go/src/runtime/mgcsweep.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Garbage collector: sweeping

package runtime

import (
	"runtime/internal/atomic"
	"unsafe"
)

var sweep sweepdata

// State of background sweep.
type sweepdata struct {
	lock    mutex
	g       *g
	parked  bool
	started bool

	spanidx uint32 // background sweeper position

	nbgsweep    uint32
	npausesweep uint32
}

//go:nowritebarrier
func finishsweep_m(stw bool) {
	// Sweeping must be complete before marking commences, so
	// sweep any unswept spans. If this is a concurrent GC, there
	// shouldn't be any spans left to sweep, so this should finish
	// instantly. If GC was forced before the concurrent sweep
	// finished, there may be spans to sweep.
	for sweepone() != ^uintptr(0) {
		sweep.npausesweep++
	}

	// There may be some other spans being swept concurrently that
	// we need to wait for. If finishsweep_m is done with the world stopped
	// this is not required because the STW must have waited for sweeps.
	//
	// TODO(austin): As of this writing, we always pass true for stw.
	// Consider removing this code.
	if !stw {
		sg := mheap_.sweepgen
		for _, s := range work.spans {
			if s.sweepgen != sg && s.state == _MSpanInUse {
				s.ensureSwept()
			}
		}
	}
	nextMarkBitArenaEpoch()
}

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

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

	for {
		for gosweepone() != ^uintptr(0) {
			sweep.nbgsweep++
			Gosched()
		}
		lock(&sweep.lock)
		if !gosweepdone() {
			// This can happen if a GC runs between
			// gosweepone returning ^0 above
			// and the lock being acquired.
			unlock(&sweep.lock)
			continue
		}
		sweep.parked = true
		goparkunlock(&sweep.lock, "GC sweep wait", traceEvGoBlock, 1)
	}
}

// sweeps one span
// returns number of pages returned to heap, or ^uintptr(0) if there is nothing to sweep
//go:nowritebarrier
func sweepone() uintptr {
	_g_ := getg()

	// 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++
	sg := mheap_.sweepgen
	for {
		idx := atomic.Xadd(&sweep.spanidx, 1) - 1
		if idx >= uint32(len(work.spans)) {
			mheap_.sweepdone = 1
			_g_.m.locks--
			if debug.gcpacertrace > 0 && idx == uint32(len(work.spans)) {
				print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
			}
			return ^uintptr(0)
		}
		s := work.spans[idx]
		if s.state != mSpanInUse {
			s.sweepgen = sg
			continue
		}
		if s.sweepgen != sg-2 || !atomic.Cas(&s.sweepgen, sg-2, sg-1) {
			continue
		}
		npages := s.npages
		if !s.sweep(false) {
			npages = 0
		}
		_g_.m.locks--
		return npages
	}
}

//go:nowritebarrier
func gosweepone() uintptr {
	var ret uintptr
	systemstack(func() {
		ret = sweepone()
	})
	return ret
}

//go:nowritebarrier
func gosweepdone() bool {
	return mheap_.sweepdone != 0
}

// Returns only when span s has been swept.
//go:nowritebarrier
func (s *mspan) ensureSwept() {
	// Caller must disable preemption.
	// Otherwise when this function returns the span can become unswept again
	// (if GC is triggered on another goroutine).
	_g_ := getg()
	if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
		throw("MSpan_EnsureSwept: m is not locked")
	}

	sg := mheap_.sweepgen
	if atomic.Load(&s.sweepgen) == sg {
		return
	}
	// The caller must be sure that the span is a MSpanInUse span.
	if atomic.Cas(&s.sweepgen, sg-2, sg-1) {
		s.sweep(false)
		return
	}
	// unfortunate condition, and we don't have efficient means to wait
	for atomic.Load(&s.sweepgen) != sg {
		osyield()
	}
}

