Source file src/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  //
    45  // In the terminology of the Go memory model, a call to Put(x) “synchronizes before”
    46  // a call to [Pool.Get] returning that same value x.
    47  // Similarly, a call to New returning x “synchronizes before”
    48  // a call to Get returning that same value x.
    49  type Pool struct {
    50  	noCopy noCopy
    51  
    52  	local     unsafe.Pointer // local fixed-size per-P pool, actual type is [P]poolLocal
    53  	localSize uintptr        // size of the local array
    54  
    55  	victim     unsafe.Pointer // local from previous cycle
    56  	victimSize uintptr        // size of victims array
    57  
    58  	// New optionally specifies a function to generate
    59  	// a value when Get would otherwise return nil.
    60  	// It may not be changed concurrently with calls to Get.
    61  	New func() any
    62  }
    63  
    64  // Local per-P Pool appendix.
    65  type poolLocalInternal struct {
    66  	private any       // Can be used only by the respective P.
    67  	shared  poolChain // Local P can pushHead/popHead; any P can popTail.
    68  }
    69  
    70  type poolLocal struct {
    71  	poolLocalInternal
    72  
    73  	// Prevents false sharing on widespread platforms with
    74  	// 128 mod (cache line size) = 0 .
    75  	pad [128 - unsafe.Sizeof(poolLocalInternal{})%128]byte
    76  }
    77  
    78  // from runtime
    79  //
    80  //go:linkname runtime_randn runtime.randn
    81  func runtime_randn(n uint32) uint32
    82  
    83  var poolRaceHash [128]uint64
    84  
    85  // poolRaceAddr returns an address to use as the synchronization point
    86  // for race detector logic. We don't use the actual pointer stored in x
    87  // directly, for fear of conflicting with other synchronization on that address.
    88  // Instead, we hash the pointer to get an index into poolRaceHash.
    89  // See discussion on golang.org/cl/31589.
    90  func poolRaceAddr(x any) unsafe.Pointer {
    91  	ptr := uintptr((*[2]unsafe.Pointer)(unsafe.Pointer(&x))[1])
    92  	h := uint32((uint64(uint32(ptr)) * 0x85ebca6b) >> 16)
    93  	return unsafe.Pointer(&poolRaceHash[h%uint32(len(poolRaceHash))])
    94  }
    95  
    96  // Put adds x to the pool.
    97  func (p *Pool) Put(x any) {
    98  	if x == nil {
    99  		return
   100  	}
   101  	if race.Enabled {
   102  		if runtime_randn(4) == 0 {
   103  			// Randomly drop x on floor.
   104  			return
   105  		}
   106  		race.ReleaseMerge(poolRaceAddr(x))
   107  		race.Disable()
   108  	}
   109  	l, _ := p.pin()
   110  	if l.private == nil {
   111  		l.private = x
   112  	} else {
   113  		l.shared.pushHead(x)
   114  	}
   115  	runtime_procUnpin()
   116  	if race.Enabled {
   117  		race.Enable()
   118  	}
   119  }
   120  
   121  // Get selects an arbitrary item from the [Pool], removes it from the
   122  // Pool, and returns it to the caller.
   123  // Get may choose to ignore the pool and treat it as empty.
   124  // Callers should not assume any relation between values passed to [Pool.Put] and
   125  // the values returned by Get.
   126  //
   127  // If Get would otherwise return nil and p.New is non-nil, Get returns
   128  // the result of calling p.New.
   129  func (p *Pool) Get() any {
   130  	if race.Enabled {
   131  		race.Disable()
   132  	}
   133  	l, pid := p.pin()
   134  	x := l.private
   135  	l.private = nil
   136  	if x == nil {
   137  		// Try to pop the head of the local shard. We prefer
   138  		// the head over the tail for temporal locality of
   139  		// reuse.
   140  		x, _ = l.shared.popHead()
   141  		if x == nil {
   142  			x = p.getSlow(pid)
   143  		}
   144  	}
   145  	runtime_procUnpin()
   146  	if race.Enabled {
   147  		race.Enable()
   148  		if x != nil {
   149  			race.Acquire(poolRaceAddr(x))
   150  		}
   151  	}
   152  	if x == nil && p.New != nil {
   153  		x = p.New()
   154  	}
   155  	return x
   156  }
   157  
   158  func (p *Pool) getSlow(pid int) any {
   159  	// See the comment in pin regarding ordering of the loads.
   160  	size := runtime_LoadAcquintptr(&p.localSize) // load-acquire
   161  	locals := p.local                            // load-consume
   162  	// Try to steal one element from other procs.
   163  	for i := 0; i < int(size); i++ {
   164  		l := indexLocal(locals, (pid+i+1)%int(size))
   165  		if x, _ := l.shared.popTail(); x != nil {
   166  			return x
   167  		}
   168  	}
   169  
   170  	// Try the victim cache. We do this after attempting to steal
   171  	// from all primary caches because we want objects in the
   172  	// victim cache to age out if at all possible.
