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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 (
"internal/race"
"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.
//
// A Pool must not be copied after first use.
type Pool struct {
noCopy noCopy
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 race.Enabled {
// 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 race.Enabled {
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 cannot 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(&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
}
p.local = nil
p.localSize = 0
}
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()