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sync/atomic: add Value

A Value provides an atomic load and store of a consistently typed value.
It's intended to be used with copy-on-write idiom (see the example).

Performance:
BenchmarkValueRead	50000000	        21.7 ns/op
BenchmarkValueRead-2	200000000	         8.63 ns/op
BenchmarkValueRead-4	300000000	         4.33 ns/op

TBR=rsc
R=golang-codereviews
CC=golang-codereviews
https://golang.org/cl/136710045
This commit is contained in:
Dmitriy Vyukov 2014-09-16 19:54:26 -07:00
parent cbf97d9103
commit 98a1e207e2
5 changed files with 331 additions and 0 deletions

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@ -166,3 +166,9 @@ TEXT runtime·main_main(SB),NOSPLIT,$0-0
TEXT runtime·timenow(SB), NOSPLIT, $0-0
JMP time·now(SB)
TEXT syncatomic·runtime_procPin(SB),NOSPLIT,$0-0
JMP sync·runtime_procPin(SB)
TEXT syncatomic·runtime_procUnpin(SB),NOSPLIT,$0-0
JMP sync·runtime_procUnpin(SB)

17
src/sync/atomic/norace.go Normal file
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@ -0,0 +1,17 @@
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build !race
package atomic
import "unsafe"
const raceenabled = false
func raceAcquire(addr unsafe.Pointer) {
}
func raceReleaseMerge(addr unsafe.Pointer) {
}

22
src/sync/atomic/race.go Normal file
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@ -0,0 +1,22 @@
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build race
package atomic
import (
"runtime"
"unsafe"
)
const raceenabled = true
func raceAcquire(addr unsafe.Pointer) {
runtime.RaceAcquire(addr)
}
func raceReleaseMerge(addr unsafe.Pointer) {
runtime.RaceReleaseMerge(addr)
}

91
src/sync/atomic/value.go Normal file
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@ -0,0 +1,91 @@
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package atomic
import (
"unsafe"
)
// A Value provides an atomic load and store of a consistently typed value.
// Values can be created as part of other data structures.
// The zero value for a Value returns nil from Load.
// Once Store has been called, a Value must not be copied.
type Value struct {
v interface{}
}
// ifaceWords is interface{} internal representation.
type ifaceWords struct {
typ unsafe.Pointer
data unsafe.Pointer
}
// Load returns the value set by the most recent Store.
// It returns nil if there has been no call to Store for this Value.
func (v *Value) Load() (x interface{}) {
vp := (*ifaceWords)(unsafe.Pointer(v))
typ := LoadPointer(&vp.typ)
if typ == nil || uintptr(typ) == ^uintptr(0) {
// First store not yet completed.
return nil
}
data := LoadPointer(&vp.data)
xp := (*ifaceWords)(unsafe.Pointer(&x))
xp.typ = typ
xp.data = data
if raceenabled {
raceAcquire(unsafe.Pointer(v))
}
return
}
// Store sets the value of the Value to v.
// All calls to Store for a given Value must use values of the same concrete type.
// Store of an inconsistent type panics, as does Store(nil).
func (v *Value) Store(x interface{}) {
if x == nil {
panic("sync/atomic: store of nil value into Value")
}
if raceenabled {
raceReleaseMerge(unsafe.Pointer(v))
}
vp := (*ifaceWords)(unsafe.Pointer(v))
xp := (*ifaceWords)(unsafe.Pointer(&x))
for {
typ := LoadPointer(&vp.typ)
if typ == nil {
// Attempt to start first store.
// Disable preemption so that other goroutines can use
// active spin wait to wait for completion; and so that
// GC does not see the fake type accidentally.
runtime_procPin()
if !CompareAndSwapPointer(&vp.typ, nil, unsafe.Pointer(^uintptr(0))) {
runtime_procUnpin()
continue
}
// Complete first store.
StorePointer(&vp.data, xp.data)
StorePointer(&vp.typ, xp.typ)
runtime_procUnpin()
return
}
if uintptr(typ) == ^uintptr(0) {
// First store in progress. Wait.
// Since we disable preemption around the first store,
// we can wait with active spinning.
continue
}
// First store completed. Check type and overwrite data.
if typ != xp.typ {
panic("sync/atomic: store of inconsistently typed value into Value")
}
StorePointer(&vp.data, xp.data)
return
}
}
// Disable/enable preemption, implemented in runtime.
func runtime_procPin()
func runtime_procUnpin()

