attempts to create + * pools with greater than the maximum number result in + * {@code IllegalArgumentException}. + * + * <p>This implementation rejects submitted tasks (that is, by throwing + * {@link RejectedExecutionException}) only when the pool is shut down + * or internal resources have been exhausted. + * + * @since 1.7 + * @author Doug Lea + */ +public class ForkJoinPool extends AbstractExecutorService { + + /* + * Implementation Overview + * + * This class and its nested classes provide the main + * functionality and control for a set of worker threads: + * Submissions from non-FJ threads enter into submission queues. + * Workers take these tasks and typically split them into subtasks + * that may be stolen by other workers. Preference rules give + * first priority to processing tasks from their own queues (LIFO + * or FIFO, depending on mode), then to randomized FIFO steals of + * tasks in other queues. + * + * WorkQueues + * ========== + * + * Most operations occur within work-stealing queues (in nested + * class WorkQueue). These are special forms of Deques that + * support only three of the four possible end-operations -- push, + * pop, and poll (aka steal), under the further constraints that + * push and pop are called only from the owning thread (or, as + * extended here, under a lock), while poll may be called from + * other threads. (If you are unfamiliar with them, you probably + * want to read Herlihy and Shavit's book "The Art of + * Multiprocessor programming", chapter 16 describing these in + * more detail before proceeding.) The main work-stealing queue + * design is roughly similar to those in the papers "Dynamic + * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005 + * (http://research.sun.com/scalable/pubs/index.html) and + * "Idempotent work stealing" by Michael, Saraswat, and Vechev, + * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186). + * See also "Correct and Efficient Work-Stealing for Weak Memory + * Models" by Le, Pop, Cohen, and Nardelli, PPoPP 2013 + * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an + * analysis of memory ordering (atomic, volatile etc) issues. The + * main differences ultimately stem from GC requirements that we + * null out taken slots as soon as we can, to maintain as small a + * footprint as possible even in programs generating huge numbers + * of tasks. To accomplish this, we shift the CAS arbitrating pop + * vs poll (steal) from being on the indices ("base" and "top") to + * the slots themselves. So, both a successful pop and poll + * mainly entail a CAS of a slot from non-null to null. Because + * we rely on CASes of references, we do not need tag bits on base + * or top. They are simple ints as used in any circular + * array-based queue (see for example ArrayDeque). Updates to the + * indices must still be ordered in a way that guarantees that top + * == base means the queue is empty, but otherwise may err on the + * side of possibly making the queue appear nonempty when a push, + * pop, or poll have not fully committed. Note that this means + * that the poll operation, considered individually, is not + * wait-free. One thief cannot successfully continue until another + * in-progress one (or, if previously empty, a push) completes. + * However, in the aggregate, we ensure at least probabilistic + * non-blockingness. If an attempted steal fails, a thief always + * chooses a different random victim target to try next. So, in + * order for one thief to progress, it suffices for any + * in-progress poll or new push on any empty queue to + * complete. (This is why we normally use method pollAt and its + * variants that try once at the apparent base index, else + * consider alternative actions, rather than method poll.) + * + * This approach also enables support of a user mode in which local + * task processing is in FIFO, not LIFO order, simply by using + * poll rather than pop. This can be useful in message-passing + * frameworks in which tasks are never joined. However neither + * mode considers affinities, loads, cache localities, etc, so + * rarely provide the best possible performance on a given + * machine, but portably provide good throughput by averaging over + * these factors. (Further, even if we did try to use such + * information, we do not usually have a basis for exploiting it. + * For example, some sets of tasks profit from cache affinities, + * but others are harmed by cache pollution effects.) + * + * WorkQueues are also used in a similar way for tasks submitted + * to the pool. We cannot mix these tasks in the same queues used + * for work-stealing (this would contaminate lifo/fifo + * processing). Instead, we randomly associate submission queues + * with submitting threads, using a form of hashing. The + * Submitter probe value serves as a hash code for + * choosing existing queues, and may be randomly repositioned upon + * contention with other submitters. In essence, submitters act + * like workers except that they are restricted to executing local + * tasks that they submitted (or in the case of CountedCompleters, + * others with the same root task). However, because most + * shared/external queue operations are more expensive than + * internal, and because, at steady state, external submitters + * will compete for CPU with workers, ForkJoinTask.join and + * related methods disable them from repeatedly helping to process + * tasks if all workers are active. Insertion of tasks in shared + * mode requires a lock (mainly to protect in the case of + * resizing) but we use only a simple spinlock (using bits in + * field qlock), because submitters encountering a busy queue move + * on to try or create other queues -- they block only when + * creating and registering new queues. + * + * Management + * ========== + * + * The main throughput advantages of work-stealing stem from + * decentralized control -- workers mostly take tasks from + * themselves or each other. We cannot negate this in the + * implementation of other management responsibilities. The main + * tactic for avoiding bottlenecks is packing nearly all + * essentially atomic control state into two volatile variables + * that are by far most often read (not written) as status and + * consistency checks. + * + * Field "ctl" contains 64 bits holding all the information needed + * to atomically decide to add, inactivate, enqueue (on an event + * queue), dequeue, and/or re-activate workers. To enable this + * packing, we restrict maximum parallelism to (1<<15)-1 (which is + * far in excess of normal operating range) to allow ids, counts, + * and their negations (used for thresholding) to fit into 16bit + * fields. + * + * Field "plock" is a form of sequence lock with a saturating + * shutdown bit (similarly for per-queue "qlocks"), mainly + * protecting updates to the workQueues array, as well as to + * enable shutdown. When used as a lock, it is normally only very + * briefly held, so is nearly always available after at most a + * brief spin, but we use a monitor-based backup strategy to + * block when needed. + * + * Recording WorkQueues. WorkQueues are recorded in the + * "workQueues" array that is created upon first use and expanded + * if necessary. Updates to the array while recording new workers + * and unrecording terminated ones are protected from each other + * by a lock but the array is otherwise concurrently readable, and + * accessed directly. To simplify index-based operations, the + * array size is always a power of two, and all readers must + * tolerate null slots. Worker queues are at odd indices. Shared + * (submission) queues are at even indices, up to a maximum of 64 + * slots, to limit growth even if array needs to expand to add + * more workers. Grouping them together in this way simplifies and + * speeds up task scanning. + * + * All worker thread creation is on-demand, triggered by task + * submissions, replacement of terminated workers, and/or + * compensation for blocked workers. However, all other support + * code is set up to work with other policies. To ensure that we + * do not hold on to worker references that would prevent GC, ALL + * accesses to workQueues are via indices into the workQueues + * array (which is one source of some of the messy code + * constructions here). In essence, the workQueues array serves as + * a weak reference mechanism. Thus for example the wait queue + * field of ctl stores indices, not references. Access to the + * workQueues in associated methods (for example signalWork) must + * both index-check and null-check the IDs. All such accesses + * ignore bad IDs by returning out early from what they are doing, + * since this can only be associated with termination, in which + * case it is OK to give up. All uses of the workQueues array + * also check that it is non-null (even if previously + * non-null). This allows nulling during termination, which is + * currently not necessary, but remains an option for + * resource-revocation-based shutdown schemes. It also helps + * reduce JIT issuance of uncommon-trap code, which tends to + * unnecessarily complicate control flow in some methods. + * + * Event Queuing. Unlike HPC work-stealing frameworks, we cannot + * let workers spin indefinitely scanning for tasks when none can + * be found immediately, and we cannot start/resume workers unless + * there appear to be tasks available. On the other hand, we must + * quickly prod them into action when new tasks are submitted or + * generated. In many usages, ramp-up time to activate workers is + * the main limiting factor in overall performance (this is + * compounded at program start-up by JIT compilation and + * allocation). So we try to streamline this as much as possible. + * We park/unpark workers after placing in an event wait queue + * when they cannot find work. This "queue" is actually a simple + * Treiber stack, headed by the "id" field of ctl, plus a 15bit + * counter value (that reflects the number of times a worker has + * been inactivated) to avoid ABA effects (we need only as many + * version numbers as worker threads). Successors are held in + * field WorkQueue.nextWait. Queuing deals with several intrinsic + * races, mainly that a task-producing thread can miss seeing (and + * signalling) another thread that gave up looking for work but + * has not yet entered the wait queue. We solve this by requiring + * a full sweep of all workers (via repeated calls to method + * scan()) both before and after a newly waiting worker is added + * to the wait queue. Because enqueued workers may actually be + * rescanning rather than waiting, we set and clear the "parker" + * field of WorkQueues to reduce unnecessary calls to unpark. + * (This requires a secondary recheck to avoid missed signals.) + * Note the unusual conventions about Thread.interrupts + * surrounding parking and other blocking: Because interrupts are + * used solely to alert threads to check termination, which is + * checked anyway upon blocking, we clear status (using + * Thread.interrupted) before any call to park, so that park does + * not immediately return due to status being set via some other + * unrelated call to interrupt in user code. + * + * Signalling. We create or wake up workers only when there + * appears to be at least one task they might be able to find and + * execute. When a submission is added or another worker adds a + * task to a queue that has fewer than two tasks, they signal + * waiting workers (or trigger creation of new ones if fewer than + * the given parallelism level -- signalWork). These primary + * signals are buttressed by others whenever other threads remove + * a task from a queue and notice that there are other tasks there + * as well. So in general, pools will be over-signalled. On most + * platforms, signalling (unpark) overhead time is noticeably + * long, and the time between signalling a thread and it actually + * making progress can be very noticeably long, so it is worth + * offloading these delays from critical paths as much as + * possible. Additionally, workers spin-down gradually, by staying + * alive so long as they see the ctl state changing. Similar + * stability-sensing techniques are also used before blocking in + * awaitJoin and helpComplete. + * + * Trimming workers. To release resources after periods of lack of + * use, a worker starting to wait when the pool is quiescent will + * time out and terminate if the pool has remained quiescent for a + * given period -- a short period if there are more threads than + * parallelism, longer as the number of threads decreases. This + * will slowly propagate, eventually terminating all workers after + * periods of non-use. + * + * Shutdown and Termination. A call to shutdownNow atomically sets + * a plock bit and then (non-atomically) sets each worker's + * qlock status, cancels all unprocessed tasks, and wakes up + * all waiting workers. Detecting whether termination should + * commence after a non-abrupt shutdown() call requires more work + * and bookkeeping. We need consensus about quiescence (i.e., that + * there is no more work). The active count provides a primary + * indication but non-abrupt shutdown still requires a rechecking + * scan for any workers that are inactive but not queued. + * + * Joining Tasks + * ============= + * + * Any of several actions may be taken when one worker is waiting + * to join a task stolen (or always held) by another. Because we + * are multiplexing many tasks on to a pool of workers, we can't + * just let them block (as in Thread.join). We also cannot just + * reassign the joiner's run-time stack with another and replace + * it later, which would be a form of "continuation", that even if + * possible is not necessarily a good idea