syncmap: add a synchronized map implementation.
This is a draft for the sync.Map API proposed in golang/go#18177. It supports fast-path loads via an atomic variable, falling back to a Mutex for stores. In order to keep stores amortized to O(1), loads following a store follow the Mutex path until enough loads have occurred to offset the cost of a deep copy. For mostly-read loads, such as the maps in the reflect package in the standard library, this significantly reduces cache-line contention vs. a plain RWMutex with a map. goos: linux goarch: amd64 pkg: golang.org/x/sync/syncmap BenchmarkLoadMostlyHits/*syncmap_test.DeepCopyMap 20000000 73.1 ns/op BenchmarkLoadMostlyHits/*syncmap_test.DeepCopyMap-48 100000000 13.8 ns/op BenchmarkLoadMostlyHits/*syncmap_test.RWMutexMap 20000000 87.7 ns/op BenchmarkLoadMostlyHits/*syncmap_test.RWMutexMap-48 10000000 154 ns/op BenchmarkLoadMostlyHits/*syncmap.Map 20000000 72.1 ns/op BenchmarkLoadMostlyHits/*syncmap.Map-48 100000000 11.2 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.DeepCopyMap 20000000 63.2 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.DeepCopyMap-48 200000000 14.2 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.RWMutexMap 20000000 72.7 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.RWMutexMap-48 10000000 150 ns/op BenchmarkLoadMostlyMisses/*syncmap.Map 30000000 56.4 ns/op BenchmarkLoadMostlyMisses/*syncmap.Map-48 200000000 9.77 ns/op BenchmarkLoadOrStoreBalanced/*syncmap_test.RWMutexMap 2000000 683 ns/op BenchmarkLoadOrStoreBalanced/*syncmap_test.RWMutexMap-48 1000000 1394 ns/op BenchmarkLoadOrStoreBalanced/*syncmap.Map 2000000 645 ns/op BenchmarkLoadOrStoreBalanced/*syncmap.Map-48 1000000 1253 ns/op BenchmarkLoadOrStoreUnique/*syncmap_test.RWMutexMap 1000000 1015 ns/op BenchmarkLoadOrStoreUnique/*syncmap_test.RWMutexMap-48 1000000 1911 ns/op BenchmarkLoadOrStoreUnique/*syncmap.Map 1000000 1018 ns/op BenchmarkLoadOrStoreUnique/*syncmap.Map-48 1000000 1776 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.DeepCopyMap 50000000 30.2 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.DeepCopyMap-48 2000000000 1.24 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.RWMutexMap 30000000 50.1 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.RWMutexMap-48 5000000 451 ns/op BenchmarkLoadOrStoreCollision/*syncmap.Map 30000000 36.8 ns/op BenchmarkLoadOrStoreCollision/*syncmap.Map-48 2000000000 1.24 ns/op BenchmarkAdversarialAlloc/*syncmap_test.DeepCopyMap 10000000 213 ns/op BenchmarkAdversarialAlloc/*syncmap_test.DeepCopyMap-48 1000000 5012 ns/op BenchmarkAdversarialAlloc/*syncmap_test.RWMutexMap 20000000 68.8 ns/op BenchmarkAdversarialAlloc/*syncmap_test.RWMutexMap-48 5000000 429 ns/op BenchmarkAdversarialAlloc/*syncmap.Map 5000000 229 ns/op BenchmarkAdversarialAlloc/*syncmap.Map-48 2000000 600 ns/op BenchmarkAdversarialDelete/*syncmap_test.DeepCopyMap 5000000 314 ns/op BenchmarkAdversarialDelete/*syncmap_test.DeepCopyMap-48 2000000 726 ns/op BenchmarkAdversarialDelete/*syncmap_test.RWMutexMap 20000000 63.2 ns/op BenchmarkAdversarialDelete/*syncmap_test.RWMutexMap-48 5000000 469 ns/op BenchmarkAdversarialDelete/*syncmap.Map 10000000 203 ns/op BenchmarkAdversarialDelete/*syncmap.Map-48 10000000 253 ns/op goos: linux goarch: ppc64le pkg: golang.org/x/sync/syncmap BenchmarkLoadMostlyHits/*syncmap_test.DeepCopyMap 5000000 253 ns/op BenchmarkLoadMostlyHits/*syncmap_test.DeepCopyMap-48 50000000 26.2 ns/op BenchmarkLoadMostlyHits/*syncmap_test.RWMutexMap 5000000 505 ns/op BenchmarkLoadMostlyHits/*syncmap_test.RWMutexMap-48 3000000 443 ns/op BenchmarkLoadMostlyHits/*syncmap.Map 10000000 200 ns/op BenchmarkLoadMostlyHits/*syncmap.Map-48 100000000 18.1 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.DeepCopyMap 10000000 162 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.DeepCopyMap-48 100000000 23.8 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.RWMutexMap 10000000 195 ns/op BenchmarkLoadMostlyMisses/*syncmap_test.RWMutexMap-48 3000000 531 ns/op BenchmarkLoadMostlyMisses/*syncmap.Map 10000000 182 ns/op BenchmarkLoadMostlyMisses/*syncmap.Map-48 100000000 15.8 ns/op BenchmarkLoadOrStoreBalanced/*syncmap_test.RWMutexMap 1000000 1664 ns/op BenchmarkLoadOrStoreBalanced/*syncmap_test.RWMutexMap-48 1000000 1768 ns/op BenchmarkLoadOrStoreBalanced/*syncmap.Map 1000000 2128 ns/op BenchmarkLoadOrStoreBalanced/*syncmap.Map-48 1000000 1903 ns/op BenchmarkLoadOrStoreUnique/*syncmap_test.RWMutexMap 1000000 2657 ns/op BenchmarkLoadOrStoreUnique/*syncmap_test.RWMutexMap-48 1000000 2577 