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书接上文 修改 golang 源代码获取 goroutine id 实现 ThreadLocal。上文实现的版本由于 map 是多个 goroutine 共享的，存在竞争，影响了性能，实现思路类似 java 初期的 ThreadLocal，今天我们借鉴现代版 java 的 ThreadLocal 来实现。
思路
先看看 java 里面怎么实现的可以看到每个线程实例都引用了一个 map，map 的 key 是 ThreadLocal 对象，value 是实际存储的数据。下面我们也按照这个思路来实现，golang 中 g 实例相当于 java 的 Thread 实例，我们可以修改 g 的结构来达到目的。
实现
修改 g 结构
修改 $GOROOT/src/runtime/runtime2.go 文件，为 g 结构体添加 localMap *goroutineLocalMap 字段
type g struct {
// Stack parameters.
// stack describes the actual stack memory: [stack.lo, stack.hi).
// stackguard0 is the stack pointer compared in the Go stack growth prologue.
// It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
// stackguard1 is the stack pointer compared in the C stack growth prologue.
// It is stack.lo+StackGuard on g0 and gsignal stacks.
// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
stack stack // offset known to runtime/cgo
stackguard0 uintptr // offset known to liblink
stackguard1 uintptr // offset known to liblink
_panic *_panic // innermost panic – offset known to liblink
_defer *_defer // innermost defer
m *m // current m; offset known to arm liblink
sched gobuf
syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
stktopsp uintptr // expected sp at top of stack, to check in traceback
param unsafe.Pointer // passed parameter on wakeup
atomicstatus uint32
stackLock uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
goid int64
schedlink guintptr
waitsince int64 // approx time when the g become blocked
waitreason waitReason // if status==Gwaiting
preempt bool // preemption signal, duplicates stackguard0 = stackpreempt
paniconfault bool // panic (instead of crash) on unexpected fault address
preemptscan bool // preempted g does scan for gc
gcscandone bool // g has scanned stack; protected by _Gscan bit in status
gcscanvalid bool // false at start of gc cycle, true if G has not run since last scan; TODO: remove?
throwsplit bool // must not split stack
raceignore int8 // ignore race detection events
sysblocktraced bool // StartTrace has emitted EvGoInSyscall about this goroutine
sysexitticks int64 // cputicks when syscall has returned (for tracing)
traceseq uint64 // trace event sequencer
tracelastp puintptr // last P emitted an event for this goroutine
lockedm muintptr
sig uint32
writebuf []byte
sigcode0 uintptr
sigcode1 uintptr
sigpc uintptr
gopc uintptr // pc of go statement that created this goroutine
ancestors *[]ancestorInfo // ancestor information goroutine(s) that created this goroutine (only used if debug.tracebackancestors)
startpc uintptr // pc of goroutine function
racectx uintptr
waiting *sudog // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
cgoCtxt []uintptr // cgo traceback context
labels unsafe.Pointer // profiler labels
timer *timer // cached timer for time.Sleep
selectDone uint32 // are we participating in a select and did someone win the race?
// Per-G GC state
// gcAssistBytes is this G’s GC assist credit in terms of
// bytes allocated. If this is positive, then the G has credit
// to allocate gcAssistBytes bytes without assisting. If this
// is negative, then the G must correct this by performing
// scan work. We track this in bytes to make it fast to update
// and check for debt in the malloc hot path. The assist ratio
// determines how this corresponds to scan work debt.
