From 5d2a4bbc4948292501aeb7fe23f6bc5f7c6ea346 Mon Sep 17 00:00:00 2001 From: Rob Pike Date: Sat, 31 Oct 2009 18:29:06 -0700 Subject: concurrency R=go-dev, iant, rsc http://go/go-review/1018004 --- doc/effective_go.html | 244 +++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 240 insertions(+), 4 deletions(-) (limited to 'doc/effective_go.html') diff --git a/doc/effective_go.html b/doc/effective_go.html index 05ac22368..1bd8655fa 100644 --- a/doc/effective_go.html +++ b/doc/effective_go.html @@ -837,7 +837,7 @@ new instance each time it is evaluated. 
 func NewFile(fd int, name string) *File {
-	if file < 0 {
+	if fd < 0 {
 		return nil
 	}
 	f := File{fd, name, nil, 0};
@@ -1335,7 +1335,7 @@ In Go, enumerated constants are created using the iota
 enumerator.  Since iota can be part of an expression and
 expressions can be implicitly repeated, it is easy to build intricate
 sets of values.
-

+

 
 type ByteSize float64
 const (
@@ -1962,6 +1962,10 @@ is never used.
 
 
Share by communicating

 
+

+Concurrent programming is a large topic and there is space only for some
+Go-specific highlights here.
+

 

 Concurrent programming in many environments is made difficult by the
 subtleties required to implement correct access to shared variables.  Go encourages
@@ -1986,16 +1990,248 @@ Another way to think about this model is to consider a typical single-threaded
 program running on one CPU. It has no need for synchronization primitives.
 Now run another such instance; it too needs no synchronization.  Now let those
 two communicate; if the communication is the synchronizer, there's still no need
-for other synchronization.  Consider Unix pipelines: they fit this model just
-fine.  Although Go's approach to concurrency originates in Hoare's
+for other synchronization.  Consider Unix pipelines: they fit this model
+perfectly.  Although Go's approach to concurrency originates in Hoare's
 Communicating Sequential Processes (CSP),
 it can also be seen as a type-safe generalization of Unix pipes.
 

 
 
Goroutines

 
+

+They're called goroutines because the existing
+terms—threads, coroutines, processes, and so on—convey
+inaccurate connotations.  A goroutine has a simple model: it is a
+function executing in parallel with other goroutines in the same
+address space.  It is lightweight, costing little more than the
+allocation of stack space.
+And the stacks start small, so they are cheap, and grow
+by allocating (and freeing) heap storage as required.
+

+

+Goroutines are multiplexed onto multiple OS threads so if one should
+block, such as while waiting for I/O, others continue to run.  Their
+design hides many of the complexities of thread creation and
+management.
+

+

+Prefix a function or method call with the go
+keyword to run the call in a new goroutine.
+When the call completes, the goroutine
+exits, silently.  (The effect is similar to the Unix shell's
+& notation for running a command in the
+background.)
+

+
+go list.Sort();  // run list.Sort in parallel; don't wait for it. 
+

+

+A function literal can be handy in a goroutine invocation.
+
+func Announce(message string, delay int64) {
+	go func() {
+		time.Sleep(delay);
+		fmt.Println(message);
+	}()  // Note the parentheses - must call the function.
+}
+

+

+In Go function literals are closures: the implementation makes
+sure the variables referred to by the function survive as long as they are active.
+

+These examples aren't too practical because the functions have no way of signaling
+completion.  For that, we need channels.
+

+
 
Channels

 
+

+Like maps, channels are a reference type and are allocated with make.
+If an optional integer parameter is provided, it sets the buffer size for the channel.
+The default is zero, for an unbuffered or synchronous channel.
+

+
+ci := make(chan int);            // unbuffered channel of integers
+cj := make(chan int, 0);         // unbuffered channel of integers
+cs := make(chan *os.File, 100);  // buffered channel of pointers to Files
+

+

+Channels combine communication—the exchange of a value—with
+synchronization—guaranteeing that two calculations (goroutines) are in
+a known state.
+

+

+There are lots of nice idioms using channels.  Here's one to get us started.
+In the previous section we launched a sort in the background. A channel
+can allow the launching goroutine to wait for the sort to complete.
+

+
+c := make(chan int);  // Allocate a channel.
+// Start the sort in a goroutine; when it completes, signal on the channel.
+go func() {
+    list.Sort();
+    c <- 1;  // Send a signal; value does not matter. 
+}();
+doSomethingForAWhile();
+<-c;   // Wait for sort to finish; discard sent value.
+

