Golang GC算法

概括

Go的垃圾回收官方形容为 非分代 非紧缩 写屏障 三色并发标记清理算法。

非分代:不像Java那样分为年轻代和年老代,自然也没有minor gc和maj o gc的区别。

非紧缩:在垃圾回收之后不会进行内存整理以清除内存碎片。

写屏障:在并发标记的过程中,如果应用程序(mutator)修改了对象图,就可能出现标记遗漏的可能,写屏障就是为了处理标记遗漏的问题。

三色:将GC中的对象按照搜索的情况分成三种:

  1. 黑色: 对象在这次GC中已标记,且这个对象包含的子对象也已标记
  2. 灰色: 对象在这次GC中已标记, 但这个对象包含的子对象未标记
  3. 白色: 对象在这次GC中未标记
    并发 :可以和应用程序(mutator)在一定程度上并发执行。
    标记清理 :GC算法分为两个大步骤:标记阶段找出要回收的对象,清理阶段则回收未被标记的对象(要被回收的对象)

触发时机

  • gcTriggerAlways: 强制触发GC,没找到什么情况下使用这个
  • gcTriggerHeap: 当前分配的内存达到一定值(动态计算)就触发GC
  • gcTriggerTime: 当一定时间(2分钟)没有执行过GC就触发GC
  • gcTriggerCycle: 要求启动新一轮的GC, 已启动则跳过, 手动触发GC的runtime.GC()会使用这个条件
func gcStart(mode gcMode, trigger gcTrigger) {
    // Since this is called from malloc and malloc is called in
    // the guts of a number of libraries that might be holding
    // locks, don't attempt to start GC in non-preemptible or
    // potentially unstable situations.
    mp := acquirem()
    if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" {
        releasem(mp)
        return
    }
    releasem(mp)
    mp = nil

    // 检查GC条件是否满足,和下面的test()构成双检查锁,如果满足GC条件但目前处于GC清理阶段,那就参与清理
    for trigger.test() && gosweepone() != ^uintptr(0) {
        sweep.nbgsweep++
    }

    // 加锁检查
    semacquire(&work.startSema)
    if !trigger.test() {
        semrelease(&work.startSema)
        return
    }
    /***************  .....  *****************/
    
}

在trigger.test()函数中,检查是否满足GC触发的条件

func (t gcTrigger) test() bool {
    if !memstats.enablegc || panicking != 0 {
        return false
    }
    if t.kind == gcTriggerAlways {
        return true
    }
    if gcphase != _GCoff {
        return false
    }
    switch t.kind {
    case gcTriggerHeap:
        // Non-atomic access to heap_live for performance. If
        // we are going to trigger on this, this thread just
        // atomically wrote heap_live anyway and we'll see our
        // own write.
        return memstats.heap_live >= memstats.gc_trigger
    case gcTriggerTime:
        if gcpercent < 0 {
            return false
        }
        lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
        // forcegcperiod = 2分钟
        return lastgc != 0 && t.now-lastgc > forcegcperiod
    case gcTriggerCycle:
        // t.n > work.cycles, but accounting for wraparound.
        return int32(t.n-work.cycles) > 0
    }
    return true
}
const (
    // gcTriggerAlways indicates that a cycle should be started
    // unconditionally, even if GOGC is off or we're in a cycle
    // right now. This cannot be consolidated with other cycles.
    gcTriggerAlways gcTriggerKind = iota

    // gcTriggerHeap indicates that a cycle should be started when
    // the heap size reaches the trigger heap size computed by the
    // controller.
    gcTriggerHeap

    // gcTriggerTime indicates that a cycle should be started when
    // it's been more than forcegcperiod nanoseconds since the
    // previous GC cycle.
    gcTriggerTime

    // gcTriggerCycle indicates that a cycle should be started if
    // we have not yet started cycle number gcTrigger.n (relative
    // to work.cycles).
    gcTriggerCycle
)

算法过程

  1. Sweep Termination: 对未清扫的span进行清扫, 只有上一轮的GC的清扫工作完成才可以开始新一轮的GC
  2. Mark: 扫描所有根对象, 和根对象可以到达的所有对象, 标记它们不被回收
  3. Mark Termination: 完成标记工作, 重新扫描部分根对象(要求STW)
  4. Sweep: 按标记结果清扫span

    golang_gc.jpg

func gcStart(mode gcMode, trigger gcTrigger) {
    // 拿到锁,保证只有一个执行流进入到这个临界区
    semacquire(&worldsema)

