sweep.md

垃圾回收器：清扫

[TOC]

var sweep sweepdata

type sweepdata struct {
	lock    mutex
	g       *g
	parked  bool
	started bool

	nbgsweep    uint32
	npausesweep uint32
}

func bgsweep(c chan int) {
	sweep.g = getg()

	lock(&sweep.lock)
	sweep.parked = true
	c <- 1
	goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1) // "GC sweep wait"

	for {
		for sweepone() != ^uintptr(0) {
			sweep.nbgsweep++
			Gosched()
		}
		for freeSomeWbufs(true) {
			Gosched()
		}
		lock(&sweep.lock)
		if !isSweepDone() {
			unlock(&sweep.lock)
			continue
		}
		sweep.parked = true
		goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
	}
}

func sweepone() uintptr {
	_g_ := getg()
	sweepRatio := mheap_.sweepPagesPerByte // For debugging

	// increment locks to ensure that the goroutine is not preempted
	// in the middle of sweep thus leaving the span in an inconsistent state for next GC
	_g_.m.locks++
	if atomic.Load(&mheap_.sweepdone) != 0 {
		_g_.m.locks--
		return ^uintptr(0)
	}
	atomic.Xadd(&mheap_.sweepers, +1)

	// Find a span to sweep.
	var s *mspan
	sg := mheap_.sweepgen
	for {
		s = mheap_.sweepSpans[1-sg/2%2].pop()
		if s == nil {
			atomic.Store(&mheap_.sweepdone, 1)
			break
		}
		if s.state != mSpanInUse {
			// This can happen if direct sweeping already
			// swept this span, but in that case the sweep
			// generation should always be up-to-date.
			if !(s.sweepgen == sg || s.sweepgen == sg+3) {
				print("runtime: bad span s.state=", s.state, " s.sweepgen=", s.sweepgen, " sweepgen=", sg, "\n")
				throw("non in-use span in unswept list")
			}
			continue
		}
		if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
			break
		}
	}

	// Sweep the span we found.
	npages := ^uintptr(0)
	if s != nil {
		npages = s.npages
		if s.sweep(false) {
			// Whole span was freed. Count it toward the
			// page reclaimer credit since these pages can
			// now be used for span allocation.
			atomic.Xadduintptr(&mheap_.reclaimCredit, npages)
		} else {
			// Span is still in-use, so this returned no
			// pages to the heap and the span needs to
			// move to the swept in-use list.
			npages = 0
		}
	}

	// Decrement the number of active sweepers and if this is the
	// last one print trace information.
	if atomic.Xadd(&mheap_.sweepers, -1) == 0 && atomic.Load(&mheap_.sweepdone) != 0 {
		if debug.gcpacertrace > 0 {
			print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", (memstats.heap_live-mheap_.sweepHeapLiveBasis)>>20, "MB during sweep; swept ", mheap_.pagesSwept, " pages at ", sweepRatio, " pages/byte\n")
		}
	}
	_g_.m.locks--
	return npages
}

func prepareFreeWorkbufs() {
	lock(&work.wbufSpans.lock)
	if work.full != 0 {
		throw("cannot free workbufs when work.full != 0")
	}
	// Since all workbufs are on the empty list, we don't care
	// which ones are in which spans. We can wipe the entire empty
	// list and move all workbuf spans to the free list.
	work.empty = 0
	work.wbufSpans.free.takeAll(&work.wbufSpans.busy)
	unlock(&work.wbufSpans.lock)
}

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