   173  	size = atomic.LoadUintptr(&p.victimSize)
   174  	if uintptr(pid) >= size {
   175  		return nil
   176  	}
   177  	locals = p.victim
   178  	l := indexLocal(locals, pid)
   179  	if x := l.private; x != nil {
   180  		l.private = nil
   181  		return x
   182  	}
   183  	for i := 0; i < int(size); i++ {
   184  		l := indexLocal(locals, (pid+i)%int(size))
   185  		if x, _ := l.shared.popTail(); x != nil {
   186  			return x
   187  		}
   188  	}
   189  
   190  	// Mark the victim cache as empty for future gets don't bother
   191  	// with it.
   192  	atomic.StoreUintptr(&p.victimSize, 0)
   193  
   194  	return nil
   195  }
   196  
   197  // pin pins the current goroutine to P, disables preemption and
   198  // returns poolLocal pool for the P and the P's id.
   199  // Caller must call runtime_procUnpin() when done with the pool.
   200  func (p *Pool) pin() (*poolLocal, int) {
   201  	// Check whether p is nil to get a panic.
   202  	// Otherwise the nil dereference happens while the m is pinned,
   203  	// causing a fatal error rather than a panic.
   204  	if p == nil {
   205  		panic("nil Pool")
   206  	}
   207  
   208  	pid := runtime_procPin()
   209  	// In pinSlow we store to local and then to localSize, here we load in opposite order.
   210  	// Since we've disabled preemption, GC cannot happen in between.
   211  	// Thus here we must observe local at least as large localSize.
   212  	// We can observe a newer/larger local, it is fine (we must observe its zero-initialized-ness).
   213  	s := runtime_LoadAcquintptr(&p.localSize) // load-acquire
   214  	l := p.local                              // load-consume
   215  	if uintptr(pid) < s {
   216  		return indexLocal(l, pid), pid
   217  	}
   218  	return p.pinSlow()
   219  }
   220  
   221  func (p *Pool) pinSlow() (*poolLocal, int) {
   222  	// Retry under the mutex.
   223  	// Can not lock the mutex while pinned.
   224  	runtime_procUnpin()
   225  	allPoolsMu.Lock()
   226  	defer allPoolsMu.Unlock()
   227  	pid := runtime_procPin()
   228  	// poolCleanup won't be called while we are pinned.
   229  	s := p.localSize
   230  	l := p.local
   231  	if uintptr(pid) < s {
   232  		return indexLocal(l, pid), pid
   233  	}
   234  	if p.local == nil {
   235  		allPools = append(allPools, p)
   236  	}
   237  	// If GOMAXPROCS changes between GCs, we re-allocate the array and lose the old one.
   238  	size := runtime.GOMAXPROCS(0)
   239  	local := make([]poolLocal, size)
   240  	atomic.StorePointer(&p.local, unsafe.Pointer(&local[0])) // store-release
   241  	runtime_StoreReluintptr(&p.localSize, uintptr(size))     // store-release
   242  	return &local[pid], pid
   243  }
   244  
   245  func poolCleanup() {
   246  	// This function is called with the world stopped, at the beginning of a garbage collection.
   247  	// It must not allocate and probably should not call any runtime functions.
   248  
   249  	// Because the world is stopped, no pool user can be in a
   250  	// pinned section (in effect, this has all Ps pinned).
   251  
   252  	// Drop victim caches from all pools.
   253  	for _, p := range oldPools {
   254  		p.victim = nil
   255  		p.victimSize = 0
   256  	}
   257  
   258  	// Move primary cache to victim cache.
   259  	for _, p := range allPools {
   260  		p.victim = p.local
   261  		p.victimSize = p.localSize
   262  		p.local = nil
   263  		p.localSize = 0
   264  	}
   265  
   266  	// The pools with non-empty primary caches now have non-empty
   267  	// victim caches and no pools have primary caches.
   268  	oldPools, allPools = allPools, nil
   269  }
   270  
   271  var (
   272  	allPoolsMu Mutex
   273  
   274  	// allPools is the set of pools that have non-empty primary
   275  	// caches. Protected by either 1) allPoolsMu and pinning or 2)
   276  	// STW.
   277  	allPools []*Pool
   278  
   279  	// oldPools is the set of pools that may have non-empty victim
   280  	// caches. Protected by STW.
   281  	oldPools []*Pool
   282  )
   283  
   284  func init() {
   285  	runtime_registerPoolCleanup(poolCleanup)
   286  }
   287  
   288  func indexLocal(l unsafe.Pointer, i int) *poolLocal {
   289  	lp := unsafe.Pointer(uintptr(l) + uintptr(i)*unsafe.Sizeof(poolLocal{}))
   290  	return (*poolLocal)(lp)
   291  }
   292  
   293  // Implemented in runtime.
   294  func runtime_registerPoolCleanup(cleanup func())
   295  func runtime_procPin() int
   296  func runtime_procUnpin()
   297  
   298  // The below are implemented in internal/runtime/atomic and the
   299  // compiler also knows to intrinsify the symbol we linkname into this
   300  // package.
   301  
   302  //go:linkname runtime_LoadAcquintptr internal/runtime/atomic.LoadAcquintptr
   303  func runtime_LoadAcquintptr(ptr *uintptr) uintptr
   304  
   305  //go:linkname runtime_StoreReluintptr internal/runtime/atomic.StoreReluintptr
   306  func runtime_StoreReluintptr(ptr *uintptr, val uintptr) uintptr
   307  

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