    // 启动后台扫描任务(G)
    if mode == gcBackgroundMode {
        gcBgMarkStartWorkers()
    }

    gcResetMarkState()

    work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs
    if work.stwprocs > ncpu {
        work.stwprocs = ncpu
    }
    work.heap0 = atomic.Load64(&memstats.heap_live)
    work.pauseNS = 0
    work.mode = mode

    now := nanotime()
    work.tSweepTerm = now
    work.pauseStart = now
    if trace.enabled {
        traceGCSTWStart(1)
    }
    systemstack(stopTheWorldWithSema)
    // Finish sweep before we start concurrent scan.
    systemstack(func() {
        finishsweep_m()
    })
    // clearpools before we start the GC. If we wait they memory will not be
    // reclaimed until the next GC cycle.
    clearpools()

    work.cycles++
    if mode == gcBackgroundMode { // Do as much work concurrently as possible
        gcController.startCycle()
        work.heapGoal = memstats.next_gc

        // Enter concurrent mark phase and enable
        // write barriers.
        setGCPhase(_GCmark)

        gcBgMarkPrepare() // Must happen before assist enable.
        gcMarkRootPrepare()

        // Mark all active tinyalloc blocks. Since we're
        // allocating from these, they need to be black like
        // other allocations. The alternative is to blacken
        // the tiny block on every allocation from it, which
        // would slow down the tiny allocator.
        gcMarkTinyAllocs()

        // At this point all Ps have enabled the write
        // barrier, thus maintaining the no white to
        // black invariant. Enable mutator assists to
        // put back-pressure on fast allocating
        // mutators.
        atomic.Store(&gcBlackenEnabled, 1)

        // Assists and workers can start the moment we start
        // the world.
        gcController.markStartTime = now

        // Concurrent mark.
        systemstack(func() {
            now = startTheWorldWithSema(trace.enabled)
        })
        work.pauseNS += now - work.pauseStart
        work.tMark = now
    }

    semrelease(&work.startSema)
}

关键函数及路径:

  1. gcBgMarkStartWorkers():准备后台标记工作goroutine(allp), 启动后等待该任务通知信号量bgMarkReady再继续,notewakeup(&work.bgMarkReady)
  2. gcResetMarkState():重置一些全局状态和所有gorontine的栈(一种根对象)扫描状态
  3. systemstack(stopTheWorldWithSema):启动stop the world
  4. systemstack(func(){finishsweep_m()}): 不断去除要清理的span进行清理,然后重置gcmark位
  5. clearpools(): 清扫sched.sudogcache和sched.deferpool,不知道在干嘛......
  6. gcController.startCycle():启动新一轮GC,设置gc controller的状态位和计算一些估计值
  7. setGCPhase(_GCmark):设置GC阶段,启用写屏障
  8. gcBgMarkPrepare():设置后台标记任务计数;work.nproc = ^uint32(0),work.nwait = ^uint32(0)
  9. gcMarkRootPrepare(): 计算扫描根对象的任务数量
  10. gcMarkTinyAllocs(): 标记所有tiny alloc等待合并的对象
  11. atomic.Store(&gcBlackenEnabled, 1): 启用辅助GC
  12. systemstack(func(){now=startTheWorldWithSema(trace.enable)}): 停止stop the world
func gcBgMarkWorker(_p_ *p) {
    /**********  .......  ***********/
    // 通知gcBgMarkStartWorkers可以继续处理
    notewakeup(&work.bgMarkReady)

    for {

        // 切换到g0运行
        systemstack(func() {
            // Mark our goroutine preemptible so its stack
            // can be scanned. This lets two mark workers
            // scan each other (otherwise, they would
            // deadlock). We must not modify anything on
            // the G stack. However, stack shrinking is
            // disabled for mark workers, so it is safe to
            // read from the G stack.
            casgstatus(gp, _Grunning, _Gwaiting)
            switch _p_.gcMarkWorkerMode {
            default:
                throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
            case gcMarkWorkerDedicatedMode:
                gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
                if gp.preempt {
                    lock(&sched.lock)
                    for {
                        gp, _ := runqget(_p_)
                        if gp == nil {
                            break
                        }
                        globrunqput(gp)
                    }
                    unlock(&sched.lock)
                }
                // Go back to draining, this time
                // without preemption.
                gcDrain(&_p_.gcw, gcDrainNoBlock|gcDrainFlushBgCredit)
            case gcMarkWorkerFractionalMode:
                gcDrain(&_p_.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
            case gcMarkWorkerIdleMode:
                gcDrain(&_p_.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
            }
            casgstatus(gp, _Gwaiting, _Grunning)
        })

