Imported Upstream version 1.3upstream/1.3

1 files changed, 223 insertions, 0 deletions

diff --git a/src/pkg/sync/pool.go b/src/pkg/sync/pool.go
new file mode 100644
index 000000000..1f08707cd
--- /dev/null
+++ b/src/pkg/sync/pool.go
@@ -0,0 +1,223 @@
+// 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()