libgo/go/sync/pool.go

   1 // Copyright 2013 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package sync
   6
   7 import (
   8         "internal/race"
   9         "runtime"
  10         "sync/atomic"
  11         "unsafe"
  12 )
  13
  14 // A Pool is a set of temporary objects that may be individually saved and
  15 // retrieved.
  16 //
  17 // Any item stored in the Pool may be removed automatically at any time without
  18 // notification. If the Pool holds the only reference when this happens, the
  19 // item might be deallocated.
  20 //
  21 // A Pool is safe for use by multiple goroutines simultaneously.
  22 //
  23 // Pool's purpose is to cache allocated but unused items for later reuse,
  24 // relieving pressure on the garbage collector. That is, it makes it easy to
  25 // build efficient, thread-safe free lists. However, it is not suitable for all
  26 // free lists.
  27 //
  28 // An appropriate use of a Pool is to manage a group of temporary items
  29 // silently shared among and potentially reused by concurrent independent
  30 // clients of a package. Pool provides a way to amortize allocation overhead
  31 // across many clients.
  32 //
  33 // An example of good use of a Pool is in the fmt package, which maintains a
  34 // dynamically-sized store of temporary output buffers. The store scales under
  35 // load (when many goroutines are actively printing) and shrinks when
  36 // quiescent.
  37 //
  38 // On the other hand, a free list maintained as part of a short-lived object is
  39 // not a suitable use for a Pool, since the overhead does not amortize well in
  40 // that scenario. It is more efficient to have such objects implement their own
  41 // free list.
  42 //
  43 // A Pool must not be copied after first use.
  44 type Pool struct {
  45         noCopy noCopy
  46
  47         local     unsafe.Pointer // local fixed-size per-P pool, actual type is [P]poolLocal
  48         localSize uintptr        // size of the local array
  49
  50         // New optionally specifies a function to generate
  51         // a value when Get would otherwise return nil.
  52         // It may not be changed concurrently with calls to Get.
  53         New func() interface{}
  54 }
  55
  56 // Local per-P Pool appendix.
  57 type poolLocal struct {
  58         private interface{}   // Can be used only by the respective P.
  59         shared  []interface{} // Can be used by any P.
  60         Mutex                 // Protects shared.
  61         pad     [128]byte     // Prevents false sharing.
  62 }
  63
  64 // from runtime
  65 func fastrand() uint32
  66
  67 var poolRaceHash [128]uint64
  68
  69 // poolRaceAddr returns an address to use as the synchronization point
  70 // for race detector logic. We don't use the actual pointer stored in x
  71 // directly, for fear of conflicting with other synchronization on that address.
  72 // Instead, we hash the pointer to get an index into poolRaceHash.
  73 // See discussion on golang.org/cl/31589.
  74 func poolRaceAddr(x interface{}) unsafe.Pointer {
  75         ptr := uintptr((*[2]unsafe.Pointer)(unsafe.Pointer(&x))[1])
  76         h := uint32((uint64(uint32(ptr)) * 0x85ebca6b) >> 16)
  77         return unsafe.Pointer(&poolRaceHash[h%uint32(len(poolRaceHash))])
  78 }
  79
  80 // Put adds x to the pool.
  81 func (p *Pool) Put(x interface{}) {
  82         if x == nil {
  83                 return
  84         }
  85         if race.Enabled {
  86                 if fastrand()%4 == 0 {
  87                         // Randomly drop x on floor.
  88                         return
  89                 }
  90                 race.ReleaseMerge(poolRaceAddr(x))
  91                 race.Disable()
  92         }
  93         l := p.pin()
  94         if l.private == nil {
  95                 l.private = x
  96                 x = nil
  97         }
  98         runtime_procUnpin()
  99         if x != nil {
 100                 l.Lock()
 101                 l.shared = append(l.shared, x)
 102                 l.Unlock()
 103         }
 104         if race.Enabled {
 105                 race.Enable()
 106         }
 107 }
 108
 109 // Get selects an arbitrary item from the Pool, removes it from the
 110 // Pool, and returns it to the caller.
 111 // Get may choose to ignore the pool and treat it as empty.
 112 // Callers should not assume any relation between values passed to Put and
 113 // the values returned by Get.
 114 //
 115 // If Get would otherwise return nil and p.New is non-nil, Get returns
 116 // the result of calling p.New.