+

+Receivers always block until there is data to receive.
+If the channel is unbuffered, the sender blocks until the receiver has
+received the value.
+If the channel has a buffer, the sender blocks only until the
+value has been copied to the buffer.
+

+

+A buffered channel can be used like a semaphore, for instance to
+limit throughput.  In this example, incoming requests are passed
+to handle, which sends a value into the channel, processes
+the request, and then receives a value out of the channel.
+The capacity of the channel buffer limits the number of
+simultaneous calls to process.
+

+
+var sem = make(chan int, MaxOutstanding)
+
+func handle(r *Request) {
+    sem <- 1;    // Wait for active queue to drain.
+    process(r);  // May take a long time.
+    <-sem;       // Done; enable next request to run.
+}
+
+func Serve(queue chan *Request) {
+    for {
+        req := <-queue;
+        go handle(req);  // Don't wait for handle to finish.
+    }
+}
+

+

+Here's the same idea implemented by starting a fixed
+number of handle goroutines all reading from the request
+channel.
+The number of goroutines limits the number of simultaneous
+calls to process.
+This Serve function also accepts a channel on which
+it will be told to exit; after launching the goroutines it blocks
+receiving from that channel.
+

+
+func handle(queue chan *Request) {
+	for r := range queue {
+		process(r);
+	}
+}
+
+func Serve(clientRequests chan *clientRequests, quit chan bool) {
+	// Start handlers
+	for i := 0; i < MaxOutstanding; i++ {
+		go handle(clientRequests)
+	}
+	<-quit;	// Wait to be told to exit.
+}
+

+
+
Channels of channels

+

+One of the most important properties of Go is that
+a channel is a first-class value that can be allocated and passed
+around like any other.  A common use of this property is
+to implement safe, parallel demultiplexing.
+

+In the example in the previous section, handle was
+an idealized handler for a request but we didn't define the
+type it was handling.  If that type includes a channel on which
+to reply, each client can provide its own path for the answer.
+Here's a schematic definition of type Request.
+

+
+type Request struct {
+    args  []int;
+    f    func([]int) int;
+    resultChan	<-chan int;
+}
+

+

+The client provides a function and its arguments, as well as
+a channel inside the request object on which to receive the answer.
+

+
+func sum(a []int) (s int) {
+	for _, v := range a {
+		s += v
+	}
+	return
+}
+
+request := &Request{[]int{3, 4, 5}, sum, make(chan int)}
+// Send request
+client Requests <- request;
+// Wait for response.
+fmt.Printf("answer: %d\n", <-request.resultChan);
+

+

+On the server side, the handler function is the only thing that changes.
+

+
+func handle(queue chan *Request) {
+	for req := range queue {
+		req.resultChan <- req.f(req.args);
+	}
+}
+

+

+There's clearly a lot more to do to make it realistic, but this
+code is a framework for a rate-limited, parallel, non-blocking RPC
+system, and there's not a mutex in sight.
+

+
+
Parallelization

+

+Another application of these ideas is to parallelize a calculation
+across multiple CPU cores.  If the calculation can be broken into
+separate pieces, it can be parallelized, with a channel to signal
+when each piece completes.
+

+

+Let's say we have an expensive operation to perform on an array of items,
+and that the value of the operation on each item is independent,
+as in this idealized example.
+

+
+type Vec []float64
+
+// Apply the operation to n elements of v starting at i.
+func (v Vec) DoSome(i, n int, u Vec, c chan int) {
+    for ; i < n; i++ {
+        v[i] += u.Op(v[i])
+    }
+    c <- 1;    // signal that this piece is done
+}
+

+

+We launch the pieces independently in a loop, one per CPU.
+They can complete in any order but it doesn't matter; we just
+count the completion signals by draining the channel after
+launching all the goroutines.
+

+
+const NCPU = 4	// number of CPU cores
+
+func (v Vec) DoAll(u Vec) {
+    c := make(chan int, NCPU);  // Buffering optional but sensible.
+    for i := 0; i < NCPU; i++ {
+        go v.DoSome(i*len(v)/NCPU, (i+1)*len(v)/NCPU, u, c);
+    }
+    // Drain the channel.
+    for i := 0; i < NCPU; i++ {
+        <-c    // wait for one task to complete
+    }
+    // All done.
+}
+
+

+
 
A leaky buffer

 
 

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