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-rw-r--r--src/pkg/sync/pool.go223
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diff --git a/src/pkg/sync/pool.go b/src/pkg/sync/pool.go
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+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync
+
+import (
+ "runtime"
+ "sync/atomic"
+ "unsafe"
+)
+
+// A Pool is a set of temporary objects that may be individually saved and
+// retrieved.
+//
+// Any item stored in the Pool may be removed automatically at any time without
+// notification. If the Pool holds the only reference when this happens, the
+// item might be deallocated.
+//
+// A Pool is safe for use by multiple goroutines simultaneously.
+//
+// Pool's purpose is to cache allocated but unused items for later reuse,
+// relieving pressure on the garbage collector. That is, it makes it easy to
+// build efficient, thread-safe free lists. However, it is not suitable for all
+// free lists.
+//
+// An appropriate use of a Pool is to manage a group of temporary items
+// silently shared among and potentially reused by concurrent independent
+// clients of a package. Pool provides a way to amortize allocation overhead
+// across many clients.
+//
+// An example of good use of a Pool is in the fmt package, which maintains a
+// dynamically-sized store of temporary output buffers. The store scales under
+// load (when many goroutines are actively printing) and shrinks when
+// quiescent.
+//
+// On the other hand, a free list maintained as part of a short-lived object is
+// not a suitable use for a Pool, since the overhead does not amortize well in
+// that scenario. It is more efficient to have such objects implement their own
+// free list.
+//
+type Pool struct {
+ local unsafe.Pointer // local fixed-size per-P pool, actual type is [P]poolLocal
+ localSize uintptr // size of the local array
+
+ // New optionally specifies a function to generate
+ // a value when Get would otherwise return nil.
+ // It may not be changed concurrently with calls to Get.
+ New func() interface{}
+}
+
+// Local per-P Pool appendix.
+type poolLocal struct {
+ private interface{} // Can be used only by the respective P.
+ shared []interface{} // Can be used by any P.
+ Mutex // Protects shared.
+ pad [128]byte // Prevents false sharing.
+}
+
+// Put adds x to the pool.
+func (p *Pool) Put(x interface{}) {
+ if raceenabled {
+ // Under race detector the Pool degenerates into no-op.
+ // It's conforming, simple and does not introduce excessive
+ // happens-before edges between unrelated goroutines.
+ return
+ }
+ if x == nil {
+ return
+ }
+ l := p.pin()
+ if l.private == nil {
+ l.private = x
+ x = nil
+ }
+ runtime_procUnpin()
+ if x == nil {
+ return
+ }
+ l.Lock()
+ l.shared = append(l.shared, x)
+ l.Unlock()
+}
+
+// Get selects an arbitrary item from the Pool, removes it from the
+// Pool, and returns it to the caller.
+// Get may choose to ignore the pool and treat it as empty.
+// Callers should not assume any relation between values passed to Put and
+// the values returned by Get.
+//
+// If Get would otherwise return nil and p.New is non-nil, Get returns
+// the result of calling p.New.
+func (p *Pool) Get() interface{} {
+ if raceenabled {
+ if p.New != nil {
+ return p.New()
+ }
+ return nil
+ }
+ l := p.pin()
+ x := l.private
+ l.private = nil
+ runtime_procUnpin()
+ if x != nil {
+ return x
+ }
+ l.Lock()
+ last := len(l.shared) - 1
+ if last >= 0 {
+ x = l.shared[last]
+ l.shared = l.shared[:last]
+ }
+ l.Unlock()
+ if x != nil {
+ return x
+ }
+ return p.getSlow()
+}
+
+func (p *Pool) getSlow() (x interface{}) {
+ // See the comment in pin regarding ordering of the loads.
+ size := atomic.LoadUintptr(&p.localSize) // load-acquire
+ local := p.local // load-consume
+ // Try to steal one element from other procs.
+ pid := runtime_procPin()
+ runtime_procUnpin()
+ for i := 0; i < int(size); i++ {
+ l := indexLocal(local, (pid+i+1)%int(size))
+ l.Lock()
+ last := len(l.shared) - 1
+ if last >= 0 {
+ x = l.shared[last]
+ l.shared = l.shared[:last]
+ l.Unlock()
+ break
+ }
+ l.Unlock()
+ }
+
+ if x == nil && p.New != nil {
+ x = p.New()
+ }
+ return x
+}
+
+// pin pins the current goroutine to P, disables preemption and returns poolLocal pool for the P.
+// Caller must call runtime_procUnpin() when done with the pool.
+func (p *Pool) pin() *poolLocal {
+ pid := runtime_procPin()
+ // In pinSlow we store to localSize and then to local, here we load in opposite order.
+ // Since we've disabled preemption, GC can not happen in between.
+ // Thus here we must observe local at least as large localSize.
+ // We can observe a newer/larger local, it is fine (we must observe its zero-initialized-ness).
+ s := atomic.LoadUintptr(&p.localSize) // load-acquire
+ l := p.local // load-consume
+ if uintptr(pid) < s {
+ return indexLocal(l, pid)
+ }
+ return p.pinSlow()
+}
+
+func (p *Pool) pinSlow() *poolLocal {
+ // Retry under the mutex.
+ // Can not lock the mutex while pinned.
+ runtime_procUnpin()
+ allPoolsMu.Lock()
+ defer allPoolsMu.Unlock()
+ pid := runtime_procPin()
+ // poolCleanup won't be called while we are pinned.
+ s := p.localSize
+ l := p.local
+ if uintptr(pid) < s {
+ return indexLocal(l, pid)
+ }
+ if p.local == nil {
+ allPools = append(allPools, p)
+ }
+ // If GOMAXPROCS changes between GCs, we re-allocate the array and lose the old one.
+ size := runtime.GOMAXPROCS(0)
+ local := make([]poolLocal, size)
+ atomic.StorePointer((*unsafe.Pointer)(&p.local), unsafe.Pointer(&local[0])) // store-release
+ atomic.StoreUintptr(&p.localSize, uintptr(size)) // store-release
+ return &local[pid]
+}
+
+func poolCleanup() {
+ // This function is called with the world stopped, at the beginning of a garbage collection.
+ // It must not allocate and probably should not call any runtime functions.
+ // Defensively zero out everything, 2 reasons:
+ // 1. To prevent false retention of whole Pools.
+ // 2. If GC happens while a goroutine works with l.shared in Put/Get,
+ // it will retain whole Pool. So next cycle memory consumption would be doubled.
+ for i, p := range allPools {
+ allPools[i] = nil
+ for i := 0; i < int(p.localSize); i++ {
+ l := indexLocal(p.local, i)
+ l.private = nil
+ for j := range l.shared {
+ l.shared[j] = nil
+ }
+ l.shared = nil
+ }
+ }
+ allPools = []*Pool{}
+}
+
+var (
+ allPoolsMu Mutex
+ allPools []*Pool
+)
+
+func init() {
+ runtime_registerPoolCleanup(poolCleanup)
+}
+
+func indexLocal(l unsafe.Pointer, i int) *poolLocal {
+ return &(*[1000000]poolLocal)(l)[i]
+}
+
+// Implemented in runtime.
+func runtime_registerPoolCleanup(cleanup func())
+func runtime_procPin() int
+func runtime_procUnpin()