ns/op BenchmarkLoadOrStoreUnique/*syncmap.Map 1000000 1714 ns/op BenchmarkLoadOrStoreUnique/*syncmap.Map-48 1000000 2484 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.DeepCopyMap 10000000 130 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.DeepCopyMap-48 100000000 11.3 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.RWMutexMap 3000000 426 ns/op BenchmarkLoadOrStoreCollision/*syncmap_test.RWMutexMap-48 2000000 930 ns/op BenchmarkLoadOrStoreCollision/*syncmap.Map 10000000 131 ns/op BenchmarkLoadOrStoreCollision/*syncmap.Map-48 300000000 4.07 ns/op BenchmarkAdversarialAlloc/*syncmap_test.DeepCopyMap 3000000 447 ns/op BenchmarkAdversarialAlloc/*syncmap_test.DeepCopyMap-48 300000 4159 ns/op BenchmarkAdversarialAlloc/*syncmap_test.RWMutexMap 10000000 191 ns/op BenchmarkAdversarialAlloc/*syncmap_test.RWMutexMap-48 3000000 535 ns/op BenchmarkAdversarialAlloc/*syncmap.Map 2000000 525 ns/op BenchmarkAdversarialAlloc/*syncmap.Map-48 1000000 1000 ns/op BenchmarkAdversarialDelete/*syncmap_test.DeepCopyMap 2000000 711 ns/op BenchmarkAdversarialDelete/*syncmap_test.DeepCopyMap-48 2000000 900 ns/op BenchmarkAdversarialDelete/*syncmap_test.RWMutexMap 3000000 354 ns/op BenchmarkAdversarialDelete/*syncmap_test.RWMutexMap-48 3000000 473 ns/op BenchmarkAdversarialDelete/*syncmap.Map 2000000 1357 ns/op BenchmarkAdversarialDelete/*syncmap.Map-48 5000000 334 ns/op Updates golang/go#18177 Change-Id: I8d561b617b1cd2ca03a8e68a5d5a28a519a0ce38 Reviewed-on: https://go-review.googlesource.com/33912 Reviewed-by: Russ Cox <rsc@golang.org>
This commit is contained in:
Bryan C. Mills
Bryan Mills
parent
450f422ab2
commit
54b13b0b03
4 changed files with 649 additions and 0 deletions
192
syncmap/map.go
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192
syncmap/map.go
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// Copyright 2016 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package syncmap provides a concurrent map implementation.
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// It is a prototype for a proposed addition to the sync package
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// in the standard library.
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// (https://golang.org/issue/18177)
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package syncmap
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import (
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"sync"
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"sync/atomic"
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)
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// Map is a concurrent map with amortized-constant-time operations.
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// It is safe for multiple goroutines to call a Map's methods concurrently.
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//
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// The zero Map is valid and empty.
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//
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// A Map must not be copied after first use.
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type Map struct {
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mu sync.Mutex
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// clean is a copy of the map's contents that will never be overwritten, and
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// is thus safe for concurrent lookups without additional synchronization.
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//
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// A nil clean map indicates that the current map contents are stored in the
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// dirty field instead.
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// If clean is non-nil, its contents are up-to-date.
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//
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// clean is always safe to load, but must only be stored with mu held.
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clean atomic.Value // map[interface{}]interface{}
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// dirty is a copy of the map to which all writes occur.
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//
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// A nil dirty map indicates that the current map contents are either empty or
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// stored in the clean field.
