标记

垃圾回收的代码主要集中在函数 gcStart() 中。

// gcStart 是 GC 的入口函数，根据 gcMode 做处理。
// 1. gcMode == gcBackgroundMode（后台运行，也就是并行）, _GCoff -> _GCmark
// 2. 否则 GCoff -> _GCmarktermination，这个时候就是主动 GC 
func gcStart(mode gcMode, forceTrigger bool) {
    ...
}

• STW phase 1

在 GC 开始之前的准备工作。

func gcStart(mode gcMode, forceTrigger bool) {
    ...
    //在后台启动 mark worker 
    if mode == gcBackgroundMode {
        gcBgMarkStartWorkers()
    }
    ...
    // Stop The World
    systemstack(stopTheWorldWithSema)
    ...
    if mode == gcBackgroundMode {
        // GC 开始前的准备工作
        //处理设置 GCPhase，setGCPhase 还会 enable write barrier
        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()
        // Start The World
        systemstack(startTheWorldWithSema)
    } else {
        ...
    }
}

• Mark

Mark 阶段是并行的运行，通过在后台一直运行 mark worker 来实现。

func gcStart(mode gcMode, forceTrigger bool) {
    ...
    //在后台启动 mark worker 
    if mode == gcBackgroundMode {
        gcBgMarkStartWorkers()
    }
}
func gcBgMarkStartWorkers() {
    // Background marking is performed by per-P G's. Ensure that
    // each P has a background GC G.
    for _, p := range &allp {
        if p == nil || p.status == _Pdead {
            break
        }
        if p.gcBgMarkWorker == 0 {
            go gcBgMarkWorker(p)
            notetsleepg(&work.bgMarkReady, -1)
            noteclear(&work.bgMarkReady)
        }
    }
}
// gcBgMarkWorker 是一直在后台运行的，大部分时候是休眠状态，通过 gcController 来调度
func gcBgMarkWorker(_p_ *p) {
    for {
        // 将当前 goroutine 休眠，直到满足某些条件
        gopark(...)
        ...
        // mark 过程
        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, gcDrainNoBlock|gcDrainFlushBgCredit)
        case gcMarkWorkerFractionalMode:
            gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
        case gcMarkWorkerIdleMode:
            gcDrain(&_p_.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
        }
        casgstatus(gp, _Gwaiting, _Grunning)
        })
        ...
    }
}

Mark 阶段的标记代码主要在函数 gcDrain() 中实现。

// gcDrain scans roots and objects in work buffers, blackening grey
// objects until all roots and work buffers have been drained.
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
    ...    
    // Drain root marking jobs.
    if work.markrootNext < work.markrootJobs {
        for !(preemptible && gp.preempt) {
            job := atomic.Xadd(&work.markrootNext, +1) - 1
            if job >= work.markrootJobs {
                break
            }
            markroot(gcw, job)
            if idle && pollWork() {
                goto done
            }
        }
    }
    // 处理 heap 标记
    // Drain heap marking jobs.
    for !(preemptible && gp.preempt) {
        ...
        //从灰色列队中取出对象
        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
        }
        //扫描灰色对象的引用对象，标记为灰色，入灰色队列
        scanobject(b, gcw)
    }
}

• Mark termination (STW phase 2)

mark termination 阶段会 stop the world。函数实现在 gcMarkTermination()。

func gcMarkTermination() {
    // World is stopped.
    // Run gc on the g0 stack. We do this so that the g stack
    // we're currently running on will no longer change. Cuts
    // the root set down a bit (g0 stacks are not scanned, and
    // we don't need to scan gc's internal state).  We also
    // need to switch to g0 so we can shrink the stack.
    systemstack(func() {
        gcMark(startTime)
        // Must return immediately.
        // The outer function's stack may have moved
        // during gcMark (it shrinks stacks, including the
        // outer function's stack), so we must not refer
        // to any of its variables. Return back to the
        // non-system stack to pick up the new addresses
        // before continuing.
    })
    ...
}