since we sometimes need + * both an unblocked task and its continuation to progress. + * Instead we combine two tactics: + * + * Helping: Arranging for the joiner to execute some task that it + * would be running if the steal had not occurred. + * + * Compensating: Unless there are already enough live threads, + * method tryCompensate() may create or re-activate a spare + * thread to compensate for blocked joiners until they unblock. + * + * A third form (implemented in tryRemoveAndExec) amounts to + * helping a hypothetical compensator: If we can readily tell that + * a possible action of a compensator is to steal and execute the + * task being joined, the joining thread can do so directly, + * without the need for a compensation thread (although at the + * expense of larger run-time stacks, but the tradeoff is + * typically worthwhile). + * + * The ManagedBlocker extension API can't use helping so relies + * only on compensation in method awaitBlocker. + * + * The algorithm in tryHelpStealer entails a form of "linear" + * helping: Each worker records (in field currentSteal) the most + * recent task it stole from some other worker. Plus, it records + * (in field currentJoin) the task it is currently actively + * joining. Method tryHelpStealer uses these markers to try to + * find a worker to help (i.e., steal back a task from and execute + * it) that could hasten completion of the actively joined task. + * In essence, the joiner executes a task that would be on its own + * local deque had the to-be-joined task not been stolen. This may + * be seen as a conservative variant of the approach in Wagner & + * Calder "Leapfrogging: a portable technique for implementing + * efficient futures" SIGPLAN Notices, 1993 + * (http://portal.acm.org/citation.cfm?id=155354). It differs in + * that: (1) We only maintain dependency links across workers upon + * steals, rather than use per-task bookkeeping. This sometimes + * requires a linear scan of workQueues array to locate stealers, + * but often doesn't because stealers leave hints (that may become + * stale/wrong) of where to locate them. It is only a hint + * because a worker might have had multiple steals and the hint + * records only one of them (usually the most current). Hinting + * isolates cost to when it is needed, rather than adding to + * per-task overhead. (2) It is "shallow", ignoring nesting and + * potentially cyclic mutual steals. (3) It is intentionally + * racy: field currentJoin is updated only while actively joining, + * which means that we miss links in the chain during long-lived + * tasks, GC stalls etc (which is OK since blocking in such cases + * is usually a good idea). (4) We bound the number of attempts + * to find work (see MAX_HELP) and fall back to suspending the + * worker and if necessary replacing it with another. + * + * Helping actions for CountedCompleters are much simpler: Method + * helpComplete can take and execute any task with the same root + * as the task being waited on. However, this still entails some + * traversal of completer chains, so is less efficient than using + * CountedCompleters without explicit joins. + * + * It is impossible to keep exactly the target parallelism number + * of threads running at any given time. Determining the + * existence of conservatively safe helping targets, the + * availability of already-created spares, and the apparent need + * to create new spares are all racy, so we rely on multiple + * retries of each. Compensation in the apparent absence of + * helping opportunities is challenging to control on JVMs, where + * GC and other activities can stall progress of tasks that in + * turn stall out many other dependent tasks, without us being + * able to determine whether they will ever require compensation. + * Even though work-stealing otherwise encounters little + * degradation in the presence of more threads than cores, + * aggressively adding new threads in such cases entails risk of + * unwanted positive feedback control loops in which more threads + * cause more dependent stalls (as well as delayed progress of + * unblocked threads to the point that we know they are available) + * leading to more situations requiring more threads, and so + * on. This aspect of control can be seen as an (analytically + * intractable) game with an opponent that may choose the worst + * (for us) active thread to stall at any time. We take several + * precautions to bound losses (and thus bound gains), mainly in + * methods tryCompensate and awaitJoin. + * + * Common Pool + * =========== + * + * The static common pool always exists after static + * initialization. Since it (or any other created pool) need + * never be used, we minimize initial construction overhead and + * footprint to the setup of about a dozen fields, with no nested + * allocation. Most bootstrapping occurs within method + * fullExternalPush during the first submission to the pool. + * + * When external threads submit to the common pool, they can + * perform subtask processing (see externalHelpJoin and related + * methods). This caller-helps policy makes it sensible to set + * common pool parallelism level to one (or more) less than the + * total number of available cores, or even zero for pure + * caller-runs. We do not need to record whether external + * submissions are to the common pool -- if not, externalHelpJoin + * returns quickly (at the most helping to signal some common pool + * workers). These submitters would otherwise be blocked waiting + * for completion, so the extra effort (with liberally sprinkled + * task status checks) in inapplicable cases amounts to an odd + * form of limited spin-wait before blocking in ForkJoinTask.join. + * + * Style notes + * =========== + * + * There is a lot of representation-level coupling among classes + * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask. The + * fields of WorkQueue maintain data structures managed by + * ForkJoinPool, so are directly accessed. There is little point + * trying to reduce this, since any associated future changes in + * representations will need to be accompanied by algorithmic + * changes anyway. Several methods intrinsically sprawl because + * they must accumulate sets of consistent reads of volatiles held + * in local variables. Methods signalWork() and scan() are the + * main bottlenecks, so are especially heavily + * micro-optimized/mangled. There are lots of inline assignments + * (of form "while ((local = field) != 0)") which are usually the + * simplest way to ensure the required read orderings (which are + * sometimes critical). This leads to a "C"-like style of listing + * declarations of these locals at the heads of methods or blocks. + * There are several occurrences of the unusual "do {} while + * (!cas...)" which is the simplest way to force an update of a + * CAS'ed variable. There are also other coding oddities (including + * several unnecessary-looking hoisted null checks) that help + * some methods perform reasonably even when interpreted (not + * compiled). + * + * The order of declarations in this file is: + * (1) Static utility functions + * (2) Nested (static) classes + * (3) Static fields + * (4) Fields, along with constants used when unpacking some of them + * (5) Internal control methods + * (6) Callbacks and other support for ForkJoinTask methods + * (7) Exported methods + * (8) Static block initializing statics in minimally dependent order + */ + + // Static utilities + + /** + * If there is a security manager, makes sure caller has + * permission to modify threads. + */ + private static void checkPermission() { + SecurityManager security = System.getSecurityManager(); + if (security != null) + security.checkPermission(modifyThreadPermission); + } + + // Nested classes + + /** + * Factory for creating new {@link ForkJoinWorkerThread}s. + * A {@code ForkJoinWorkerThreadFactory} must be defined and used + * for {@code ForkJoinWorkerThread} subclasses that extend base + * functionality or initialize threads with different contexts. + */ + public static interface ForkJoinWorkerThreadFactory { + /** + * Returns a new worker thread operating in the given pool. + * + * @param pool the pool this thread works in + * @return the new worker thread + * @throws NullPointerException if the pool is null + */ + public ForkJoinWorkerThread newThread(ForkJoinPool pool); + } + + /** + * Default ForkJoinWorkerThreadFactory implementation; creates a + * new ForkJoinWorkerThread. + */ + static final class DefaultForkJoinWorkerThreadFactory + implements ForkJoinWorkerThreadFactory { + public final ForkJoinWorkerThread newThread(ForkJoinPool pool) { + return new ForkJoinWorkerThread(pool); + } + } + + /** + * Class for artificial tasks that are used to replace the target + * of local joins if they are removed from an interior queue slot + * in WorkQueue.tryRemoveAndExec. We don't need the proxy to + * actually do anything beyond having a unique identity. + */ + static final class EmptyTask extends ForkJoinTask<Void> { + private static final long serialVersionUID = -7721805057305804111L; + EmptyTask() { status = ForkJoinTask.NORMAL; } // force done + public final Void getRawResult() { return null; } + public final void setRawResult(Void x) {} + public final boolean exec() { return true; } + } + + /** + * Queues supporting work-stealing as well as external task + * submission. See above for main rationale and algorithms. + * Implementation relies heavily on "Unsafe" intrinsics + * and selective use of "volatile": + * + * Field "base" is the index (mod array.length) of the least valid + * queue slot, which is always the next position to steal (poll) + * from if nonempty. Reads and writes require volatile orderings + * but not CAS, because updates are only performed after slot + * CASes. + * + * Field "top" is the index (mod array.length) of the next queue + * slot to push to or pop from. It is written only by owner thread + * for push, or under lock for external/shared push, and accessed + * by other threads only after reading (volatile) base. Both top + * and base are allowed to wrap around on overflow, but (top - + * base) (or more commonly -(base - top) to force volatile read of + * base before top) still estimates size. The lock ("qlock") is + * forced to -1 on termination, causing all further lock attempts + * to fail. (Note: we don't need CAS for termination state because + * upon pool shutdown, all shared-queues will stop being used + * anyway.) Nearly all lock bodies are set up so that exceptions + * within lock bodies are "impossible" (modulo JVM errors that + * would cause failure anyway.) + * + * The array slots are read and written using the emulation of + * volatiles/atomics provided by Unsafe. Insertions must in + * general use putOrderedObject as a form of releasing store to + * ensure that all writes to the task object are ordered before + * its publication in the queue. All removals entail a CAS to + * null. The array is always a power of two. To ensure safety of + * Unsafe array operations, all accesses perform explicit null + * checks and implicit bounds checks via power-of-two masking. + * + * In addition to basic queuing support, this class contains + * fields described elsewhere to control execution. It turns out + * to work better memory-layout-wise to include them in this class + * rather than a separate class. + * + * Performance on most platforms is very sensitive to placement of + * instances of both WorkQueues and their arrays -- we absolutely + * do not want multiple WorkQueue instances or multiple queue + * arrays sharing cache lines. (It would be best for queue objects + * and their arrays to share, but there is nothing available to + * help arrange that). The @Contended annotation alerts JVMs to + * try to keep instances apart. + */ + static final class WorkQueue { + /** + * Capacity of work-stealing queue array upon initialization. + * Must be a power of two; at least 4, but should be larger to + * reduce or eliminate cacheline sharing among queues. + * Currently, it is much larger, as a partial workaround for + * the fact that JVMs often place arrays in locations that + * share GC bookkeeping (especially cardmarks) such that + * per-write accesses encounter serious memory contention. + */ + static final int INITIAL_QUEUE_CAPACITY = 1 << 13; + + /** + * Maximum size for queue arrays. Must be a power of two less + * than or equal to 1 << (31 - width of array entry) to ensure + * lack of wraparound of index calculations, but defined to a + * value a bit less than this to help users trap runaway + * programs before saturating systems. + */ + static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M + + // Heuristic padding to ameliorate unfortunate memory placements + volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06; + + volatile int eventCount; // encoded inactivation count; < 0 if inactive + int nextWait; // encoded record of next event waiter + int nsteals; // number of steals + int hint; // steal index hint + short poolIndex; // index of this queue in pool + final short mode; // 0: lifo, > 0: fifo, < 0: shared + volatile int qlock; // 1: locked, -1: terminate; else 0 + volatile int base; // index of next slot for poll + int top; // index of next slot for push + ForkJoinTask<?>[] array; // the elements (initially unallocated) + final ForkJoinPool pool; // the containing pool (may be null) + final ForkJoinWorkerThread owner; // owning thread or null if shared + volatile Thread parker; // == owner during call to park; else null + volatile ForkJoinTask<?> currentJoin; // task being joined in awaitJoin + ForkJoinTask<?