// Sweep frees or collects finalizers for blocks not marked in the mark phase.
// It clears the mark bits in preparation for the next GC round.
// Returns true if the span was returned to heap.
// If preserve=true, don't return it to heap nor relink in MCentral lists;
// caller takes care of it.
//TODO go:nowritebarrier
func (s *mspan) sweep(preserve bool) bool {
	// It's critical that we enter this function with preemption disabled,
	// GC must not start while we are in the middle of this function.
	_g_ := getg()
	if _g_.m.locks == 0 && _g_.m.mallocing == 0 && _g_ != _g_.m.g0 {
		throw("MSpan_Sweep: m is not locked")
	}
	sweepgen := mheap_.sweepgen
	if s.state != mSpanInUse || s.sweepgen != sweepgen-1 {
		print("MSpan_Sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
		throw("MSpan_Sweep: bad span state")
	}

	if trace.enabled {
		traceGCSweepStart()
	}

	atomic.Xadd64(&mheap_.pagesSwept, int64(s.npages))

	cl := s.sizeclass
	size := s.elemsize
	res := false
	nfree := 0

	c := _g_.m.mcache
	freeToHeap := false

	// The allocBits indicate which unmarked objects don't need to be
	// processed since they were free at the end of the last GC cycle
	// and were not allocated since then.
	// If the allocBits index is >= s.freeindex and the bit
	// is not marked then the object remains unallocated
	// since the last GC.
	// This situation is analogous to being on a freelist.

	// Unlink & free special records for any objects we're about to free.
	// Two complications here:
	// 1. An object can have both finalizer and profile special records.
	//    In such case we need to queue finalizer for execution,
	//    mark the object as live and preserve the profile special.
	// 2. A tiny object can have several finalizers setup for different offsets.
	//    If such object is not marked, we need to queue all finalizers at once.
	// Both 1 and 2 are possible at the same time.
	specialp := &s.specials
	special := *specialp
	for special != nil {
		// A finalizer can be set for an inner byte of an object, find object beginning.
		objIndex := uintptr(special.offset) / size
		p := s.base() + objIndex*size
		mbits := s.markBitsForIndex(objIndex)
		if !mbits.isMarked() {
			// This object is not marked and has at least one special record.
			// Pass 1: see if it has at least one finalizer.
			hasFin := false
			endOffset := p - s.base() + size
			for tmp := special; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
				if tmp.kind == _KindSpecialFinalizer {
					// Stop freeing of object if it has a finalizer.
					mbits.setMarkedNonAtomic()
					hasFin = true
					break
				}
			}
			// Pass 2: queue all finalizers _or_ handle profile record.
			for special != nil && uintptr(special.offset) < endOffset {
				// Find the exact byte for which the special was setup
				// (as opposed to object beginning).
				p := s.base() + uintptr(special.offset)
				if special.kind == _KindSpecialFinalizer || !hasFin {
					// Splice out special record.
					y := special
					special = special.next
					*specialp = special
					freespecial(y, unsafe.Pointer(p), size)
				} else {
					// This is profile record, but the object has finalizers (so kept alive).
					// Keep special record.
					specialp = &special.next
					special = *specialp
				}
			}
		} else {
			// object is still live: keep special record
			specialp = &special.next
			special = *specialp
		}
	}

	if debug.allocfreetrace != 0 || raceenabled || msanenabled {
		// Find all newly freed objects. This doesn't have to
		// efficient; allocfreetrace has massive overhead.
		mbits := s.markBitsForBase()
		abits := s.allocBitsForIndex(0)
		for i := uintptr(0); i < s.nelems; i++ {
			if !mbits.isMarked() && (abits.index < s.freeindex || abits.isMarked()) {
				x := s.base() + i*s.elemsize
				if debug.allocfreetrace != 0 {
					tracefree(unsafe.Pointer(x), size)
				}
				if raceenabled {
					racefree(unsafe.Pointer(x), size)
				}
				if msanenabled {
					msanfree(unsafe.Pointer(x), size)
				}
			}
			mbits.advance()
			abits.advance()
		}
	}

	// Count the number of free objects in this span.
	nfree = s.countFree()
	if cl == 0 && nfree != 0 {
		s.needzero = 1
		freeToHeap = true
	}
	nalloc := uint16(s.nelems) - uint16(nfree)
	nfreed := s.allocCount - nalloc
	if nalloc > s.allocCount {
		print("runtime: nelems=", s.nelems, " nfree=", nfree, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
		throw("sweep increased allocation count")
	}

	s.allocCount = nalloc
	wasempty := s.nextFreeIndex() == s.nelems
	s.freeindex = 0 // reset allocation index to start of span.

