Source file src/cmd/go/internal/par/work.go
Documentation: cmd/go/internal/par
1 // Copyright 2018 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 par implements parallel execution helpers. 6 package par 7 8 import ( 9 "math/rand" 10 "sync" 11 "sync/atomic" 12 ) 13 14 // Work manages a set of work items to be executed in parallel, at most once each. 15 // The items in the set must all be valid map keys. 16 type Work struct { 17 f func(interface{}) // function to run for each item 18 running int // total number of runners 19 20 mu sync.Mutex 21 added map[interface{}]bool // items added to set 22 todo []interface{} // items yet to be run 23 wait sync.Cond // wait when todo is empty 24 waiting int // number of runners waiting for todo 25 } 26 27 func (w *Work) init() { 28 if w.added == nil { 29 w.added = make(map[interface{}]bool) 30 } 31 } 32 33 // Add adds item to the work set, if it hasn't already been added. 34 func (w *Work) Add(item interface{}) { 35 w.mu.Lock() 36 w.init() 37 if !w.added[item] { 38 w.added[item] = true 39 w.todo = append(w.todo, item) 40 if w.waiting > 0 { 41 w.wait.Signal() 42 } 43 } 44 w.mu.Unlock() 45 } 46 47 // Do runs f in parallel on items from the work set, 48 // with at most n invocations of f running at a time. 49 // It returns when everything added to the work set has been processed. 50 // At least one item should have been added to the work set 51 // before calling Do (or else Do returns immediately), 52 // but it is allowed for f(item) to add new items to the set. 53 // Do should only be used once on a given Work. 54 func (w *Work) Do(n int, f func(item interface{})) { 55 if n < 1 { 56 panic("par.Work.Do: n < 1") 57 } 58 if w.running >= 1 { 59 panic("par.Work.Do: already called Do") 60 } 61 62 w.running = n 63 w.f = f 64 w.wait.L = &w.mu 65 66 for i := 0; i < n-1; i++ { 67 go w.runner() 68 } 69 w.runner() 70 } 71 72 // runner executes work in w until both nothing is left to do 73 // and all the runners are waiting for work. 74 // (Then all the runners return.) 75 func (w *Work) runner() { 76 for { 77 // Wait for something to do. 78 w.mu.Lock() 79 for len(w.todo) == 0 { 80 w.waiting++ 81 if w.waiting == w.running { 82 // All done. 83 w.wait.Broadcast() 84 w.mu.Unlock() 85 return 86 } 87 w.wait.Wait() 88 w.waiting-- 89 } 90 91 // Pick something to do at random, 92 // to eliminate pathological contention 93 // in case items added at about the same time 94 // are most likely to contend. 95 i := rand.Intn(len(w.todo)) 96 item := w.todo[i] 97 w.todo[i] = w.todo[len(w.todo)-1] 98 w.todo = w.todo[:len(w.todo)-1] 99 w.mu.Unlock() 100 101 w.f(item) 102 } 103 } 104 105 // Cache runs an action once per key and caches the result. 106 type Cache struct { 107 m sync.Map 108 } 109 110 type cacheEntry struct { 111 done uint32 112 mu sync.Mutex 113 result interface{} 114 } 115 116 // Do calls the function f if and only if Do is being called for the first time with this key. 117 // No call to Do with a given key returns until the one call to f returns. 118 // Do returns the value returned by the one call to f. 119 func (c *Cache) Do(key interface{}, f func() interface{}) interface{} { 120 entryIface, ok := c.m.Load(key) 121 if !ok { 122 entryIface, _ = c.m.LoadOrStore(key, new(cacheEntry)) 123 } 124 e := entryIface.(*cacheEntry) 125 if atomic.LoadUint32(&e.done) == 0 { 126 e.mu.Lock() 127 if atomic.LoadUint32(&e.done) == 0 { 128 e.result = f() 129 atomic.StoreUint32(&e.done, 1) 130 } 131 e.mu.Unlock() 132 } 133 return e.result 134 } 135 136 // Get returns the cached result associated with key. 137 // It returns nil if there is no such result. 138 // If the result for key is being computed, Get does not wait for the computation to finish. 139 func (c *Cache) Get(key interface{}) interface{} { 140 entryIface, ok := c.m.Load(key) 141 if !ok { 142 return nil 143 } 144 e := entryIface.(*cacheEntry) 145 if atomic.LoadUint32(&e.done) == 0 { 146 return nil 147 } 148 return e.result 149 } 150 151 // Clear removes all entries in the cache. 152 // 153 // Concurrent calls to Get may return old values. Concurrent calls to Do 154 // may return old values or store results in entries that have been deleted. 155 // 156 // TODO(jayconrod): Delete this after the package cache clearing functions 157 // in internal/load have been removed. 158 func (c *Cache) Clear() { 159 c.m.Range(func(key, value interface{}) bool { 160 c.m.Delete(key) 161 return true 162 }) 163 } 164 165 // Delete removes an entry from the map. It is safe to call Delete for an 166 // entry that does not exist. Delete will return quickly, even if the result 167 // for a key is still being computed; the computation will finish, but the 168 // result won't be accessible through the cache. 169 // 170 // TODO(jayconrod): Delete this after the package cache clearing functions 171 // in internal/load have been removed. 172 func (c *Cache) Delete(key interface{}) { 173 c.m.Delete(key) 174 } 175 176 // DeleteIf calls pred for each key in the map. If pred returns true for a key, 177 // DeleteIf removes the corresponding entry. If the result for a key is 178 // still being computed, DeleteIf will remove the entry without waiting for 179 // the computation to finish. The result won't be accessible through the cache. 180 // 181 // TODO(jayconrod): Delete this after the package cache clearing functions 182 // in internal/load have been removed. 183 func (c *Cache) DeleteIf(pred func(key interface{}) bool) { 184 c.m.Range(func(key, _ interface{}) bool { 185 if pred(key) { 186 c.Delete(key) 187 } 188 return true 189 }) 190 } 191