> currentSteal; // current non-local task being executed + + volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17; + volatile Object pad18, pad19, pad1a, pad1b, pad1c, pad1d; + + WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode, + int seed) { + this.pool = pool; + this.owner = owner; + this.mode = (short)mode; + this.hint = seed; // store initial seed for runWorker + // Place indices in the center of array (that is not yet allocated) + base = top = INITIAL_QUEUE_CAPACITY >>> 1; + } + + /** + * Returns the approximate number of tasks in the queue. + */ + final int queueSize() { + int n = base - top; // non-owner callers must read base first + return (n >= 0) ? 0 : -n; // ignore transient negative + } + + /** + * Provides a more accurate estimate of whether this queue has + * any tasks than does queueSize, by checking whether a + * near-empty queue has at least one unclaimed task. + */ + final boolean isEmpty() { + ForkJoinTask<?>[] a; int m, s; + int n = base - (s = top); + return (n >= 0 || + (n == -1 && + ((a = array) == null || + (m = a.length - 1) < 0 || + U.getObject + (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null))); + } + + /** + * Pushes a task. Call only by owner in unshared queues. (The + * shared-queue version is embedded in method externalPush.) + * + * @param task the task. Caller must ensure non-null. + * @throws RejectedExecutionException if array cannot be resized + */ + final void push(ForkJoinTask<?> task) { + ForkJoinTask<?>[] a; ForkJoinPool p; + int s = top, n; + if ((a = array) != null) { // ignore if queue removed + int m = a.length - 1; + U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task); + if ((n = (top = s + 1) - base) <= 2) + (p = pool).signalWork(p.workQueues, this); + else if (n >= m) + growArray(); + } + } + + /** + * Initializes or doubles the capacity of array. Call either + * by owner or with lock held -- it is OK for base, but not + * top, to move while resizings are in progress. + */ + final ForkJoinTask<?>[] growArray() { + ForkJoinTask<?>[] oldA = array; + int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY; + if (size > MAXIMUM_QUEUE_CAPACITY) + throw new RejectedExecutionException("Queue capacity exceeded"); + int oldMask, t, b; + ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size]; + if (oldA != null && (oldMask = oldA.length - 1) >= 0 && + (t = top) - (b = base) > 0) { + int mask = size - 1; + do { + ForkJoinTask<?> x; + int oldj = ((b & oldMask) << ASHIFT) + ABASE; + int j = ((b & mask) << ASHIFT) + ABASE; + x = (ForkJoinTask<?>)U.getObjectVolatile(oldA, oldj); + if (x != null && + U.compareAndSwapObject(oldA, oldj, x, null)) + U.putObjectVolatile(a, j, x); + } while (++b != t); + } + return a; + } + + /** + * Takes next task, if one exists, in LIFO order. Call only + * by owner in unshared queues. + */ + final ForkJoinTask<?> pop() { + ForkJoinTask<?>[] a; ForkJoinTask<?> t; int m; + if ((a = array) != null && (m = a.length - 1) >= 0) { + for (int s; (s = top - 1) - base >= 0;) { + long j = ((m & s) << ASHIFT) + ABASE; + if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null) + break; + if (U.compareAndSwapObject(a, j, t, null)) { + top = s; + return t; + } + } + } + return null; + } + + /** + * Takes a task in FIFO order if b is base of queue and a task + * can be claimed without contention. Specialized versions + * appear in ForkJoinPool methods scan and tryHelpStealer. + */ + final ForkJoinTask<?> pollAt(int b) { + ForkJoinTask<?> t; ForkJoinTask<?>[] a; + if ((a = array) != null) { + int j = (((a.length - 1) & b) << ASHIFT) + ABASE; + if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) != null && + base == b && U.compareAndSwapObject(a, j, t, null)) { + U.putOrderedInt(this, QBASE, b + 1); + return t; + } + } + return null; + } + + /** + * Takes next task, if one exists, in FIFO order. + */ + final ForkJoinTask<?> poll() { + ForkJoinTask<?>[] a; int b; ForkJoinTask<?> t; + while ((b = base) - top < 0 && (a = array) != null) { + int j = (((a.length - 1) & b) << ASHIFT) + ABASE; + t = (ForkJoinTask<?>)U.getObjectVolatile(a, j); + if (t != null) { + if (U.compareAndSwapObject(a, j, t, null)) { + U.putOrderedInt(this, QBASE, b + 1); + return t; + } + } + else if (base == b) { + if (b + 1 == top) + break; + Thread.yield(); // wait for lagging update (very rare) + } + } + return null; + } + + /** + * Takes next task, if one exists, in order specified by mode. + */ + final ForkJoinTask<?> nextLocalTask() { + return mode == 0 ? pop() : poll(); + } + + /** + * Returns next task, if one exists, in order specified by mode. + */ + final ForkJoinTask<?> peek() { + ForkJoinTask<?>[] a = array; int m; + if (a == null || (m = a.length - 1) < 0) + return null; + int i = mode == 0 ? top - 1 : base; + int j = ((i & m) << ASHIFT) + ABASE; + return (ForkJoinTask<?>)U.getObjectVolatile(a, j); + } + + /** + * Pops the given task only if it is at the current top. + * (A shared version is available only via FJP.tryExternalUnpush) + */ + final boolean tryUnpush(ForkJoinTask<?> t) { + ForkJoinTask<?>[] a; int s; + if ((a = array) != null && (s = top) != base && + U.compareAndSwapObject + (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) { + top = s; + return true; + } + return false; + } + + /** + * Removes and cancels all known tasks, ignoring any exceptions. + */ + final void cancelAll() { + ForkJoinTask.cancelIgnoringExceptions(currentJoin); + ForkJoinTask.cancelIgnoringExceptions(currentSteal); + for (ForkJoinTask<?> t; (t = poll()) != null; ) + ForkJoinTask.cancelIgnoringExceptions(t); + } + + // Specialized execution methods + + /** + * Polls and runs tasks until empty. + */ + final void pollAndExecAll() { + for (ForkJoinTask<?> t; (t = poll()) != null;) + t.doExec(); + } + + /** + * Executes a top-level task and any local tasks remaining + * after execution. + */ + final void runTask(ForkJoinTask<?> task) { + if ((currentSteal = task) != null) { + task.doExec(); + ForkJoinTask<?>[] a = array; + int md = mode; + ++nsteals; + currentSteal = null; + if (md != 0) + pollAndExecAll(); + else if (a != null) { + int s, m = a.length - 1; + while ((s = top - 1) - base >= 0) { + long i = ((m & s) << ASHIFT) + ABASE; + ForkJoinTask<?> t = (ForkJoinTask<?>)U.getObject(a, i); + if (t == null) + break; + if (U.compareAndSwapObject(a, i, t, null)) { + top = s; + t.doExec(); + } + } + } + } + } + + /** + * If present, removes from queue and executes the given task, + * or any other cancelled task. Returns (true) on any CAS + * or consistency check failure so caller can retry. + * + * @return false if no progress can be made, else true + */ + final boolean tryRemoveAndExec(ForkJoinTask<?> task) { + boolean stat; + ForkJoinTask<?>[] a; int m, s, b, n; + if (task != null && (a = array) != null && (m = a.length - 1) >= 0 && + (n = (s = top) - (b = base)) > 0) { + boolean removed = false, empty = true; + stat = true; + for (ForkJoinTask<?> t;;) { // traverse from s to b + long j = ((--s & m) << ASHIFT) + ABASE; + t = (ForkJoinTask<?>)U.getObject(a, j); + if (t == null) // inconsistent length + break; + else if (t == task) { + if (s + 1 == top) { // pop + if (!U.compareAndSwapObject(a, j, task, null)) + break; + top = s; + removed = true; + } + else if (base == b) // replace with proxy + removed = U.compareAndSwapObject(a, j, task, + new EmptyTask()); + break; + } + else if (t.status >= 0) + empty = false; + else if (s + 1 == top) { // pop and throw away + if (U.compareAndSwapObject(a, j, t, null)) + top = s; + break; + } + if (--n == 0) { + if (!empty && base == b) + stat = false; + break; + } + } + if (removed) + task.doExec(); + } + else + stat = false; + return stat; + } + + /** + * Tries to poll for and execute the given task or any other + * task in its CountedCompleter computation. + */ + final boolean pollAndExecCC(CountedCompleter<?> root) { + ForkJoinTask<?>[] a; int b; Object o; CountedCompleter<?> t, r; + if ((b = base) - top < 0 && (a = array) != null) { + long j = (((a.length - 1) & b) << ASHIFT) + ABASE; + if ((o = U.getObjectVolatile(a, j)) == null) + return true; // retry + if (o instanceof CountedCompleter) { + for (t = (CountedCompleter<?>)o, r = t;;) { + if (r == root) { + if (base == b && + U.compareAndSwapObject(a, j, t, null)) { + U.putOrderedInt(this, QBASE, b + 1); + t.doExec(); + } + return true; + } + else if ((r = r.completer) == null) + break; // not part of root computation + } + } + } + return false; + } + + /** + * Tries to pop and execute the given task or any other task + * in its CountedCompleter computation. + */ + final boolean externalPopAndExecCC(CountedCompleter<?> root) { + ForkJoinTask<?>[] a; int s; Object o; CountedCompleter<?> t, r; + if (base - (s = top) < 0 && (a = array) != null) { + long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE; + if ((o = U.getObject(a, j)) instanceof CountedCompleter) { + for (t = (CountedCompleter<?>)o, r = t;;) { + if (r == root) { + if (U.compareAndSwapInt(this, QLOCK, 0, 1)) { + if (top == s && array == a && + U.compareAndSwapObject(a, j, t, null)) { + top = s - 1; + qlock = 0; + t.doExec(); + } + else + qlock = 0; + } + return true; + } + else if ((r = r.completer) == null) + break; + } + } + } + return false; + } + + /** + * Internal version + */ + final boolean internalPopAndExecCC(CountedCompleter<?> root) { + ForkJoinTask<?>[] a; int s; Object o; CountedCompleter<?> t, r; + if (base - (s = top) < 0 && (a = array) != null) { + long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE; + if ((o = U.getObject(a, j)) instanceof CountedCompleter) { + for (t = (CountedCompleter<?>)o, r = t;;) { + if (r == root) { + if (U.compareAndSwapObject(a, j, t, null)) { + top = s - 1; + t.doExec(); + } + return true; + } + else if ((r = r.completer) == null) + break; + } + } + } + return false; + } + + /** + * Returns true if owned and not known to be blocked. + */ + final boolean isApparentlyUnblocked() { + Thread wt; Thread.State s; + return (eventCount >= 0 && + (wt = owner) != null && + (s = wt.getState()) != Thread.State.BLOCKED && + s != Thread.State.WAITING && + s != Thread.State.TIMED_WAITING); + } + + // Unsafe mechanics + private static final sun.misc.Unsafe U; + private static final long QBASE; + private static final long QLOCK; + private static final int ABASE; + private static final int ASHIFT; + static { + try { + U = getUnsafe(); + Class<?> k = WorkQueue.class; + Class<?> ak = ForkJoinTask[].class; + QBASE = U.objectFieldOffset + (k.getDeclaredField("base")); + QLOCK = U.objectFieldOffset + (k.getDeclaredField("qlock")); + ABASE = U.arrayBaseOffset(ak); + int scale = U.arrayIndexScale(ak); + if ((scale & (scale - 1)) != 0) + throw new Error("data type scale not a power of two"); + ASHIFT = 31 - Integer.numberOfLeadingZeros(scale); + } catch (Exception e) { + throw new Error(e); + } + } + } + + // static fields (initialized in static initializer below) + + /** + * Per-thread submission bookkeeping. Shared across all pools + * to reduce ThreadLocal pollution and because random motion + * to avoid contention in one pool is likely to hold for others. + * Lazily initialized on first submission (but null-checked + * in other contexts to avoid unnecessary initialization). + */ + static final ThreadLocal<Submitter> submitters; + + /** + * Creates a new ForkJoinWorkerThread. This factory is used unless + * overridden in ForkJoinPool constructors. + */ + public static final ForkJoinWorkerThreadFactory + defaultForkJoinWorkerThreadFactory; + + /** + * Permission required for callers of methods that may start or + * kill threads. + */ + private static final RuntimePermission modifyThreadPermission; + + /** + * Common (static) pool. Non-null for public use unless a static + * construction exception, but internal usages null-check on use + * to paranoically avoid potential initialization circularities + * as well as to simplify generated code. + */ + static final ForkJoinPool common; + + /** + * Common pool parallelism. To allow simpler use and management + * when common pool threads are disabled, we allow the underlying + * common.parallelism field to be zero, but in that case still report + * parallelism as 1 to reflect resulting caller-runs mechanics. + */ + static final int commonParallelism; + + /** + * Sequence number for creating workerNamePrefix. + */ + private static int poolNumberSequence; + + /** + * Returns the next sequence number. We don't expect this to + * ever contend, so use simple builtin sync. + */ + private static final synchronized int nextPoolId() { + return ++poolNumberSequence; + } + + // static constants + + /** + * Initial timeout value (in nanoseconds) for the thread + * triggering quiescence to park waiting for new work. On timeout, + * the thread will instead try to shrink the number of + * workers. The value should be large enough to avoid overly + * aggressive shrinkage during most transient stalls (long GCs + * etc). + */ + private static final long IDLE_TIMEOUT = 2000L * 1000L * 1000L; // 2sec + + /** + * Timeout value when there are more threads than parallelism level + */ + private static final long FAST_IDLE_TIMEOUT = 200L * 1000L * 1000L; + + /** + * Tolerance for idle timeouts, to cope with timer undershoots + */ + private static final long TIMEOUT_SLOP = 2000000L; + + /** + * The maximum stolen->joining link depth allowed in method + * tryHelpStealer. Must be a power of two. Depths for legitimate + * chains are unbounded, but we use a fixed constant to avoid + * (otherwise unchecked) cycles and to bound staleness of + * traversal parameters at the expense of sometimes blocking when + * we could be helping. + */ + private static final int MAX_HELP = 64; + + /** + * Increment for seed generators. See class ThreadLocal for + * explanation. + */ + private static final int SEED_INCREMENT = 0x61c88647; + + /* + * Bits and masks for control variables + * + * Field