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//
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// If the dirty map is nil, the next write to the map will initialize it by
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// making a deep copy of the clean map, then setting the clean map to nil.
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dirty map[interface{}]interface{}
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// misses counts the number of Load calls for which the clean map was nil
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// since the last write.
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//
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// Once enough Load misses have occurred to cover the cost of a copy, the
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// dirty map will be promoted to clean and any subsequent writes will make
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// a new copy.
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misses int
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}
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// Load returns the value stored in the map for a key, or nil if no
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// value is present.
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// The ok result indicates whether value was found in the map.
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func (m *Map) Load(key interface{}) (value interface{}, ok bool) {
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clean, _ := m.clean.Load().(map[interface{}]interface{})
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if clean != nil {
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value, ok = clean[key]
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return value, ok
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}
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m.mu.Lock()
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if m.dirty == nil {
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clean, _ := m.clean.Load().(map[interface{}]interface{})
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if clean == nil {
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// Completely empty — promote to clean immediately.
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m.clean.Store(map[interface{}]interface{}{})
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} else {
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value, ok = clean[key]
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}
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m.mu.Unlock()
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return value, ok
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}
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value, ok = m.dirty[key]
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m.missLocked()
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m.mu.Unlock()
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return value, ok
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}
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// Store sets the value for a key.
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func (m *Map) Store(key, value interface{}) {
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m.mu.Lock()
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m.dirtyLocked()
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m.dirty[key] = value
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m.mu.Unlock()
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}
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// LoadOrStore returns the existing value for the key if present.
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// Otherwise, it stores and returns the given value.
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// The loaded result is true if the value was loaded, false if stored.
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func (m *Map) LoadOrStore(key, value interface{}) (actual interface{}, loaded bool) {
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// Avoid locking if it's a clean hit.
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clean, _ := m.clean.Load().(map[interface{}]interface{})
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actual, loaded = clean[key]
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if loaded {
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return actual, true
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}
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m.mu.Lock()
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if m.dirty == nil {
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// Reload clean in case it changed while we were waiting on m.mu.
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clean, _ := m.clean.Load().(map[interface{}]interface{})
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actual, loaded = clean[key]
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if loaded {
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m.mu.Unlock()
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return actual, true
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}
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} else {
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actual, loaded = m.dirty[key]
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if loaded {
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m.missLocked()
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m.mu.Unlock()
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return actual, true
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}
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}
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m.dirtyLocked()
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m.dirty[key] = value
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actual = value
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m.mu.Unlock()
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return actual, false
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}
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// Delete deletes the value for a key.
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func (m *Map) Delete(key interface{}) {
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m.mu.Lock()
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m.dirtyLocked()
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delete(m.dirty, key)
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m.mu.Unlock()
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}
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// Range calls f sequentially for each key and value present in the map.
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// If f returns false, range stops the iteration.
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//
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// Calls to other Map methods may block until Range returns.
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// The function f must not call any other methods on the Map.
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//
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// Range does not necessarily correspond to any consistent snapshot of the Map's
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// contents: no key will be visited more than once, but if the value for any key
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// is stored or deleted concurrently, Range may reflect any mapping for that key
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// from any point during the Range call.
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func (m *Map) Range(f func(key, value interface{}) bool) {
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clean, _ := m.clean.Load().(map[interface{}]interface{})
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if clean == nil {
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m.mu.Lock()
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if m.dirty == nil {
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clean, _ = m.clean.Load().(map[interface{}]interface{})
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if clean == nil {
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// Completely empty — add an empty map to bypass m.mu next time.
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m.clean.Store(map[interface{}]interface{}{})
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}
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} else {
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// Range is already O(N), so a call to Range amortizes an entire copy of
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// the map. If it is dirty, we can promote it to clean immediately!
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clean = m.dirty
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m.clean.Store(clean)
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m.dirty = nil
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}
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m.mu.Unlock()
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}
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for k, v := range clean {
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if !f(k, v) {
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break
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}
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}
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}
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func (m *Map) missLocked() {
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if m.misses++; m.misses >= len(m.dirty) {
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m.clean.Store(m.dirty)
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m.dirty = nil
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}
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}
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// dirtyLocked prepares the map for a subsequent write.
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// It ensures that the dirty field is non-nil and clean is nil by making a deep
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// copy of clean.
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func (m *Map) dirtyLocked() {
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m.misses = 0
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if m.dirty != nil {
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return
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}
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clean, _ := m.clean.Load().(map[interface{}]interface{})
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m.dirty = make(map[interface{}]interface{}, len(clean))
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for k, v := range clean {
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m.dirty[k] = v
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}
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m.clean.Store(map[interface{}]interface{}(nil))
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}
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