Source file src/sync/pool.go

Documentation: sync

     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  	victim     unsafe.Pointer // local from previous cycle
    51  	victimSize uintptr        // size of victims array
    52  
    53  	// New optionally specifies a function to generate
    54  	// a value when Get would otherwise return nil.
    55  	// It may not be changed concurrently with calls to Get.
    56  	New func() interface{}
    57  }
    58  
    59  // Local per-P Pool appendix.
    60  type poolLocalInternal struct {
    61  	private interface{} // Can be used only by the respective P.
    62  	shared  poolChain   // Local P can pushHead/popHead; any P can popTail.
    63  }
    64  
    65  type poolLocal struct {
    66  	poolLocalInternal
    67  
    68  	// Prevents false sharing on widespread platforms with
    69  	// 128 mod (cache line size) = 0 .
    70  	pad [128 - unsafe.Sizeof(poolLocalInternal{})%128]byte
    71  }
    72  
    73  // from runtime
    74  func fastrand() uint32
    75  
    76  var poolRaceHash [128]uint64
    77  
    78  // poolRaceAddr returns an address to use as the synchronization point
    79  // for race detector logic. We don't use the actual pointer stored in x
    80  // directly, for fear of conflicting with other synchronization on that address.
    81  // Instead, we hash the pointer to get an index into poolRaceHash.
    82  // See discussion on golang.org/cl/31589.
    83  func poolRaceAddr(x interface{}) unsafe.Pointer {
    84  	ptr := uintptr((*[2]unsafe.Pointer)(unsafe.Pointer(&x))[1])
    85  	h := uint32((uint64(uint32(ptr)) * 0x85ebca6b) >> 16)
    86  	return unsafe.Pointer(&poolRaceHash[h%uint32(len(poolRaceHash))])
    87  }
    88  
    89  // Put adds x to the pool.
    90  func (p *Pool) Put(x interface{}) {
    91  	if x == nil {
    92  		return
    93  	}
    94  	if race.Enabled {
    95  		if fastrand()%4 == 0 {
    96  			// Randomly drop x on floor.
    97  			return
    98  		}
    99  		race.ReleaseMerge(poolRaceAddr(x))
   100  		race.Disable()
   101  	}
   102  	l, _ := p.pin()
   103  	if l.private == nil {
   104  		l.private = x
   105  		x = nil
   106  	}
   107  	if x != nil {
   108  		l.shared.pushHead(x)
   109  	}
   110  	runtime_procUnpin()
   111  	if race.Enabled {
   112  		race.Enable()
   113  	}
   114  }
   115  
   116  // Get selects an arbitrary item from the Pool, removes it from the
   117  // Pool, and returns it to the caller.
   118  // Get may choose to ignore the pool and treat it as empty.
   119  // Callers should not assume any relation between values passed to Put and
   120  // the values returned by Get.
   121  //
   122  // If Get would otherwise return nil and p.New is non-nil, Get returns
   123  // the result of calling p.New.
   124  func (p *Pool) Get() interface{} {
   125  	if race.Enabled {
   126  		race.Disable()
   127  	}
   128  	l, pid := p.pin()
   129  	x := l.private
   130  	l.private = nil
   131  	if x == nil {
   132  		// Try to pop the head of the local shard. We prefer
   133  		// the head over the tail for temporal locality of
   134  		// reuse.
   135  		x, _ = l.shared.popHead()
   136  		if x == nil {
   137  			x = p.getSlow(pid)
   138  		}
   139  	}
   140  	runtime_procUnpin()
   141  	if race.Enabled {
   142  		race.Enable()
   143  		if x != nil {
   144  			race.Acquire(poolRaceAddr(x))
   145  		}
   146  	}
   147  	if x == nil && p.New != nil {
   148  		x = p.New()
   149  	}
   150  	return x
   151  }
   152  
   153  func (p *Pool) getSlow(pid int) interface{} {
   154  	// See the comment in pin regarding ordering of the loads.
   155  	size := atomic.LoadUintptr(&p.localSize) // load-acquire
   156  	locals := p.local                        // load-consume
   157  	// Try to steal one element from other procs.
   158  	for i := 0; i < int(size); i++ {
   159  		l := indexLocal(locals, (pid+i+1)%int(size))
   160  		if x, _ := l.shared.popTail(); x != nil {
   161  			return x
   162  		}
   163  	}
   164  
   165  	// Try the victim cache. We do this after attempting to steal
   166  	// from all primary caches because we want objects in the
   167  	// victim cache to age out if at all possible.
   168  	size = atomic.LoadUintptr(&p.victimSize)