ctl is a long packed with: + * AC: Number of active running workers minus target parallelism (16 bits) + * TC: Number of total workers minus target parallelism (16 bits) + * ST: true if pool is terminating (1 bit) + * EC: the wait count of top waiting thread (15 bits) + * ID: poolIndex of top of Treiber stack of waiters (16 bits) + * + * When convenient, we can extract the upper 32 bits of counts and + * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e = + * (int)ctl. The ec field is never accessed alone, but always + * together with id and st. The offsets of counts by the target + * parallelism and the positionings of fields makes it possible to + * perform the most common checks via sign tests of fields: When + * ac is negative, there are not enough active workers, when tc is + * negative, there are not enough total workers, and when e is + * negative, the pool is terminating. To deal with these possibly + * negative fields, we use casts in and out of "short" and/or + * signed shifts to maintain signedness. + * + * When a thread is queued (inactivated), its eventCount field is + * set negative, which is the only way to tell if a worker is + * prevented from executing tasks, even though it must continue to + * scan for them to avoid queuing races. Note however that + * eventCount updates lag releases so usage requires care. + * + * Field plock is an int packed with: + * SHUTDOWN: true if shutdown is enabled (1 bit) + * SEQ: a sequence lock, with PL_LOCK bit set if locked (30 bits) + * SIGNAL: set when threads may be waiting on the lock (1 bit) + * + * The sequence number enables simple consistency checks: + * Staleness of read-only operations on the workQueues array can + * be checked by comparing plock before vs after the reads. + */ + + // bit positions/shifts for fields + private static final int AC_SHIFT = 48; + private static final int TC_SHIFT = 32; + private static final int ST_SHIFT = 31; + private static final int EC_SHIFT = 16; + + // bounds + private static final int SMASK = 0xffff; // short bits + private static final int MAX_CAP = 0x7fff; // max #workers - 1 + private static final int EVENMASK = 0xfffe; // even short bits + private static final int SQMASK = 0x007e; // max 64 (even) slots + private static final int SHORT_SIGN = 1 << 15; + private static final int INT_SIGN = 1 << 31; + + // masks + private static final long STOP_BIT = 0x0001L << ST_SHIFT; + private static final long AC_MASK = ((long)SMASK) << AC_SHIFT; + private static final long TC_MASK = ((long)SMASK) << TC_SHIFT; + + // units for incrementing and decrementing + private static final long TC_UNIT = 1L << TC_SHIFT; + private static final long AC_UNIT = 1L << AC_SHIFT; + + // masks and units for dealing with u = (int)(ctl >>> 32) + private static final int UAC_SHIFT = AC_SHIFT - 32; + private static final int UTC_SHIFT = TC_SHIFT - 32; + private static final int UAC_MASK = SMASK << UAC_SHIFT; + private static final int UTC_MASK = SMASK << UTC_SHIFT; + private static final int UAC_UNIT = 1 << UAC_SHIFT; + private static final int UTC_UNIT = 1 << UTC_SHIFT; + + // masks and units for dealing with e = (int)ctl + private static final int E_MASK = 0x7fffffff; // no STOP_BIT + private static final int E_SEQ = 1 << EC_SHIFT; + + // plock bits + private static final int SHUTDOWN = 1 << 31; + private static final int PL_LOCK = 2; + private static final int PL_SIGNAL = 1; + private static final int PL_SPINS = 1 << 8; + + // access mode for WorkQueue + static final int LIFO_QUEUE = 0; + static final int FIFO_QUEUE = 1; + static final int SHARED_QUEUE = -1; + + // Heuristic padding to ameliorate unfortunate memory placements + volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06; + + // Instance fields + volatile long stealCount; // collects worker counts + volatile long ctl; // main pool control + volatile int plock; // shutdown status and seqLock + volatile int indexSeed; // worker/submitter index seed + final short parallelism; // parallelism level + final short mode; // LIFO/FIFO + WorkQueue[] workQueues; // main registry + final ForkJoinWorkerThreadFactory factory; + final UncaughtExceptionHandler ueh; // per-worker UEH + final String workerNamePrefix; // to create worker name string + + volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17; + volatile Object pad18, pad19, pad1a, pad1b; + + /** + * Acquires the plock lock to protect worker array and related + * updates. This method is called only if an initial CAS on plock + * fails. This acts as a spinlock for normal cases, but falls back + * to builtin monitor to block when (rarely) needed. This would be + * a terrible idea for a highly contended lock, but works fine as + * a more conservative alternative to a pure spinlock. + */ + private int acquirePlock() { + int spins = PL_SPINS, ps, nps; + for (;;) { + if (((ps = plock) & PL_LOCK) == 0 && + U.compareAndSwapInt(this, PLOCK, ps, nps = ps + PL_LOCK)) + return nps; + else if (spins >= 0) { + if (ThreadLocalRandom.current().nextInt() >= 0) + --spins; + } + else if (U.compareAndSwapInt(this, PLOCK, ps, ps | PL_SIGNAL)) { + synchronized (this) { + if ((plock & PL_SIGNAL) != 0) { + try { + wait(); + } catch (InterruptedException ie) { + try { + Thread.currentThread().interrupt(); + } catch (SecurityException ignore) { + } + } + } + else + notifyAll(); + } + } + } + } + + /** + * Unlocks and signals any thread waiting for plock. Called only + * when CAS of seq value for unlock fails. + */ + private void releasePlock(int ps) { + plock = ps; + synchronized (this) { notifyAll(); } + } + + /** + * Tries to create and start one worker if fewer than target + * parallelism level exist. Adjusts counts etc on failure. + */ + private void tryAddWorker() { + long c; int u, e; + while ((u = (int)((c = ctl) >>> 32)) < 0 && + (u & SHORT_SIGN) != 0 && (e = (int)c) >= 0) { + long nc = ((long)(((u + UTC_UNIT) & UTC_MASK) | + ((u + UAC_UNIT) & UAC_MASK)) << 32) | (long)e; + if (U.compareAndSwapLong(this, CTL, c, nc)) { + ForkJoinWorkerThreadFactory fac; + Throwable ex = null; + ForkJoinWorkerThread wt = null; + try { + if ((fac = factory) != null && + (wt = fac.newThread(this)) != null) { + wt.start(); + break; + } + } catch (Throwable rex) { + ex = rex; + } + deregisterWorker(wt, ex); + break; + } + } + } + + // Registering and deregistering workers + + /** + * Callback from ForkJoinWorkerThread to establish and record its + * WorkQueue. To avoid scanning bias due to packing entries in + * front of the workQueues array, we treat the array as a simple + * power-of-two hash table using per-thread seed as hash, + * expanding as needed. + * + * @param wt the worker thread + * @return the worker's queue + */ + final WorkQueue registerWorker(ForkJoinWorkerThread wt) { + UncaughtExceptionHandler handler; WorkQueue[] ws; int s, ps; + wt.setDaemon(true); + if ((handler = ueh) != null) + wt.setUncaughtExceptionHandler(handler); + do {} while (!U.compareAndSwapInt(this, INDEXSEED, s = indexSeed, + s += SEED_INCREMENT) || + s == 0); // skip 0 + WorkQueue w = new WorkQueue(this, wt, mode, s); + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); + try { + if ((ws = workQueues) != null) { // skip if shutting down + int n = ws.length, m = n - 1; + int r = (s << 1) | 1; // use odd-numbered indices + if (ws[r &= m] != null) { // collision + int probes = 0; // step by approx half size + int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2; + while (ws[r = (r + step) & m] != null) { + if (++probes >= n) { + workQueues = ws = Arrays.copyOf(ws, n <<= 1); + m = n - 1; + probes = 0; + } + } + } + w.poolIndex = (short)r; + w.eventCount = r; // volatile write orders + ws[r] = w; + } + } finally { + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + wt.setName(workerNamePrefix.concat(Integer.toString(w.poolIndex >>> 1))); + return w; + } + + /** + * Final callback from terminating worker, as well as upon failure + * to construct or start a worker. Removes record of worker from + * array, and adjusts counts. If pool is shutting down, tries to + * complete termination. + * + * @param wt the worker thread, or null if construction failed + * @param ex the exception causing failure, or null if none + */ + final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) { + WorkQueue w = null; + if (wt != null && (w = wt.workQueue) != null) { + int ps; long sc; + w.qlock = -1; // ensure set + do {} while (!U.compareAndSwapLong(this, STEALCOUNT, + sc = stealCount, + sc + w.nsteals)); + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); + try { + int idx = w.poolIndex; + WorkQueue[] ws = workQueues; + if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w) + ws[idx] = null; + } finally { + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + } + + long c; // adjust ctl counts + do {} while (!U.compareAndSwapLong + (this, CTL, c = ctl, (((c - AC_UNIT) & AC_MASK) | + ((c - TC_UNIT) & TC_MASK) | + (c & ~(AC_MASK|TC_MASK))))); + + if (!tryTerminate(false, false) && w != null && w.array != null) { + w.cancelAll(); // cancel remaining tasks + WorkQueue[] ws; WorkQueue v; Thread p; int u, i, e; + while ((u = (int)((c = ctl) >>> 32)) < 0 && (e = (int)c) >= 0) { + if (e > 0) { // activate or create replacement + if ((ws = workQueues) == null || + (i = e & SMASK) >= ws.length || + (v = ws[i]) == null) + break; + long nc = (((long)(v.nextWait & E_MASK)) | + ((long)(u + UAC_UNIT) << 32)); + if (v.eventCount != (e | INT_SIGN)) + break; + if (U.compareAndSwapLong(this, CTL, c, nc)) { + v.eventCount = (e + E_SEQ) & E_MASK; + if ((p = v.parker) != null) + U.unpark(p); + break; + } + } + else { + if ((short)u < 0) + tryAddWorker(); + break; + } + } + } + if (ex == null) // help clean refs on way out + ForkJoinTask.helpExpungeStaleExceptions(); + else // rethrow + ForkJoinTask.rethrow(ex); + } + + // Submissions + + /** + * Per-thread records for threads that submit to pools. Currently + * holds only pseudo-random seed / index that is used to choose + * submission queues in method externalPush. In the future, this may + * also incorporate a means to implement different task rejection + * and resubmission policies. + * + * Seeds for submitters and workers/workQueues work in basically + * the same way but are initialized and updated using slightly + * different mechanics. Both are initialized using the same + * approach as in class ThreadLocal, where successive values are + * unlikely to collide with previous values. Seeds are then + * randomly modified upon collisions using xorshifts, which + * requires a non-zero seed. + */ + static final class Submitter { + int seed; + Submitter(int s) { seed = s; } + } + + /** + * Unless shutting down, adds the given task to a submission queue + * at submitter's current queue index (modulo submission + * range). Only the most common path is directly handled in this + * method. All others are relayed to fullExternalPush. + * + * @param task the task. Caller must ensure non-null. + */ + final void externalPush(ForkJoinTask<?> task) { + Submitter z = submitters.get(); + WorkQueue q; int r, m, s, n, am; ForkJoinTask<?>[] a; + int ps = plock; + WorkQueue[] ws = workQueues; + if (z != null && ps > 0 && ws != null && (m = (ws.length - 1)) >= 0 && + (q = ws[m & (r = z.seed) & SQMASK]) != null && r != 0 && + U.compareAndSwapInt(q, QLOCK, 0, 1)) { // lock + if ((a = q.array) != null && + (am = a.length - 1) > (n = (s = q.top) - q.base)) { + int j = ((am & s) << ASHIFT) + ABASE; + U.putOrderedObject(a, j, task); + q.top = s + 1; // push on to deque + q.qlock = 0; + if (n <= 1) + signalWork(ws, q); + return; + } + q.qlock = 0; + } + fullExternalPush(task); + } + + /** + * Full version of externalPush. This method is called, among + * other times, upon the first submission of the first task to the + * pool, so must perform secondary initialization. It also + * detects first submission by an external thread by looking up + * its ThreadLocal, and creates a new shared queue if the one at + * index if empty or contended. The plock lock body must be + * exception-free (so no try/finally) so we optimistically + * allocate new queues outside the lock and throw them away if + * (very rarely) not needed. + * + * Secondary initialization occurs when plock is zero, to create + * workQueue array and set plock to a valid value. This lock body + * must also be exception-free. Because the plock seq value can + * eventually wrap around zero, this method harmlessly fails to + * reinitialize if workQueues exists, while still advancing plock. + */ + private void fullExternalPush(ForkJoinTask<?> task) { + int r = 0; // random index seed + for (Submitter z = submitters.get();;) { + WorkQueue[] ws; WorkQueue q; int ps, m, k; + if (z == null) { + if (U.compareAndSwapInt(this, INDEXSEED, r = indexSeed, + r += SEED_INCREMENT) && r != 0) + submitters.set(z = new Submitter(r)); + } + else if (r == 0) { // move to a different index + r = z.seed; + r ^= r << 13; // same xorshift as WorkQueues + r ^= r >>> 17; + z.seed = r ^= (r << 5); + } + if ((ps = plock) < 0) + throw new RejectedExecutionException(); + else if (ps == 0 || (ws = workQueues) == null || + (m = ws.length - 1) < 0) { // initialize workQueues + int p = parallelism; // find power of two table size + int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots + n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; + n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1; + WorkQueue[] nws = ((ws = workQueues) == null || ws.length == 0 ? + new WorkQueue[n] : null); + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + if (((ws = workQueues) == null || ws.length == 0) && nws != null) + workQueues = nws; + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + else if ((q = ws[k = r & m & SQMASK]) != null) { + if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) { + ForkJoinTask<?