gcAssistBytes int64
localMap *goroutineLocalMap // 这是我们添加的
}
注意不要放在第一个字段，否则编译会出现 fatal: morestack on g0
实现 goroutineLocal
在 $GOROOT/src/runtime/ 目录下创建 go 原文件 goroutine_local.go
package runtime
type goroutineLocalMap struct {
m map[*goroutineLocal]interface{}
}
type goroutineLocal struct {
initfun func() interface{}
}
func NewGoroutineLocal(initfun func() interface{}) *goroutineLocal {
return &goroutineLocal{initfun}
}
func (gl *goroutineLocal)Get() interface{} {
if getg().localMap == nil {
getg().localMap = &goroutineLocalMap{make(map[*goroutineLocal]interface{})}
}
v, ok := getg().localMap.m[gl]
if !ok && gl.initfun != nil{
v = gl.initfun()
}
return v
}
func (gl *goroutineLocal)Set(v interface{}) {
if getg().localMap == nil {
getg().localMap = &goroutineLocalMap{make(map[*goroutineLocal]interface{})}
}
getg().localMap.m[gl] = v
}
func (gl *goroutineLocal)Remove() {
if getg().localMap != nil {
delete(getg().localMap.m, gl)
}
}
重新编译
cd ~/go/src
GOROOT_BOOTSTRAP=’/Users/qiuxudong/go1.9′ ./all.bash
写个 mian 函数测试一下
package main
import (
“fmt”
“time”
“runtime”
)
var gl = runtime.NewGoroutineLocal(func() interface{} {
return “default”
})
func main() {
gl.Set(“test0”)
fmt.Println(runtime.GetGoroutineId(), gl.Get())
go func() {
gl.Set(“test1”)
fmt.Println(runtime.GetGoroutineId(), gl.Get())
gl.Remove()
fmt.Println(runtime.GetGoroutineId(), gl.Get())
}()
time.Sleep(2 * time.Second)
}
可以看到
1 test0
18 test1
18 default
同样的，这个版本也可能会内存泄露，建议主动调用 Remove 清除数据。但是如果 goroutine 销毁了，对应的数据不再被引用，是可以被 GC 清理的，泄露的概率降低很多。
两种实现方式 GC 情况对比
修改 golang 源代码获取 goroutine id 实现 ThreadLocal 中的实现可以测试下泄露情况：
package goroutine_local
import (
“testing”
“fmt”
“time”
“runtime”
)
var gl = NewGoroutineLocal(func() interface{} {
return make([]byte, 1024*1024)
})
func TestGoroutineLocal(t *testing.T) {
var stats runtime.MemStats
runtime.ReadMemStats(&stats)
go func() {
for {
runtime.GC()
runtime.ReadMemStats(&stats)
fmt.Printf(“HeapAlloc = %d\n”, stats.HeapAlloc)
fmt.Printf(“NumGoroutine = %d\n”, runtime.NumGoroutine())
time.Sleep(1*time.Second)
}
}()
startAlloc()
time.Sleep(10000 * time.Second)
}
func startAlloc() {
for i := 0; i < 1000; i++ {
runtime.GC()
go func() {
gl.Set(make([]byte, 10*1024*1024))
fmt.Println(runtime.GetGoroutineId())
//gl.Remove() // 故意不删除数据，观察是否泄露
time.Sleep(1 * time.Second) // 模拟其它操作
}()
time.Sleep(1 * time.Second)
}
fmt.Println(“done”)
}
结果：
HeapAlloc = 98336
NumGoroutine = 2
HeapAlloc = 92280
NumGoroutine = 4
19
49
HeapAlloc = 21070408
NumGoroutine = 4
5
HeapAlloc = 31556568
NumGoroutine = 4
38
HeapAlloc = 42043600
NumGoroutine = 4
21
HeapAlloc = 52529512
NumGoroutine = 5
6
HeapAlloc = 63015760
NumGoroutine = 4
7
HeapAlloc = 73500784
NumGoroutine = 4
40
HeapAlloc = 83986616
NumGoroutine = 4
…
可以看到是持续上升的。
本文实现的泄露情况测试代码：
package main
import (
“fmt”
“time”
“runtime”
)
var gl = runtime.NewGoroutineLocal(func() interface{} {
return make([]byte, 10*1024*1024)
})
func main() {
var stats runtime.MemStats
go func() {
for {
runtime.GC()
runtime.ReadMemStats(&stats)
fmt.Printf(“HeapAlloc = %d\n”, stats.HeapAlloc)
fmt.Printf(“NumGoroutine = %d\n”, runtime.NumGoroutine())
time.Sleep(1*time.Second)
}
}()
startAlloc()
time.Sleep(10000 * time.Second)
}
func startAlloc() {
for i := 0; i < 1000; i++ {
runtime.GC()
go func() {
gl.Set(make([]byte, 10*1024*1024))
fmt.Println(runtime.GetGoroutineId())
//gl.Remove() // 故意不删除数据，观察是否泄露
time.Sleep(1 * time.Second) // 模拟其它操作
}()
time.Sleep(1 * time.Second)
}
fmt.Println(“done”)
}
结果：
…
HeapAlloc = 178351296
NumGoroutine = 3
114
HeapAlloc = 178351296
NumGoroutine = 3
112
HeapAlloc = 188837800
NumGoroutine = 3
131
HeapAlloc = 188837800
NumGoroutine = 3
145
HeapAlloc = 178351296
NumGoroutine = 3
146
HeapAlloc = 178351296
NumGoroutine = 3
HeapAlloc = 188837736
NumGoroutine = 3
132
58
HeapAlloc = 178351296
NumGoroutine = 3
…
可以看到 HeapAlloc 不是一直上升的，中间会有 GC 使其下降