   169  	if uintptr(pid) >= size {
   170  		return nil
   171  	}
   172  	locals = p.victim
   173  	l := indexLocal(locals, pid)
   174  	if x := l.private; x != nil {
   175  		l.private = nil
   176  		return x
   177  	}
   178  	for i := 0; i < int(size); i++ {
   179  		l := indexLocal(locals, (pid+i)%int(size))
   180  		if x, _ := l.shared.popTail(); x != nil {
   181  			return x
   182  		}
   183  	}
   184  
   185  	// Mark the victim cache as empty for future gets don't bother
   186  	// with it.
   187  	atomic.StoreUintptr(&p.victimSize, 0)
   188  
   189  	return nil
   190  }
   191  
   192  // pin pins the current goroutine to P, disables preemption and
   193  // returns poolLocal pool for the P and the P's id.
   194  // Caller must call runtime_procUnpin() when done with the pool.
   195  func (p *Pool) pin() (*poolLocal, int) {
   196  	pid := runtime_procPin()
   197  	// In pinSlow we store to local and then to localSize, here we load in opposite order.
   198  	// Since we've disabled preemption, GC cannot happen in between.
   199  	// Thus here we must observe local at least as large localSize.
   200  	// We can observe a newer/larger local, it is fine (we must observe its zero-initialized-ness).
   201  	s := atomic.LoadUintptr(&p.localSize) // load-acquire
   202  	l := p.local                          // load-consume
   203  	if uintptr(pid) < s {
   204  		return indexLocal(l, pid), pid
   205  	}
   206  	return p.pinSlow()
   207  }
   208  
   209  func (p *Pool) pinSlow() (*poolLocal, int) {
   210  	// Retry under the mutex.
   211  	// Can not lock the mutex while pinned.
   212  	runtime_procUnpin()
   213  	allPoolsMu.Lock()
   214  	defer allPoolsMu.Unlock()
   215  	pid := runtime_procPin()
   216  	// poolCleanup won't be called while we are pinned.
   217  	s := p.localSize
   218  	l := p.local
   219  	if uintptr(pid) < s {
   220  		return indexLocal(l, pid), pid
   221  	}
   222  	if p.local == nil {
   223  		allPools = append(allPools, p)
   224  	}
   225  	// If GOMAXPROCS changes between GCs, we re-allocate the array and lose the old one.
   226  	size := runtime.GOMAXPROCS(0)
   227  	local := make([]poolLocal, size)
   228  	atomic.StorePointer(&p.local, unsafe.Pointer(&local[0])) // store-release
   229  	atomic.StoreUintptr(&p.localSize, uintptr(size))         // store-release
   230  	return &local[pid], pid
   231  }
   232  
   233  func poolCleanup() {
   234  	// This function is called with the world stopped, at the beginning of a garbage collection.
   235  	// It must not allocate and probably should not call any runtime functions.
   236  
   237  	// Because the world is stopped, no pool user can be in a
   238  	// pinned section (in effect, this has all Ps pinned).
   239  
   240  	// Drop victim caches from all pools.
   241  	for _, p := range oldPools {
   242  		p.victim = nil
   243  		p.victimSize = 0
   244  	}
   245  
   246  	// Move primary cache to victim cache.
   247  	for _, p := range allPools {
   248  		p.victim = p.local
   249  		p.victimSize = p.localSize
   250  		p.local = nil
   251  		p.localSize = 0
   252  	}
   253  
   254  	// The pools with non-empty primary caches now have non-empty
   255  	// victim caches and no pools have primary caches.
   256  	oldPools, allPools = allPools, nil
   257  }
   258  
   259  var (
   260  	allPoolsMu Mutex
   261  
   262  	// allPools is the set of pools that have non-empty primary
   263  	// caches. Protected by either 1) allPoolsMu and pinning or 2)
   264  	// STW.
   265  	allPools []*Pool
   266  
   267  	// oldPools is the set of pools that may have non-empty victim
   268  	// caches. Protected by STW.
   269  	oldPools []*Pool
   270  )
   271  
   272  func init() {
   273  	runtime_registerPoolCleanup(poolCleanup)
   274  }
   275  
   276  func indexLocal(l unsafe.Pointer, i int) *poolLocal {
   277  	lp := unsafe.Pointer(uintptr(l) + uintptr(i)*unsafe.Sizeof(poolLocal{}))
   278  	return (*poolLocal)(lp)
   279  }
   280  
   281  // Implemented in runtime.
   282  func runtime_registerPoolCleanup(cleanup func())
   283  func runtime_procPin() int
   284  func runtime_procUnpin()
   285  

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