>[] a = q.array; + int s = q.top; + boolean submitted = false; + try { // locked version of push + if ((a != null && a.length > s + 1 - q.base) || + (a = q.growArray()) != null) { // must presize + int j = (((a.length - 1) & s) << ASHIFT) + ABASE; + U.putOrderedObject(a, j, task); + q.top = s + 1; + submitted = true; + } + } finally { + q.qlock = 0; // unlock + } + if (submitted) { + signalWork(ws, q); + return; + } + } + r = 0; // move on failure + } + else if (((ps = plock) & PL_LOCK) == 0) { // create new queue + q = new WorkQueue(this, null, SHARED_QUEUE, r); + q.poolIndex = (short)k; + if (((ps = plock) & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + if ((ws = workQueues) != null && k < ws.length && ws[k] == null) + ws[k] = q; + int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN); + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + else + r = 0; + } + } + + // Maintaining ctl counts + + /** + * Increments active count; mainly called upon return from blocking. + */ + final void incrementActiveCount() { + long c; + do {} while (!U.compareAndSwapLong + (this, CTL, c = ctl, ((c & ~AC_MASK) | + ((c & AC_MASK) + AC_UNIT)))); + } + + /** + * Tries to create or activate a worker if too few are active. + * + * @param ws the worker array to use to find signallees + * @param q if non-null, the queue holding tasks to be processed + */ + final void signalWork(WorkQueue[] ws, WorkQueue q) { + for (;;) { + long c; int e, u, i; WorkQueue w; Thread p; + if ((u = (int)((c = ctl) >>> 32)) >= 0) + break; + if ((e = (int)c) <= 0) { + if ((short)u < 0) + tryAddWorker(); + break; + } + if (ws == null || ws.length <= (i = e & SMASK) || + (w = ws[i]) == null) + break; + long nc = (((long)(w.nextWait & E_MASK)) | + ((long)(u + UAC_UNIT)) << 32); + int ne = (e + E_SEQ) & E_MASK; + if (w.eventCount == (e | INT_SIGN) && + U.compareAndSwapLong(this, CTL, c, nc)) { + w.eventCount = ne; + if ((p = w.parker) != null) + U.unpark(p); + break; + } + if (q != null && q.base >= q.top) + break; + } + } + + // Scanning for tasks + + /** + * Top-level runloop for workers, called by ForkJoinWorkerThread.run. + */ + final void runWorker(WorkQueue w) { + w.growArray(); // allocate queue + for (int r = w.hint; scan(w, r) == 0; ) { + r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift + } + } + + /** + * Scans for and, if found, runs one task, else possibly + * inactivates the worker. This method operates on single reads of + * volatile state and is designed to be re-invoked continuously, + * in part because it returns upon detecting inconsistencies, + * contention, or state changes that indicate possible success on + * re-invocation. + * + * The scan searches for tasks across queues starting at a random + * index, checking each at least twice. The scan terminates upon + * either finding a non-empty queue, or completing the sweep. If + * the worker is not inactivated, it takes and runs a task from + * this queue. Otherwise, if not activated, it tries to activate + * itself or some other worker by signalling. On failure to find a + * task, returns (for retry) if pool state may have changed during + * an empty scan, or tries to inactivate if active, else possibly + * blocks or terminates via method awaitWork. + * + * @param w the worker (via its WorkQueue) + * @param r a random seed + * @return worker qlock status if would have waited, else 0 + */ + private final int scan(WorkQueue w, int r) { + WorkQueue[] ws; int m; + long c = ctl; // for consistency check + if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 && w != null) { + for (int j = m + m + 1, ec = w.eventCount;;) { + WorkQueue q; int b, e; ForkJoinTask<?>[] a; ForkJoinTask<?> t; + if ((q = ws[(r - j) & m]) != null && + (b = q.base) - q.top < 0 && (a = q.array) != null) { + long i = (((a.length - 1) & b) << ASHIFT) + ABASE; + if ((t = ((ForkJoinTask<?>) + U.getObjectVolatile(a, i))) != null) { + if (ec < 0) + helpRelease(c, ws, w, q, b); + else if (q.base == b && + U.compareAndSwapObject(a, i, t, null)) { + U.putOrderedInt(q, QBASE, b + 1); + if ((b + 1) - q.top < 0) + signalWork(ws, q); + w.runTask(t); + } + } + break; + } + else if (--j < 0) { + if ((ec | (e = (int)c)) < 0) // inactive or terminating + return awaitWork(w, c, ec); + else if (ctl == c) { // try to inactivate and enqueue + long nc = (long)ec | ((c - AC_UNIT) & (AC_MASK|TC_MASK)); + w.nextWait = e; + w.eventCount = ec | INT_SIGN; + if (!U.compareAndSwapLong(this, CTL, c, nc)) + w.eventCount = ec; // back out + } + break; + } + } + } + return 0; + } + + /** + * A continuation of scan(), possibly blocking or terminating + * worker w. Returns without blocking if pool state has apparently + * changed since last invocation. Also, if inactivating w has + * caused the pool to become quiescent, checks for pool + * termination, and, so long as this is not the only worker, waits + * for event for up to a given duration. On timeout, if ctl has + * not changed, terminates the worker, which will in turn wake up + * another worker to possibly repeat this process. + * + * @param w the calling worker + * @param c the ctl value on entry to scan + * @param ec the worker's eventCount on entry to scan + */ + private final int awaitWork(WorkQueue w, long c, int ec) { + int stat, ns; long parkTime, deadline; + if ((stat = w.qlock) >= 0 && w.eventCount == ec && ctl == c && + !Thread.interrupted()) { + int e = (int)c; + int u = (int)(c >>> 32); + int d = (u >> UAC_SHIFT) + parallelism; // active count + + if (e < 0 || (d <= 0 && tryTerminate(false, false))) + stat = w.qlock = -1; // pool is terminating + else if ((ns = w.nsteals) != 0) { // collect steals and retry + long sc; + w.nsteals = 0; + do {} while (!U.compareAndSwapLong(this, STEALCOUNT, + sc = stealCount, sc + ns)); + } + else { + long pc = ((d > 0 || ec != (e | INT_SIGN)) ? 0L : + ((long)(w.nextWait & E_MASK)) | // ctl to restore + ((long)(u + UAC_UNIT)) << 32); + if (pc != 0L) { // timed wait if last waiter + int dc = -(short)(c >>> TC_SHIFT); + parkTime = (dc < 0 ? FAST_IDLE_TIMEOUT: + (dc + 1) * IDLE_TIMEOUT); + deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP; + } + else + parkTime = deadline = 0L; + if (w.eventCount == ec && ctl == c) { + Thread wt = Thread.currentThread(); + U.putObject(wt, PARKBLOCKER, this); + w.parker = wt; // emulate LockSupport.park + if (w.eventCount == ec && ctl == c) + U.park(false, parkTime); // must recheck before park + w.parker = null; + U.putObject(wt, PARKBLOCKER, null); + if (parkTime != 0L && ctl == c && + deadline - System.nanoTime() <= 0L && + U.compareAndSwapLong(this, CTL, c, pc)) + stat = w.qlock = -1; // shrink pool + } + } + } + return stat; + } + + /** + * Possibly releases (signals) a worker. Called only from scan() + * when a worker with apparently inactive status finds a non-empty + * queue. This requires revalidating all of the associated state + * from caller. + */ + private final void helpRelease(long c, WorkQueue[] ws, WorkQueue w, + WorkQueue q, int b) { + WorkQueue v; int e, i; Thread p; + if (w != null && w.eventCount < 0 && (e = (int)c) > 0 && + ws != null && ws.length > (i = e & SMASK) && + (v = ws[i]) != null && ctl == c) { + long nc = (((long)(v.nextWait & E_MASK)) | + ((long)((int)(c >>> 32) + UAC_UNIT)) << 32); + int ne = (e + E_SEQ) & E_MASK; + if (q != null && q.base == b && w.eventCount < 0 && + v.eventCount == (e | INT_SIGN) && + U.compareAndSwapLong(this, CTL, c, nc)) { + v.eventCount = ne; + if ((p = v.parker) != null) + U.unpark(p); + } + } + } + + /** + * Tries to locate and execute tasks for a stealer of the given + * task, or in turn one of its stealers, Traces currentSteal -> + * currentJoin links looking for a thread working on a descendant + * of the given task and with a non-empty queue to steal back and + * execute tasks from. The first call to this method upon a + * waiting join will often entail scanning/search, (which is OK + * because the joiner has nothing better to do), but this method + * leaves hints in workers to speed up subsequent calls. The + * implementation is very branchy to cope with potential + * inconsistencies or loops encountering chains that are stale, + * unknown, or so long that they are likely cyclic. + * + * @param joiner the joining worker + * @param task the task to join + * @return 0 if no progress can be made, negative if task + * known complete, else positive + */ + private int tryHelpStealer(WorkQueue joiner, ForkJoinTask<?> task) { + int stat = 0, steps = 0; // bound to avoid cycles + if (task != null && joiner != null && + joiner.base - joiner.top >= 0) { // hoist checks + restart: for (;;) { + ForkJoinTask<?> subtask = task; // current target + for (WorkQueue j = joiner, v;;) { // v is stealer of subtask + WorkQueue[] ws; int m, s, h; + if ((s = task.status) < 0) { + stat = s; + break restart; + } + if ((ws = workQueues) == null || (m = ws.length - 1) <= 0) + break restart; // shutting down + if ((v = ws[h = (j.hint | 1) & m]) == null || + v.currentSteal != subtask) { + for (int origin = h;;) { // find stealer + if (((h = (h + 2) & m) & 15) == 1 && + (subtask.status < 0 || j.currentJoin != subtask)) + continue restart; // occasional staleness check + if ((v = ws[h]) != null && + v.currentSteal == subtask) { + j.hint = h; // save hint + break; + } + if (h == origin) + break restart; // cannot find stealer + } + } + for (;;) { // help stealer or descend to its stealer + ForkJoinTask[] a; int b; + if (subtask.status < 0) // surround probes with + continue restart; // consistency checks + if ((b = v.base) - v.top < 0 && (a = v.array) != null) { + int i = (((a.length - 1) & b) << ASHIFT) + ABASE; + ForkJoinTask<?> t = + (ForkJoinTask<?>)U.getObjectVolatile(a, i); + if (subtask.status < 0 || j.currentJoin != subtask || + v.currentSteal != subtask) + continue restart; // stale + stat = 1; // apparent progress + if (v.base == b) { + if (t == null) + break restart; + if (U.compareAndSwapObject(a, i, t, null)) { + U.putOrderedInt(v, QBASE, b + 1); + ForkJoinTask<?> ps = joiner.currentSteal; + int jt = joiner.top; + do { + joiner.currentSteal = t; + t.doExec(); // clear local tasks too + } while (task.status >= 0 && + joiner.top != jt && + (t = joiner.pop()) != null); + joiner.currentSteal = ps; + break restart; + } + } + } + else { // empty -- try to descend + ForkJoinTask<?> next = v.currentJoin; + if (subtask.status < 0 || j.currentJoin != subtask || + v.currentSteal != subtask) + continue restart; // stale + else if (next == null || ++steps == MAX_HELP) + break restart; // dead-end or maybe cyclic + else { + subtask = next; + j = v; + break; + } + } + } + } + } + } + return stat; + } + + /** + * Analog of tryHelpStealer for CountedCompleters. Tries to steal + * and run tasks within the target's computation. + * + * @param task the task to join + */ + private int helpComplete(WorkQueue joiner, CountedCompleter<?> task) { + WorkQueue[] ws; int m; + int s = 0; + if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 && + joiner != null && task != null) { + int j = joiner.poolIndex; + int scans = m + m + 1; + long c = 0L; // for stability check + for (int k = scans; ; j += 2) { + WorkQueue q; + if ((s = task.status) < 0) + break; + else if (joiner.internalPopAndExecCC(task)) + k = scans; + else if ((s = task.status) < 0) + break; + else if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) + k = scans; + else if (--k < 0) { + if (c == (c = ctl)) + break; + k = scans; + } + } + } + return s; + } + + /** + * Tries to decrement active count (sometimes implicitly) and + * possibly release or create a compensating worker in preparation + * for blocking. Fails on contention or termination. Otherwise, + * adds a new thread if no idle workers are available and pool + * may become starved. + * + * @param c the assumed ctl value + */ + final boolean tryCompensate(long c) { + WorkQueue[] ws = workQueues; + int pc = parallelism, e = (int)c, m, tc; + if (ws != null && (m = ws.length - 1) >= 0 && e >= 0 && ctl == c) { + WorkQueue w = ws[e & m]; + if (e != 0 && w != null) { + Thread p; + long nc = ((long)(w.nextWait & E_MASK) | + (c & (AC_MASK|TC_MASK))); + int ne = (e + E_SEQ) & E_MASK; + if (w.eventCount == (e | INT_SIGN) && + U.compareAndSwapLong(this, CTL, c, nc)) { + w.eventCount = ne; + if ((p = w.parker) != null) + U.unpark(p); + return true; // replace with idle worker + } + } + else if ((tc = (short)(c >>> TC_SHIFT)) >= 0 && + (int)(c >> AC_SHIFT) + pc > 1) { + long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK); + if (U.compareAndSwapLong(this, CTL, c, nc)) + return true; // no compensation + } + else if (tc + pc < MAX_CAP) { + long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK); + if (U.compareAndSwapLong(this, CTL, c, nc)) { + ForkJoinWorkerThreadFactory fac; + Throwable ex = null; + ForkJoinWorkerThread wt = null; + try { + if ((fac = factory) != null && + (wt = fac.newThread(this)) != null) { + wt.start(); + return true; + } + } catch (Throwable rex) { + ex = rex; + } + deregisterWorker(wt, ex); // clean up and return false + } + } + } + return false; + } + + /** + * Helps and/or blocks until the given task is done. + * + * @param joiner the joining worker + * @param task the task + * @return task status on exit + */ + final int awaitJoin(WorkQueue joiner, ForkJoinTask<?> task) { + int s = 0; + if (task != null && (s = task.status) >= 0 && joiner != null) { + ForkJoinTask<?> prevJoin = joiner.currentJoin; + joiner.currentJoin = task; + do {} while (joiner.tryRemoveAndExec(task) && // process local tasks + (s = task.status) >= 0); + if (s >= 0 && (task instanceof CountedCompleter)) + s = helpComplete(joiner, (CountedCompleter<?