        /********   ......  ***********/
        // 判断是否所有后台标记任务都完成, 并且没有更多的任务
        if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
            gcMarkDone()
        }
    }
}
  1. gcDrain()是执行标记的函数
  2. 当所有标记任务完成时,执行gcMarkDone()函数
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
    initScanWork := gcw.scanWork
    // 如果根对象未扫描完,则先扫描根对象,Jobs为根对象总数,next相当于一个对象任务的取数器
    if work.markrootNext < work.markrootJobs {
        for !(preemptible && gp.preempt) {
            job := atomic.Xadd(&work.markrootNext, +1) - 1
            if job >= work.markrootJobs {
                break
            }
            // 将会扫描根对象,并把它加入到标记队列gcWork中之中,也就是把对象变成灰色
            markroot(gcw, job)
            if check != nil && check() {
                goto done
            }
        }
    }

    // 当根对象全部put到标记队列中, 消费标记队列,根据对象图进行消费
    for !(preemptible && gp.preempt) {
        if work.full == 0 {
            gcw.balance()
        }

        var b uintptr
        if blocking {
            b = gcw.get()
        } else {
            b = gcw.tryGetFast()
            if b == 0 {
                b = gcw.tryGet()
            }
        }
        if b == 0 {
            // work barrier reached or tryGet failed.
            break
        }
        scanobject(b, gcw)

        // 如果已经扫描了一定数量的对象(gcCreditSlack的值是2000)
        if gcw.scanWork >= gcCreditSlack {
            // 把扫描的对象数量添加到全局
            atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
            // 减少辅助GC的工作量和唤醒等待中的G
            if flushBgCredit {
                gcFlushBgCredit(gcw.scanWork - initScanWork)
                initScanWork = 0
            }
            idleCheck -= gcw.scanWork
            gcw.scanWork = 0
            
            // 如果是idle模式且达到了检查的扫描量, 则检查是否有其他任务(G), 如果有则跳出循环
            if idle && idleCheck <= 0 {
                idleCheck += idleCheckThreshold
                if pollWork() {
                    break
                }
            }
        }
    }

done:
    // 把扫描的对象数量添加到全局
    if gcw.scanWork > 0 {
        atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
        // 减少辅助GC的工作量和唤醒等待中的G
        if flushBgCredit {
            gcFlushBgCredit(gcw.scanWork - initScanWork)
        }
        gcw.scanWork = 0
    }
}
func gcMarkDone() {
    semacquire(&work.markDoneSema)

    // Re-check transition condition under transition lock.
    if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
        semrelease(&work.markDoneSema)
        return
    }

    // 暂时禁止启动新的后台标记任务
    atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, -0xffffffff)
    prevFractionalGoal := gcController.fractionalUtilizationGoal
    gcController.fractionalUtilizationGoal = 0
    // 转换到Mark Termination阶段,进入STW阶段
    systemstack(stopTheWorldWithSema)
    // 标记对根对象的扫描已完成
    work.markrootDone = true
    // 禁止辅助GC和后台任务
    atomic.Store(&gcBlackenEnabled, 0)
    // 唤醒所有因为辅助GC而休眠的G
    gcWakeAllAssists()

    semrelease(&work.markDoneSema)
    // 计算下一次触发gc需要的heap大小
    nextTriggerRatio := gcController.endCycle()

    // 计算下一次触发gc需要的heap大小
    gcMarkTermination(nextTriggerRatio)
}
func gcMarkTermination(nextTriggerRatio float64) {
    // 禁止辅助GC和后台标记任务的运行
    // 重新允许本地标记队列(下次GC使用)
    // 设置当前GC阶段到完成标记阶段, 并启用写屏障
    atomic.Store(&gcBlackenEnabled, 0)
    gcBlackenPromptly = false
    setGCPhase(_GCmarktermination)

    systemstack(func() {gcMark(startTime)})

    systemstack(func() {
        // 设置当前GC阶段到关闭, 并禁用写屏障
        setGCPhase(_GCoff)
        // 唤醒后台清扫任务, 将在STW结束后开始运行
        gcSweep(work.mode)
    })
    