	// gcmarkBits becomes the allocBits.
	// get a fresh cleared gcmarkBits in preparation for next GC
	s.allocBits = s.gcmarkBits
	s.gcmarkBits = newMarkBits(s.nelems)

	// Initialize alloc bits cache.
	s.refillAllocCache(0)

	// We need to set s.sweepgen = h.sweepgen only when all blocks are swept,
	// because of the potential for a concurrent free/SetFinalizer.
	// But we need to set it before we make the span available for allocation
	// (return it to heap or mcentral), because allocation code assumes that a
	// span is already swept if available for allocation.
	if freeToHeap || nfreed == 0 {
		// The span must be in our exclusive ownership until we update sweepgen,
		// check for potential races.
		if s.state != mSpanInUse || s.sweepgen != sweepgen-1 {
			print("MSpan_Sweep: state=", s.state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
			throw("MSpan_Sweep: bad span state after sweep")
		}
		// Serialization point.
		// At this point the mark bits are cleared and allocation ready
		// to go so release the span.
		atomic.Store(&s.sweepgen, sweepgen)
	}

	if nfreed > 0 && cl != 0 {
		c.local_nsmallfree[cl] += uintptr(nfreed)
		res = mheap_.central[cl].mcentral.freeSpan(s, preserve, wasempty)
		// MCentral_FreeSpan updates sweepgen
	} else if freeToHeap {
		// Free large span to heap

		// NOTE(rsc,dvyukov): The original implementation of efence
		// in CL 22060046 used SysFree instead of SysFault, so that
		// the operating system would eventually give the memory
		// back to us again, so that an efence program could run
		// longer without running out of memory. Unfortunately,
		// calling SysFree here without any kind of adjustment of the
		// heap data structures means that when the memory does
		// come back to us, we have the wrong metadata for it, either in
		// the MSpan structures or in the garbage collection bitmap.
		// Using SysFault here means that the program will run out of
		// memory fairly quickly in efence mode, but at least it won't
		// have mysterious crashes due to confused memory reuse.
		// It should be possible to switch back to SysFree if we also
		// implement and then call some kind of MHeap_DeleteSpan.
		if debug.efence > 0 {
			s.limit = 0 // prevent mlookup from finding this span
			sysFault(unsafe.Pointer(s.base()), size)
		} else {
			mheap_.freeSpan(s, 1)
		}
		c.local_nlargefree++
		c.local_largefree += size
		res = true
	}
	if trace.enabled {
		traceGCSweepDone()
	}
	return res
}

// deductSweepCredit deducts sweep credit for allocating a span of
// size spanBytes. This must be performed *before* the span is
// allocated to ensure the system has enough credit. If necessary, it
// performs sweeping to prevent going in to debt. If the caller will
// also sweep pages (e.g., for a large allocation), it can pass a
// non-zero callerSweepPages to leave that many pages unswept.
//
// deductSweepCredit makes a worst-case assumption that all spanBytes
// bytes of the ultimately allocated span will be available for object
// allocation. The caller should call reimburseSweepCredit if that
// turns out not to be the case once the span is allocated.
//
// deductSweepCredit is the core of the "proportional sweep" system.
// It uses statistics gathered by the garbage collector to perform
// enough sweeping so that all pages are swept during the concurrent
// sweep phase between GC cycles.
//
// mheap_ must NOT be locked.
func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
	if mheap_.sweepPagesPerByte == 0 {
		// Proportional sweep is done or disabled.
		return
	}

	// Account for this span allocation.
	spanBytesAlloc := atomic.Xadd64(&mheap_.spanBytesAlloc, int64(spanBytes))

	// Fix debt if necessary.
	pagesOwed := int64(mheap_.sweepPagesPerByte * float64(spanBytesAlloc))
	for pagesOwed-int64(atomic.Load64(&mheap_.pagesSwept)) > int64(callerSweepPages) {
		if gosweepone() == ^uintptr(0) {
			mheap_.sweepPagesPerByte = 0
			break
		}
	}
}

// reimburseSweepCredit records that unusableBytes bytes of a
// just-allocated span are not available for object allocation. This
// offsets the worst-case charge performed by deductSweepCredit.
func reimburseSweepCredit(unusableBytes uintptr) {
	if mheap_.sweepPagesPerByte == 0 {
		// Nobody cares about the credit. Avoid the atomic.
		return
	}
	if int64(atomic.Xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))) < 0 {
		throw("spanBytesAlloc underflow")
	}
}