修改golang源代码实现无竞争版ThreadLocal

开篇

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

image.png

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。

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。