    // 更新下一次触发gc需要的heap大小(gc_trigger)
    gcSetTriggerRatio(nextTriggerRatio)

    // 重置清扫状态
    sweep.nbgsweep = 0
    sweep.npausesweep = 0

    // 统计执行GC的次数然后唤醒等待清扫的G
    lock(&work.sweepWaiters.lock)
    memstats.numgc++
    injectglist(work.sweepWaiters.head.ptr())
    work.sweepWaiters.head = 0
    unlock(&work.sweepWaiters.lock)
    
    // 重新启动世界
    systemstack(func() { startTheWorldWithSema(true) })
    // 移动标记队列使用的缓冲区到自由列表, 使得它们可以被回收
    prepareFreeWorkbufs()
    // 释放未使用的栈
    systemstack(freeStackSpans)
   
    semrelease(&worldsema)
    // 重新允许当前的G被抢占
    releasem(mp)
    mp = nil

当标记的扫描工作完成之后,会进入到GC Mark Termination阶段,也就是gcMarkDone()函数,关键路径:

  1. systemstack(stopTheWorldWithSema):启动STW
  2. gcWakeAllAssists():唤醒所有因辅助gc而休眠的G
  3. nextTriggerRatio:=gcController.endCycle():计算下一次触发gc需要的heap大小
  4. setGCPhase(_GCmarktermination):启用写屏障
  5. systemstack(func() {gcMark(startTime)}): 再次执行标记
  6. systemstack(func(){setGCPhase(_GCoff);gcSweep(work.mode)}):关闭写屏障,唤醒后台清扫任务,将在STW结束后开始运行
  7. gcSetTriggerRatio(nextTriggerRatio):更新下次触发gc时的heap大小
  8. systemstack(func() { startTheWorldWithSema(true) }): 停止STW

STW分析:web程序中,我们关注最大停顿时间

STW出现在两个位置,分别是在初始标记阶段Mark和并发标记完成后重标记Mark Termination:

初始标记阶段:

  • systemstack(stopTheWorldWithSema):启动stop the world
  • systemstack(func(){finishsweep_m()}): 不断去除要清理的span进行清理,然后重置gcmark位
  • clearpools(): 清扫sched.sudogcache和sched.deferpool,不知道在干嘛......
  • gcController.startCycle():启动新一轮GC,设置gc controller的状态位和计算一些估计值
  • gcMarkRootPrepare(): 计算扫描根对象的任务数量
  • gcMarkTinyAllocs(): 涂灰所有tiny alloc等待合并的对象
  • systemstack(func(){now=startTheWorldWithSema(trace.enable)}): 停止stop the world

找出其中比较耗时的阶段:

  • finishsweep_m():如果上一次GC清扫阶段没有完成,那么在新的一轮GC阶段中就会在阻塞在这里,使得原本可以和应用程序并行的清扫阶段被放进STW。所以,如果频繁的执行GC,就可能会使得GC的最大停顿时间变长。
  • clearpools():时间复杂度大概为:O(5*L),L为_defer中链表的长度。
  • gcController.startCycle():O(P),P为go的P的数量,和cpu数有关,时间复杂度可以忽略
  • gcMarkRootPrepare(): O(全局变量区),包括bss段和data段
  • gcMarkTinyAllocs(): O(P)

个人觉得,对STW影响最大的是finishsweep_m()阶段,所有我们应该尽量避免让go在清扫期执行新一轮的GC。

重新标记阶段

  • systemstack(stopTheWorldWithSema):启动STW
  • gcWakeAllAssists():唤醒所有因辅助gc而休眠的G
  • nextTriggerRatio:=gcController.endCycle():计算下一次触发gc需要的heap大小
  • setGCPhase(_GCmarktermination):启用写屏障
  • systemstack(func() {gcMark(startTime)}): 再次执行标记
  • systemstack(func(){setGCPhase(_GCoff);gcSweep(work.mode)}):关闭写屏障,唤醒后台清扫任务,将在STW结束后开始运行
  • gcSetTriggerRatio(nextTriggerRatio):更新下次触发gc时的heap大小
  • systemstack(func() { startTheWorldWithSema(true) }): 停止STW

找出其中比较耗时的阶段:

  • gcWakeAllAssists():O(G),将所有可运行的G插入到调度链表
  • systemstack(func() {gcMark(startTime)}):
我来评几句
登录后评论

已发表评论数()

相关站点

热门文章