>)task); + long cc = 0; // for stability checks + while (s >= 0 && (s = task.status) >= 0) { + if ((s = tryHelpStealer(joiner, task)) == 0 && + (s = task.status) >= 0) { + if (!tryCompensate(cc)) + cc = ctl; + else { + if (task.trySetSignal() && (s = task.status) >= 0) { + synchronized (task) { + if (task.status >= 0) { + try { // see ForkJoinTask + task.wait(); // for explanation + } catch (InterruptedException ie) { + } + } + else + task.notifyAll(); + } + } + long c; // reactivate + do {} while (!U.compareAndSwapLong + (this, CTL, c = ctl, + ((c & ~AC_MASK) | + ((c & AC_MASK) + AC_UNIT)))); + } + } + } + joiner.currentJoin = prevJoin; + } + return s; + } + + /** + * Stripped-down variant of awaitJoin used by timed joins. Tries + * to help join only while there is continuous progress. (Caller + * will then enter a timed wait.) + * + * @param joiner the joining worker + * @param task the task + */ + final void helpJoinOnce(WorkQueue joiner, ForkJoinTask<?> task) { + int s; + if (joiner != null && task != null && (s = task.status) >= 0) { + ForkJoinTask<?> prevJoin = joiner.currentJoin; + joiner.currentJoin = task; + do {} while (joiner.tryRemoveAndExec(task) && // process local tasks + (s = task.status) >= 0); + if (s >= 0) { + if (task instanceof CountedCompleter) + helpComplete(joiner, (CountedCompleter<?>)task); + do {} while (task.status >= 0 && + tryHelpStealer(joiner, task) > 0); + } + joiner.currentJoin = prevJoin; + } + } + + /** + * Returns a (probably) non-empty steal queue, if one is found + * during a scan, else null. This method must be retried by + * caller if, by the time it tries to use the queue, it is empty. + */ + private WorkQueue findNonEmptyStealQueue() { + int r = ThreadLocalRandom.current().nextInt(); + for (;;) { + int ps = plock, m; WorkQueue[] ws; WorkQueue q; + if ((ws = workQueues) != null && (m = ws.length - 1) >= 0) { + for (int j = (m + 1) << 2; j >= 0; --j) { + if ((q = ws[(((r - j) << 1) | 1) & m]) != null && + q.base - q.top < 0) + return q; + } + } + if (plock == ps) + return null; + } + } + + /** + * Runs tasks until {@code isQuiescent()}. We piggyback on + * active count ctl maintenance, but rather than blocking + * when tasks cannot be found, we rescan until all others cannot + * find tasks either. + */ + final void helpQuiescePool(WorkQueue w) { + ForkJoinTask<?> ps = w.currentSteal; + for (boolean active = true;;) { + long c; WorkQueue q; ForkJoinTask<?> t; int b; + while ((t = w.nextLocalTask()) != null) + t.doExec(); + if ((q = findNonEmptyStealQueue()) != null) { + if (!active) { // re-establish active count + active = true; + do {} while (!U.compareAndSwapLong + (this, CTL, c = ctl, + ((c & ~AC_MASK) | + ((c & AC_MASK) + AC_UNIT)))); + } + if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) { + (w.currentSteal = t).doExec(); + w.currentSteal = ps; + } + } + else if (active) { // decrement active count without queuing + long nc = ((c = ctl) & ~AC_MASK) | ((c & AC_MASK) - AC_UNIT); + if ((int)(nc >> AC_SHIFT) + parallelism == 0) + break; // bypass decrement-then-increment + if (U.compareAndSwapLong(this, CTL, c, nc)) + active = false; + } + else if ((int)((c = ctl) >> AC_SHIFT) + parallelism <= 0 && + U.compareAndSwapLong + (this, CTL, c, ((c & ~AC_MASK) | + ((c & AC_MASK) + AC_UNIT)))) + break; + } + } + + /** + * Gets and removes a local or stolen task for the given worker. + * + * @return a task, if available + */ + final ForkJoinTask<?> nextTaskFor(WorkQueue w) { + for (ForkJoinTask<?> t;;) { + WorkQueue q; int b; + if ((t = w.nextLocalTask()) != null) + return t; + if ((q = findNonEmptyStealQueue()) == null) + return null; + if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) + return t; + } + } + + /** + * Returns a cheap heuristic guide for task partitioning when + * programmers, frameworks, tools, or languages have little or no + * idea about task granularity. In essence by offering this + * method, we ask users only about tradeoffs in overhead vs + * expected throughput and its variance, rather than how finely to + * partition tasks. + * + * In a steady state strict (tree-structured) computation, each + * thread makes available for stealing enough tasks for other + * threads to remain active. Inductively, if all threads play by + * the same rules, each thread should make available only a + * constant number of tasks. + * + * The minimum useful constant is just 1. But using a value of 1 + * would require immediate replenishment upon each steal to + * maintain enough tasks, which is infeasible. Further, + * partitionings/granularities of offered tasks should minimize + * steal rates, which in general means that threads nearer the top + * of computation tree should generate more than those nearer the + * bottom. In perfect steady state, each thread is at + * approximately the same level of computation tree. However, + * producing extra tasks amortizes the uncertainty of progress and + * diffusion assumptions. + * + * So, users will want to use values larger (but not much larger) + * than 1 to both smooth over transient shortages and hedge + * against uneven progress; as traded off against the cost of + * extra task overhead. We leave the user to pick a threshold + * value to compare with the results of this call to guide + * decisions, but recommend values such as 3. + * + * When all threads are active, it is on average OK to estimate + * surplus strictly locally. In steady-state, if one thread is + * maintaining say 2 surplus tasks, then so are others. So we can + * just use estimated queue length. However, this strategy alone + * leads to serious mis-estimates in some non-steady-state + * conditions (ramp-up, ramp-down, other stalls). We can detect + * many of these by further considering the number of "idle" + * threads, that are known to have zero queued tasks, so + * compensate by a factor of (#idle/#active) threads. + * + * Note: The approximation of #busy workers as #active workers is + * not very good under current signalling scheme, and should be + * improved. + */ + static int getSurplusQueuedTaskCount() { + Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q; + if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) { + int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).parallelism; + int n = (q = wt.workQueue).top - q.base; + int a = (int)(pool.ctl >> AC_SHIFT) + p; + return n - (a > (p >>>= 1) ? 0 : + a > (p >>>= 1) ? 1 : + a > (p >>>= 1) ? 2 : + a > (p >>>= 1) ? 4 : + 8); + } + return 0; + } + + // Termination + + /** + * Possibly initiates and/or completes termination. The caller + * triggering termination runs three passes through workQueues: + * (0) Setting termination status, followed by wakeups of queued + * workers; (1) cancelling all tasks; (2) interrupting lagging + * threads (likely in external tasks, but possibly also blocked in + * joins). Each pass repeats previous steps because of potential + * lagging thread creation. + * + * @param now if true, unconditionally terminate, else only + * if no work and no active workers + * @param enable if true, enable shutdown when next possible + * @return true if now terminating or terminated + */ + private boolean tryTerminate(boolean now, boolean enable) { + int ps; + if (this == common) // cannot shut down + return false; + if ((ps = plock) >= 0) { // enable by setting plock + if (!enable) + return false; + if ((ps & PL_LOCK) != 0 || + !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK)) + ps = acquirePlock(); + int nps = ((ps + PL_LOCK) & ~SHUTDOWN) | SHUTDOWN; + if (!U.compareAndSwapInt(this, PLOCK, ps, nps)) + releasePlock(nps); + } + for (long c;;) { + if (((c = ctl) & STOP_BIT) != 0) { // already terminating + if ((short)(c >>> TC_SHIFT) + parallelism <= 0) { + synchronized (this) { + notifyAll(); // signal when 0 workers + } + } + return true; + } + if (!now) { // check if idle & no tasks + WorkQueue[] ws; WorkQueue w; + if ((int)(c >> AC_SHIFT) + parallelism > 0) + return false; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; ++i) { + if ((w = ws[i]) != null && + (!w.isEmpty() || + ((i & 1) != 0 && w.eventCount >= 0))) { + signalWork(ws, w); + return false; + } + } + } + } + if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT)) { + for (int pass = 0; pass < 3; ++pass) { + WorkQueue[] ws; WorkQueue w; Thread wt; + if ((ws = workQueues) != null) { + int n = ws.length; + for (int i = 0; i < n; ++i) { + if ((w = ws[i]) != null) { + w.qlock = -1; + if (pass > 0) { + w.cancelAll(); + if (pass > 1 && (wt = w.owner) != null) { + if (!wt.isInterrupted()) { + try { + wt.interrupt(); + } catch (Throwable ignore) { + } + } + U.unpark(wt); + } + } + } + } + // Wake up workers parked on event queue + int i, e; long cc; Thread p; + while ((e = (int)(cc = ctl) & E_MASK) != 0 && + (i = e & SMASK) < n && i >= 0 && + (w = ws[i]) != null) { + long nc = ((long)(w.nextWait & E_MASK) | + ((cc + AC_UNIT) & AC_MASK) | + (cc & (TC_MASK|STOP_BIT))); + if (w.eventCount == (e | INT_SIGN) && + U.compareAndSwapLong(this, CTL, cc, nc)) { + w.eventCount = (e + E_SEQ) & E_MASK; + w.qlock = -1; + if ((p = w.parker) != null) + U.unpark(p); + } + } + } + } + } + } + } + + // external operations on common pool + + /** + * Returns common pool queue for a thread that has submitted at + * least one task. + */ + static WorkQueue commonSubmitterQueue() { + Submitter z; ForkJoinPool p; WorkQueue[] ws; int m, r; + return ((z = submitters.get()) != null && + (p = common) != null && + (ws = p.workQueues) != null && + (m = ws.length - 1) >= 0) ? + ws[m & z.seed & SQMASK] : null; + } + + /** + * Tries to pop the given task from submitter's queue in common pool. + */ + final boolean tryExternalUnpush(ForkJoinTask<?> task) { + WorkQueue joiner; ForkJoinTask<?>[] a; int m, s; + Submitter z = submitters.get(); + WorkQueue[] ws = workQueues; + boolean popped = false; + if (z != null && ws != null && (m = ws.length - 1) >= 0 && + (joiner = ws[z.seed & m & SQMASK]) != null && + joiner.base != (s = joiner.top) && + (a = joiner.array) != null) { + long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE; + if (U.getObject(a, j) == task && + U.compareAndSwapInt(joiner, QLOCK, 0, 1)) { + if (joiner.top == s && joiner.array == a && + U.compareAndSwapObject(a, j, task, null)) { + joiner.top = s - 1; + popped = true; + } + joiner.qlock = 0; + } + } + return popped; + } + + final int externalHelpComplete(CountedCompleter<?> task) { + WorkQueue joiner; int m, j; + Submitter z = submitters.get(); + WorkQueue[] ws = workQueues; + int s = 0; + if (z != null && ws != null && (m = ws.length - 1) >= 0 && + (joiner = ws[(j = z.seed) & m & SQMASK]) != null && task != null) { + int scans = m + m + 1; + long c = 0L; // for stability check + j |= 1; // poll odd queues + for (int k = scans; ; j += 2) { + WorkQueue q; + if ((s = task.status) < 0) + break; + else if (joiner.externalPopAndExecCC(task)) + k = scans; + else if ((s = task.status) < 0) + break; + else if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) + k = scans; + else if (--k < 0) { + if (c == (c = ctl)) + break; + k = scans; + } + } + } + return s; + } + + // Exported methods + + // Constructors + + /** + * Creates a {@code ForkJoinPool} with parallelism equal to {@link + * java.lang.Runtime#availableProcessors}, using the {@linkplain + * #defaultForkJoinWorkerThreadFactory default thread factory}, + * no UncaughtExceptionHandler, and non-async LIFO processing mode. + * + * @throws SecurityException if a security manager exists and + * the caller is not permitted to modify threads + * because it does not hold {@link + * java.lang.RuntimePermission}{@code ("modifyThread")} + */ + public ForkJoinPool() { + this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()), + defaultForkJoinWorkerThreadFactory, null, false); + } + + /** + * Creates a {@code ForkJoinPool} with the indicated parallelism + * level, the {@linkplain + * #defaultForkJoinWorkerThreadFactory default thread factory}, + * no UncaughtExceptionHandler, and non-async LIFO processing mode. + * + * @param parallelism the parallelism level + * @throws IllegalArgumentException if parallelism less than or + * equal to zero, or greater than implementation limit + * @throws SecurityException if a security manager exists and + * the caller is not permitted to modify threads + * because it does not hold {@link + * java.lang.RuntimePermission}{@code ("modifyThread")} + */ + public ForkJoinPool(int parallelism) { + this(parallelism, defaultForkJoinWorkerThreadFactory, null, false); + } + + /** + * Creates a {@code ForkJoinPool} with the given parameters. + * + * @param parallelism the parallelism level. For default value, + * use {@link java.lang.Runtime#availableProcessors}. + * @param factory the factory for creating new threads. For default value, + * use {@link #defaultForkJoinWorkerThreadFactory}. + * @param handler the handler for internal worker threads that + * terminate due to unrecoverable errors encountered while executing + * tasks. For default value, use {@code null}. + * @param asyncMode if true, + * establishes local first-in-first-out scheduling mode for forked + * tasks that are never joined. This mode may be more appropriate + * than default locally stack-based mode in applications in which + * worker threads only process event-style asynchronous tasks. + * For default value, use {@code false}. + * @throws IllegalArgumentException if parallelism less than or + * equal to zero, or greater than implementation limit + * @throws NullPointerException if the factory is null + * @throws SecurityException if a security manager exists and + * the caller is not permitted to modify threads + * because it does not hold {@link + * java.lang.RuntimePermission}{@code ("modifyThread")} + */ + public ForkJoinPool(int parallelism, + ForkJoinWorkerThreadFactory factory, + UncaughtExceptionHandler