 117 func (p *Pool) Get() interface{} {
 118         if race.Enabled {
 119                 race.Disable()
 120         }
 121         l := p.pin()
 122         x := l.private
 123         l.private = nil
 124         runtime_procUnpin()
 125         if x == nil {
 126                 l.Lock()
 127                 last := len(l.shared) - 1
 128                 if last >= 0 {
 129                         x = l.shared[last]
 130                         l.shared = l.shared[:last]
 131                 }
 132                 l.Unlock()
 133                 if x == nil {
 134                         x = p.getSlow()
 135                 }
 136         }
 137         if race.Enabled {
 138                 race.Enable()
 139                 if x != nil {
 140                         race.Acquire(poolRaceAddr(x))
 141                 }
 142         }
 143         if x == nil && p.New != nil {
 144                 x = p.New()
 145         }
 146         return x
 147 }
 148
 149 func (p *Pool) getSlow() (x interface{}) {
 150         // See the comment in pin regarding ordering of the loads.
 151         size := atomic.LoadUintptr(&p.localSize) // load-acquire
 152         local := p.local                         // load-consume
 153         // Try to steal one element from other procs.
 154         pid := runtime_procPin()
 155         runtime_procUnpin()
 156         for i := 0; i < int(size); i++ {
 157                 l := indexLocal(local, (pid+i+1)%int(size))
 158                 l.Lock()
 159                 last := len(l.shared) - 1
 160                 if last >= 0 {
 161                         x = l.shared[last]
 162                         l.shared = l.shared[:last]
 163                         l.Unlock()
 164                         break
 165                 }
 166                 l.Unlock()
 167         }
 168         return x
 169 }
 170
 171 // pin pins the current goroutine to P, disables preemption and returns poolLocal pool for the P.
 172 // Caller must call runtime_procUnpin() when done with the pool.
 173 func (p *Pool) pin() *poolLocal {
 174         pid := runtime_procPin()
 175         // In pinSlow we store to localSize and then to local, here we load in opposite order.
 176         // Since we've disabled preemption, GC cannot happen in between.
 177         // Thus here we must observe local at least as large localSize.
 178         // We can observe a newer/larger local, it is fine (we must observe its zero-initialized-ness).
 179         s := atomic.LoadUintptr(&p.localSize) // load-acquire
 180         l := p.local                          // load-consume
 181         if uintptr(pid) < s {
 182                 return indexLocal(l, pid)
 183         }
 184         return p.pinSlow()
 185 }
 186
 187 func (p *Pool) pinSlow() *poolLocal {
 188         // Retry under the mutex.
 189         // Can not lock the mutex while pinned.
 190         runtime_procUnpin()
 191         allPoolsMu.Lock()
 192         defer allPoolsMu.Unlock()
 193         pid := runtime_procPin()
 194         // poolCleanup won't be called while we are pinned.
 195         s := p.localSize
 196         l := p.local
 197         if uintptr(pid) < s {
 198                 return indexLocal(l, pid)
 199         }
 200         if p.local == nil {
 201                 allPools = append(allPools, p)
 202         }
 203         // If GOMAXPROCS changes between GCs, we re-allocate the array and lose the old one.
 204         size := runtime.GOMAXPROCS(0)
 205         local := make([]poolLocal, size)
 206         atomic.StorePointer(&p.local, unsafe.Pointer(&local[0])) // store-release
 207         atomic.StoreUintptr(&p.localSize, uintptr(size))         // store-release
 208         return &local[pid]
 209 }
 210
 211 func poolCleanup() {
 212         // This function is called with the world stopped, at the beginning of a garbage collection.
 213         // It must not allocate and probably should not call any runtime functions.
 214         // Defensively zero out everything, 2 reasons:
 215         // 1. To prevent false retention of whole Pools.
 216         // 2. If GC happens while a goroutine works with l.shared in Put/Get,
 217         //    it will retain whole Pool. So next cycle memory consumption would be doubled.
 218         for i, p := range allPools {
 219                 allPools[i] = nil
 220                 for i := 0; i < int(p.localSize); i++ {
 221                         l := indexLocal(p.local, i)
 222                         l.private = nil
 223                         for j := range l.shared {
 224                                 l.shared[j] = nil
 225                         }
 226                         l.shared = nil
 227                 }
 228                 p.local = nil
 229                 p.localSize = 0
 230         }
 231         allPools = []*Pool{}
 232 }
 233
 234 var (
 235         allPoolsMu Mutex
 236         allPools   []*Pool
 237 )
 238
 239 func init() {
 240         runtime_registerPoolCleanup(poolCleanup)
 241 }
 242
 243 func indexLocal(l unsafe.Pointer, i int) *poolLocal {
 244         return &(*[1000000]poolLocal)(l)[i]
 245 }
 246
 247 // Implemented in runtime.
 248 func runtime_registerPoolCleanup(cleanup func())
 249 func runtime_procPin() int
 250 func runtime_procUnpin()