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@ -0,0 +1,195 @@
// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package atomic_test
import (
"math/rand"
"runtime"
"sync"
. "sync/atomic"
"testing"
"time"
)
func TestValue(t *testing.T) {
var v Value
if v.Load() != nil {
t.Fatal("initial Value is not nil")
}
v.Store(42)
x := v.Load()
if xx, ok := x.(int); !ok || xx != 42 {
t.Fatalf("wrong value: got %+v, want 42", x)
}
v.Store(84)
x = v.Load()
if xx, ok := x.(int); !ok || xx != 84 {
t.Fatalf("wrong value: got %+v, want 84", x)
}
}
func TestValueLarge(t *testing.T) {
var v Value
v.Store("foo")
x := v.Load()
if xx, ok := x.(string); !ok || xx != "foo" {
t.Fatalf("wrong value: got %+v, want foo", x)
}
v.Store("barbaz")
x = v.Load()
if xx, ok := x.(string); !ok || xx != "barbaz" {
t.Fatalf("wrong value: got %+v, want barbaz", x)
}
}
func TestValuePanic(t *testing.T) {
const nilErr = "sync/atomic: store of nil value into Value"
const badErr = "sync/atomic: store of inconsistently typed value into Value"
var v Value
func() {
defer func() {
err := recover()
if err != nilErr {
t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, nilErr)
}
}()
v.Store(nil)
}()
v.Store(42)
func() {
defer func() {
err := recover()
if err != badErr {
t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, badErr)
}
}()
v.Store("foo")
}()
func() {
defer func() {
err := recover()
if err != nilErr {
t.Fatalf("inconsistent store panic: got '%v', want '%v'", err, nilErr)
}
}()
v.Store(nil)
}()
}
func TestValueConcurrent(t *testing.T) {
tests := [][]interface{}{
{uint16(0), ^uint16(0), uint16(1 + 2<<8), uint16(3 + 4<<8)},
{uint32(0), ^uint32(0), uint32(1 + 2<<16), uint32(3 + 4<<16)},
{uint64(0), ^uint64(0), uint64(1 + 2<<32), uint64(3 + 4<<32)},
{complex(0, 0), complex(1, 2), complex(3, 4), complex(5, 6)},
}
p := 4 * runtime.GOMAXPROCS(0)
for _, test := range tests {
var v Value
done := make(chan bool)
for i := 0; i < p; i++ {
go func() {
r := rand.New(rand.NewSource(rand.Int63()))
loop:
for j := 0; j < 1e5; j++ {
x := test[r.Intn(len(test))]
v.Store(x)
x = v.Load()
for _, x1 := range test {
if x == x1 {
continue loop
}
}
t.Logf("loaded unexpected value %+v, want %+v", x, test)
done <- false
}
done <- true
}()
}
for i := 0; i < p; i++ {
if !<-done {
t.FailNow()
}
}
}
}
func BenchmarkValueRead(b *testing.B) {
var v Value
v.Store(new(int))
b.RunParallel(func(pb *testing.PB) {
for pb.Next() {
x := v.Load().(*int)
if *x != 0 {
b.Fatalf("wrong value: got %v, want 0", *x)
}
}
})
}
// The following example shows how to use Value for periodic program config updates
// and propagation of the changes to worker goroutines.
func ExampleValue_config() {
var config Value // holds current server configuration
// Create initial config value and store into config.
config.Store(loadConfig())
go func() {
// Reload config every 10 seconds
// and update config value with the new version.
for {
time.Sleep(10 * time.Second)
config.Store(loadConfig())
}
}()
// Create worker goroutines that handle incoming requests
// using the latest config value.
for i := 0; i < 10; i++ {
go func() {
for r := range requests() {
c := config.Load()
// Handle request r using config c.
_, _ = r, c
}
}()
}
}
func loadConfig() map[string]string {
return make(map[string]string)
}
func requests() chan int {
return make(chan int)
}
// The following example shows how to maintain a scalable frequently read,
// but infrequently updated data structure using copy-on-write idiom.
func ExampleValue_readMostly() {
type Map map[string]string
var m Value
m.Store(make(Map))
var mu sync.Mutex // used only by writers
// read function can be used to read the data without further synchronization
read := func(key string) (val string) {
m1 := m.Load().(Map)
return m1[key]
}
// insert function can be used to update the data without further synchronization
insert := func(key, val string) {
mu.Lock() // synchronize with other potential writers
defer mu.Unlock()
m1 := m.Load().(Map) // load current value of the data structure
m2 := make(Map) // create a new value
for k, v := range m1 {
m2[k] = v // copy all data from the current object to the new one
}
m2[key] = val // do the update that we need
m.Store(m2) // atomically replace the current object with the new one
// At this point all new readers start working with the new version.
// The old version will be garbage collected once the existing readers
// (if any) are done with it.
}
_, _ = read, insert
}