handler, + boolean asyncMode) { + this(checkParallelism(parallelism), + checkFactory(factory), + handler, + (asyncMode ? FIFO_QUEUE : LIFO_QUEUE), + "ForkJoinPool-" + nextPoolId() + "-worker-"); + checkPermission(); + } + + private static int checkParallelism(int parallelism) { + if (parallelism <= 0 || parallelism > MAX_CAP) + throw new IllegalArgumentException(); + return parallelism; + } + + private static ForkJoinWorkerThreadFactory checkFactory + (ForkJoinWorkerThreadFactory factory) { + if (factory == null) + throw new NullPointerException(); + return factory; + } + + /** + * Creates a {@code ForkJoinPool} with the given parameters, without + * any security checks or parameter validation. Invoked directly by + * makeCommonPool. + */ + private ForkJoinPool(int parallelism, + ForkJoinWorkerThreadFactory factory, + UncaughtExceptionHandler handler, + int mode, + String workerNamePrefix) { + this.workerNamePrefix = workerNamePrefix; + this.factory = factory; + this.ueh = handler; + this.mode = (short)mode; + this.parallelism = (short)parallelism; + long np = (long)(-parallelism); // offset ctl counts + this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); + } + + /** + * Returns the common pool instance. This pool is statically + * constructed; its run state is unaffected by attempts to {@link + * #shutdown} or {@link #shutdownNow}. However this pool and any + * ongoing processing are automatically terminated upon program + * {@link System#exit}. Any program that relies on asynchronous + * task processing to complete before program termination should + * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence}, + * before exit. + * + * @return the common pool instance + * @since 1.8 + */ + public static ForkJoinPool commonPool() { + // assert common != null : "static init error"; + return common; + } + + // Execution methods + + /** + * Performs the given task, returning its result upon completion. + * If the computation encounters an unchecked Exception or Error, + * it is rethrown as the outcome of this invocation. Rethrown + * exceptions behave in the same way as regular exceptions, but, + * when possible, contain stack traces (as displayed for example + * using {@code ex.printStackTrace()}) of both the current thread + * as well as the thread actually encountering the exception; + * minimally only the latter. + * + * @param task the task + * @param <T> the type of the task's result + * @return the task's result + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public <T> T invoke(ForkJoinTask<T> task) { + if (task == null) + throw new NullPointerException(); + externalPush(task); + return task.join(); + } + + /** + * Arranges for (asynchronous) execution of the given task. + * + * @param task the task + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public void execute(ForkJoinTask<?> task) { + if (task == null) + throw new NullPointerException(); + externalPush(task); + } + + // AbstractExecutorService methods + + /** + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public void execute(Runnable task) { + if (task == null) + throw new NullPointerException(); + ForkJoinTask<?> job; + if (task instanceof ForkJoinTask<?>) // avoid re-wrap + job = (ForkJoinTask<?>) task; + else + job = new ForkJoinTask.RunnableExecuteAction(task); + externalPush(job); + } + + /** + * Submits a ForkJoinTask for execution. + * + * @param task the task to submit + * @param <T> the type of the task's result + * @return the task + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) { + if (task == null) + throw new NullPointerException(); + externalPush(task); + return task; + } + + /** + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public <T> ForkJoinTask<T> submit(Callable<T> task) { + ForkJoinTask<T> job = new ForkJoinTask.AdaptedCallable<T>(task); + externalPush(job); + return job; + } + + /** + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public <T> ForkJoinTask<T> submit(Runnable task, T result) { + ForkJoinTask<T> job = new ForkJoinTask.AdaptedRunnable<T>(task, result); + externalPush(job); + return job; + } + + /** + * @throws NullPointerException if the task is null + * @throws RejectedExecutionException if the task cannot be + * scheduled for execution + */ + public ForkJoinTask<?> submit(Runnable task) { + if (task == null) + throw new NullPointerException(); + ForkJoinTask<?> job; + if (task instanceof ForkJoinTask<?>) // avoid re-wrap + job = (ForkJoinTask<?>) task; + else + job = new ForkJoinTask.AdaptedRunnableAction(task); + externalPush(job); + return job; + } + + /** + * @throws NullPointerException {@inheritDoc} + * @throws RejectedExecutionException {@inheritDoc} + */ + public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) { + // In previous versions of this class, this method constructed + // a task to run ForkJoinTask.invokeAll, but now external + // invocation of multiple tasks is at least as efficient. + ArrayList<Future<T>> futures = new ArrayList<Future<T>>(tasks.size()); + + boolean done = false; + try { + for (Callable<T> t : tasks) { + ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t); + futures.add(f); + externalPush(f); + } + for (int i = 0, size = futures.size(); i < size; i++) + ((ForkJoinTask<?>)futures.get(i)).quietlyJoin(); + done = true; + return futures; + } finally { + if (!done) + for (int i = 0, size = futures.size(); i < size; i++) + futures.get(i).cancel(false); + } + } + + /** + * Returns the factory used for constructing new workers. + * + * @return the factory used for constructing new workers + */ + public ForkJoinWorkerThreadFactory getFactory() { + return factory; + } + + /** + * Returns the handler for internal worker threads that terminate + * due to unrecoverable errors encountered while executing tasks. + * + * @return the handler, or {@code null} if none + */ + public UncaughtExceptionHandler getUncaughtExceptionHandler() { + return ueh; + } + + /** + * Returns the targeted parallelism level of this pool. + * + * @return the targeted parallelism level of this pool + */ + public int getParallelism() { + int par; + return ((par = parallelism) > 0) ? par : 1; + } + + /** + * Returns the targeted parallelism level of the common pool. + * + * @return the targeted parallelism level of the common pool + * @since 1.8 + */ + public static int getCommonPoolParallelism() { + return commonParallelism; + } + + /** + * Returns the number of worker threads that have started but not + * yet terminated. The result returned by this method may differ + * from {@link #getParallelism} when threads are created to + * maintain parallelism when others are cooperatively blocked. + * + * @return the number of worker threads + */ + public int getPoolSize() { + return parallelism + (short)(ctl >>> TC_SHIFT); + } + + /** + * Returns {@code true} if this pool uses local first-in-first-out + * scheduling mode for forked tasks that are never joined. + * + * @return {@code true} if this pool uses async mode + */ + public boolean getAsyncMode() { + return mode == FIFO_QUEUE; + } + + /** + * Returns an estimate of the number of worker threads that are + * not blocked waiting to join tasks or for other managed + * synchronization. This method may overestimate the + * number of running threads. + * + * @return the number of worker threads + */ + public int getRunningThreadCount() { + int rc = 0; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 1; i < ws.length; i += 2) { + if ((w = ws[i]) != null && w.isApparentlyUnblocked()) + ++rc; + } + } + return rc; + } + + /** + * Returns an estimate of the number of threads that are currently + * stealing or executing tasks. This method may overestimate the + * number of active threads. + * + * @return the number of active threads + */ + public int getActiveThreadCount() { + int r = parallelism + (int)(ctl >> AC_SHIFT); + return (r <= 0) ? 0 : r; // suppress momentarily negative values + } + + /** + * Returns {@code true} if all worker threads are currently idle. + * An idle worker is one that cannot obtain a task to execute + * because none are available to steal from other threads, and + * there are no pending submissions to the pool. This method is + * conservative; it might not return {@code true} immediately upon + * idleness of all threads, but will eventually become true if + * threads remain inactive. + * + * @return {@code true} if all threads are currently idle + */ + public boolean isQuiescent() { + return parallelism + (int)(ctl >> AC_SHIFT) <= 0; + } + + /** + * Returns an estimate of the total number of tasks stolen from + * one thread's work queue by another. The reported value + * underestimates the actual total number of steals when the pool + * is not quiescent. This value may be useful for monitoring and + * tuning fork/join programs: in general, steal counts should be + * high enough to keep threads busy, but low enough to avoid + * overhead and contention across threads. + * + * @return the number of steals + */ + public long getStealCount() { + long count = stealCount; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 1; i < ws.length; i += 2) { + if ((w = ws[i]) != null) + count += w.nsteals; + } + } + return count; + } + + /** + * Returns an estimate of the total number of tasks currently held + * in queues by worker threads (but not including tasks submitted + * to the pool that have not begun executing). This value is only + * an approximation, obtained by iterating across all threads in + * the pool. This method may be useful for tuning task + * granularities. + * + * @return the number of queued tasks + */ + public long getQueuedTaskCount() { + long count = 0; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 1; i < ws.length; i += 2) { + if ((w = ws[i]) != null) + count += w.queueSize(); + } + } + return count; + } + + /** + * Returns an estimate of the number of tasks submitted to this + * pool that have not yet begun executing. This method may take + * time proportional to the number of submissions. + * + * @return the number of queued submissions + */ + public int getQueuedSubmissionCount() { + int count = 0; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; i += 2) { + if ((w = ws[i]) != null) + count += w.queueSize(); + } + } + return count; + } + + /** + * Returns {@code true} if there are any tasks submitted to this + * pool that have not yet begun executing. + * + * @return {@code true} if there are any queued submissions + */ + public boolean hasQueuedSubmissions() { + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; i += 2) { + if ((w = ws[i]) != null && !w.isEmpty()) + return true; + } + } + return false; + } + + /** + * Removes and returns the next unexecuted submission if one is + * available. This method may be useful in extensions to this + * class that re-assign work in systems with multiple pools. + * + * @return the next submission, or {@code null} if none + */ + protected ForkJoinTask<?> pollSubmission() { + WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; i += 2) { + if ((w = ws[i]) != null && (t = w.poll()) != null) + return t; + } + } + return null; + } + + /** + * Removes all available unexecuted submitted and forked tasks + * from scheduling queues and adds them to the given collection, + * without altering their execution status. These may include + * artificially generated or wrapped tasks. This method is + * designed to be invoked only when the pool is known to be + * quiescent. Invocations at other times may not remove all + * tasks. A failure encountered while attempting to add elements + * to collection {@code c} may result in elements being in + * neither, either or both collections when the associated + * exception is thrown. The behavior of this operation is + * undefined if the specified collection is modified while the + * operation is in progress. + * + * @param c the collection to transfer elements into + * @return the number of elements transferred + */ + protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) { + int count = 0; + WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; ++i) { + if ((w = ws[i]) != null) { + while ((t = w.poll()) != null) { + c.add(t); + ++count; + } + } + } + } + return count; + } + + /** + * Returns a string identifying this pool, as well as its state, + * including indications of run state, parallelism level, and + * worker and task counts. + * + * @return a string identifying this pool, as well as its state + */ + public String toString() { + // Use a single pass through workQueues to collect counts + long qt = 0L, qs = 0L; int rc = 0; + long st = stealCount; + long c = ctl; + WorkQueue[] ws; WorkQueue w; + if ((ws = workQueues) != null) { + for (int i = 0; i < ws.length; ++i) { + if ((w = ws[i]) != null) { + int size = w.queueSize(); + if ((i & 1) == 0) + qs += size; + else { + qt += size; + st += w.nsteals; + if (w.isApparentlyUnblocked()) + ++rc; + } + } + } + } + int pc = parallelism; + int tc = pc + (short)(c >>> TC_SHIFT); + int ac = pc + (int)(c >> AC_SHIFT); + if (ac < 0) // ignore transient negative + ac = 0; + String level; + if ((c & STOP_BIT) != 0) + level = (tc == 0) ? "Terminated" : "Terminating"; + else + level = plock < 0 ? "Shutting down" : "Running"; + return super.toString() + + "[" + level + + ", parallelism = " + pc + + ", size = " + tc + + ", active = " + ac + + ", running = " + rc + + ", steals = " + st + + ", tasks = " + qt + + ", submissions = " + qs + + "]"; + } + + /** + * Possibly initiates an orderly shutdown in which previously + * submitted tasks are executed, but no new tasks will be + * accepted. Invocation has no effect on execution state if this + * is the {@link #commonPool()}, and no additional effect if + * already shut down. Tasks that are in the process of being + * submitted concurrently during the course of this method may or + * may not be rejected. + * + * @throws SecurityException if a security manager exists and + * the caller is not permitted to modify threads + * because it does not hold {@link + * java.lang.RuntimePermission}{@code ("modifyThread")} + */ + public void shutdown() { + checkPermission(); + tryTerminate(false, true); + } + + /** + * Possibly attempts to cancel and/or stop all tasks, and reject + * all subsequently submitted tasks. Invocation has no effect on + * execution state if this is the {@link #commonPool()}, and no + * additional effect if already shut down. Otherwise, tasks that + * are in the process of being submitted or executed concurrently + * during the course of this method may or may not be + * rejected. This method cancels both existing and unexecuted + * tasks, in order to permit termination in the presence of task + * dependencies. So the method always returns an empty list + * (unlike the case for some other Executors). + * + * @return an empty list + * @throws SecurityException if a security manager exists and + * the caller is not permitted to modify threads + * because it does not hold {@link + * java.lang.RuntimePermission}{@code ("modifyThread")} + */ + public List<Runnable> shutdownNow() { + checkPermission(); + tryTerminate(true, true); + return Collections.emptyList(); + } + + /** + * Returns {@code true} if all tasks have completed following shut down. + * + * @return {@code true} if all tasks have completed following shut down + */ + public boolean isTerminated() { + long c = ctl; + return ((c & STOP_BIT) != 0L && + (short)(c >>> TC_SHIFT) + parallelism <= 0); + } + + /** + * Returns {@code true} if the process of termination has + * commenced but not yet completed. This method may be useful for + * debugging. A return of {@code true} reported a sufficient + * period after shutdown may indicate that submitted tasks have + * ignored or suppressed interruption, or are waiting for I/O, + * causing this executor not to properly terminate. (See the + * advisory notes for class {@link ForkJoinTask} stating that + * tasks should not normally entail blocking operations. But if + * they do, they must abort them on interrupt.) + * + * @return {@code true} if terminating but not yet terminated + */ + public boolean isTerminating() { + long c = ctl; + return ((c & STOP_BIT) != 0L && + (short)(c >>> TC_SHIFT) + parallelism > 0); + } + + /** + * Returns {@code true} if this pool has been shut down. + * + * @return {@code true} if this pool has been shut down + */ + public boolean isShutdown() { + return plock < 0; + } + + /** + * Blocks until all tasks have completed execution after a + * shutdown request, or the timeout occurs, or the current thread + * is interrupted, whichever happens first. Because the {@link + * #commonPool()} never terminates until program shutdown, when + * applied to the common pool, this method is equivalent to {@link + * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}. + * + * @param timeout the maximum time to wait + * @param unit the time unit of the timeout argument + * @return {@code true} if this executor terminated and + * {@code false} if the timeout elapsed before termination + * @throws InterruptedException if interrupted while waiting + */ + public boolean awaitTermination(long timeout, TimeUnit unit) + throws InterruptedException { + if (Thread.interrupted()) + throw new InterruptedException(); + if (this == common) { + awaitQuiescence(timeout, unit); + return false; + } + long nanos = unit.toNanos(timeout); + if (isTerminated()) + return true; + if (nanos <= 0L) + return false; + long deadline = System.nanoTime() + nanos; + synchronized (this) { + for (;;) { + if (isTerminated()) + return true; + if (nanos <= 0L) + return false; + long millis = TimeUnit.NANOSECONDS.toMillis(nanos); + wait(millis > 0L ? millis : 1L); + nanos = deadline - System.nanoTime(); + } + } + } + + /** + * If called by a ForkJoinTask operating in this pool, equivalent + * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise, + * waits and/or attempts to assist performing tasks until this + * pool {@link #isQuiescent} or the indicated timeout elapses. + * + * @param timeout the maximum time to wait + * @param unit the time unit of the timeout argument + * @return {@code true} if quiescent; {@code false} if the + * timeout elapsed. + */ + public boolean awaitQuiescence(long timeout, TimeUnit unit) { + long nanos = unit.toNanos(timeout); + ForkJoinWorkerThread wt; + Thread thread = Thread.currentThread(); + if ((thread instanceof ForkJoinWorkerThread) && + (wt = (ForkJoinWorkerThread)thread).pool == this) { + helpQuiescePool(wt.workQueue); + return true; + } + long startTime = System.nanoTime(); + WorkQueue[] ws; + int r = 0, m; + boolean found = true; + while (!isQuiescent() && (ws = workQueues) != null && + (m = ws.length - 1) >= 0) { + if (!found) { + if ((System.nanoTime() - startTime) > nanos) + return false; + Thread.yield(); // cannot block + } + found = false; + for (int j = (m + 1) << 2; j >= 0; --j) { + ForkJoinTask<?> t; WorkQueue q; int b; + if ((q = ws[r++ & m]) != null && (b = q.base) - q.top < 0) { + found = true; + if ((t = q.pollAt(b)) != null) + t.doExec(); + break; + } + } + } + return true; + } + + /** + * Waits and/or attempts to assist performing tasks indefinitely + * until the {@link #commonPool()} {@link #isQuiescent}. + */ + static void quiesceCommonPool() { + common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS); + } + + /** + * Interface for extending managed parallelism for tasks running + * in {@link ForkJoinPool}s. + * + * <p>A {@code ManagedBlocker} provides two methods. Method + * {@code isReleasable} must return {@code true} if blocking is + * not necessary. Method {@code block} blocks the current thread + * if necessary (perhaps internally invoking {@code isReleasable} + * before actually blocking). These actions are performed by any + * thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}. + * The unusual methods in this API accommodate synchronizers that + * may, but don't usually, block for long periods. Similarly, they + * allow more efficient internal handling of cases in which + * additional workers may be, but usually are not, needed to + * ensure sufficient parallelism. Toward this end, + * implementations of method {@code isReleasable} must be amenable + * to repeated invocation. + * + * <p>For example, here is a ManagedBlocker based on a + * ReentrantLock: + * <pre> {@code + * class ManagedLocker implements ManagedBlocker { + * final ReentrantLock lock; + * boolean hasLock = false; + * ManagedLocker(ReentrantLock lock) { this.lock = lock; } + * public boolean block() { + * if (!hasLock) + * lock.lock(); + * return true; + * } + * public boolean isReleasable() { + * return hasLock || (hasLock = lock.tryLock()); + * } + * }}</pre> + * + * <p>Here is a class that possibly blocks waiting for an + * item on a given queue: + * <pre> {@code + * class QueueTaker<E> implements ManagedBlocker { + * final BlockingQueue<E> queue; + * volatile E item = null; + * QueueTaker(BlockingQueue<E> q) { this.queue = q; } + * public boolean block() throws InterruptedException { + * if (item == null) + * item = queue.take(); + * return true; + * } + * public boolean isReleasable() { + * return item != null || (item = queue.poll()) != null; + * } + * public E getItem() { // call after pool.managedBlock completes + * return item; + * } + * }}</pre> + */ + public static interface ManagedBlocker { + /** + * Possibly blocks the current thread, for example waiting for + * a lock or condition. + * + * @return {@code true} if no additional blocking is necessary + * (i.e., if isReleasable would return true) + * @throws InterruptedException if interrupted while waiting + * (the method is not required to do so, but is allowed to) + */ + boolean block() throws InterruptedException; + + /** + * Returns {@code true} if blocking is unnecessary. + * @return {@code true} if blocking is unnecessary + */ + boolean isReleasable(); + } + + /** + * Blocks in accord with the given blocker. If the current thread + * is a {@link ForkJoinWorkerThread}, this method possibly + * arranges for a spare thread to be activated if necessary to + * ensure sufficient parallelism while the current thread is blocked. + * + * <p>If the caller is not a {@link ForkJoinTask}, this method is + * behaviorally equivalent to + * <pre> {@code + * while (!blocker.isReleasable()) + * if (blocker.block()) + * return; + * }</pre> + * + * If the caller is a {@code ForkJoinTask}, then the pool may + * first be expanded to ensure parallelism, and later adjusted. + * + * @param blocker the blocker + * @throws InterruptedException if blocker.block did so + */ + public static void managedBlock(ManagedBlocker blocker) + throws InterruptedException { + Thread t = Thread.currentThread(); + if (t instanceof ForkJoinWorkerThread) { + ForkJoinPool p = ((ForkJoinWorkerThread)t).pool; + while (!blocker.isReleasable()) { + if (p.tryCompensate(p.ctl)) { + try { + do {} while (!blocker.isReleasable() && + !blocker.block()); + } finally { + p.incrementActiveCount(); + } + break; + } + } + } + else { + do {} while (!blocker.isReleasable() && + !blocker.block()); + } + } + + // AbstractExecutorService overrides. These rely on undocumented + // fact that ForkJoinTask.adapt returns ForkJoinTasks that also + // implement RunnableFuture. + + protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) { + return new ForkJoinTask.AdaptedRunnable<T>(runnable, value); + } + + protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) { + return new ForkJoinTask.AdaptedCallable<T>(callable); + } + + // Unsafe mechanics + private static final sun.misc.Unsafe U; + private static final long CTL; + private static final long PARKBLOCKER; + private static final int ABASE; + private static final int ASHIFT; + private static final long STEALCOUNT; + private static final long PLOCK; + private static final long INDEXSEED; + private static final long QBASE; + private static final long QLOCK; + + static { + // initialize field offsets for CAS etc + try { + U = getUnsafe(); + Class<?> k = ForkJoinPool.class; + CTL = U.objectFieldOffset + (k.getDeclaredField("ctl")); + STEALCOUNT = U.objectFieldOffset + (k.getDeclaredField("stealCount")); + PLOCK = U.objectFieldOffset + (k.getDeclaredField("plock")); + INDEXSEED = U.objectFieldOffset + (k.getDeclaredField("indexSeed")); + Class<?> tk = Thread.class; + PARKBLOCKER = U.objectFieldOffset + (tk.getDeclaredField("parkBlocker")); + Class<?> wk = WorkQueue.class; + QBASE = U.objectFieldOffset + (wk.getDeclaredField("base")); + QLOCK = U.objectFieldOffset + (wk.getDeclaredField("qlock")); + Class<?> ak = ForkJoinTask[].class; + ABASE = U.arrayBaseOffset(ak); + int scale = U.arrayIndexScale(ak); + if ((scale & (scale - 1)) != 0) + throw new Error("data type scale not a power of two"); + ASHIFT = 31 - Integer.numberOfLeadingZeros(scale); + } catch (Exception e) { + throw new Error(e); + } + + submitters = new ThreadLocal<Submitter>(); + defaultForkJoinWorkerThreadFactory = + new DefaultForkJoinWorkerThreadFactory(); + modifyThreadPermission = new RuntimePermission("modifyThread"); + + common = java.security.AccessController.doPrivileged + (new java.security.PrivilegedAction<ForkJoinPool>() { + public ForkJoinPool run() { return makeCommonPool(); }}); + int par = common.parallelism; // report 1 even if threads disabled + commonParallelism = par > 0 ? par : 1; + } + + /** + * Creates and returns the common pool, respecting user settings + * specified via system properties. + */ + private static ForkJoinPool makeCommonPool() { + int parallelism = -1; + ForkJoinWorkerThreadFactory factory + = defaultForkJoinWorkerThreadFactory; + UncaughtExceptionHandler handler = null; + try { // ignore exceptions in accessing/parsing properties + String pp = System.getProperty + ("java.util.concurrent.ForkJoinPool.common.parallelism"); + String fp = System.getProperty + ("java.util.concurrent.ForkJoinPool.common.threadFactory"); + String hp = System.getProperty + ("java.util.concurrent.ForkJoinPool.common.exceptionHandler"); + if (pp != null) + parallelism = Integer.parseInt(pp); + if (fp != null) + factory = ((ForkJoinWorkerThreadFactory)ClassLoader. + getSystemClassLoader().loadClass(fp).newInstance()); + if (hp != null) + handler = ((UncaughtExceptionHandler)ClassLoader. + getSystemClassLoader().loadClass(hp).newInstance()); + } catch (Exception ignore) { + } + + if (parallelism < 0 && // default 1 less than #cores + (parallelism = Runtime.getRuntime().availableProcessors() - 1) < 0) + parallelism = 0; + if (parallelism > MAX_CAP) + parallelism = MAX_CAP; + return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE, + "ForkJoinPool.commonPool-worker-"); + } + + /** + * Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. + * Replace with a simple call to Unsafe.getUnsafe when integrating + * into a jdk. + * + * @return a sun.misc.Unsafe + */ + private static sun.misc.Unsafe getUnsafe() { + try { + return sun.misc.Unsafe.getUnsafe(); + } catch (SecurityException tryReflectionInstead) {} + try { + return java.security.AccessController.doPrivileged + (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() { + public sun.misc.Unsafe run() throws Exception { + Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class; + for (java.lang.reflect.Field f : k.getDeclaredFields()) { + f.setAccessible(true); + Object x = f.get(null); + if (k.isInstance(x)) + return k.cast(x); + } + throw new NoSuchFieldError("the Unsafe"); + }}); + } catch (java.security.PrivilegedActionException e) { + throw new RuntimeException("Could not initialize intrinsics", + e.getCause()); + } + } +} |