1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Garbage collector: sweeping
7 // The sweeper consists of two different algorithms:
9 // * The object reclaimer finds and frees unmarked slots in spans. It
10 // can free a whole span if none of the objects are marked, but that
11 // isn't its goal. This can be driven either synchronously by
12 // mcentral.cacheSpan for mcentral spans, or asynchronously by
13 // sweepone, which looks at all the mcentral lists.
15 // * The span reclaimer looks for spans that contain no marked objects
16 // and frees whole spans. This is a separate algorithm because
17 // freeing whole spans is the hardest task for the object reclaimer,
18 // but is critical when allocating new spans. The entry point for
19 // this is mheap_.reclaim and it's driven by a sequential scan of
20 // the page marks bitmap in the heap arenas.
22 // Both algorithms ultimately call mspan.sweep, which sweeps a single
28 "runtime/internal/atomic"
34 // State of background sweep.
35 type sweepdata struct {
44 // active tracks outstanding sweepers and the sweep
45 // termination condition.
48 // centralIndex is the current unswept span class.
49 // It represents an index into the mcentral span
50 // sets. Accessed and updated via its load and
51 // update methods. Not protected by a lock.
53 // Reset at mark termination.
54 // Used by mheap.nextSpanForSweep.
55 centralIndex sweepClass
58 // sweepClass is a spanClass and one bit to represent whether we're currently
59 // sweeping partial or full spans.
60 type sweepClass uint32
63 numSweepClasses = numSpanClasses * 2
64 sweepClassDone sweepClass = sweepClass(^uint32(0))
67 func (s *sweepClass) load() sweepClass {
68 return sweepClass(atomic.Load((*uint32)(s)))
71 func (s *sweepClass) update(sNew sweepClass) {
72 // Only update *s if its current value is less than sNew,
73 // since *s increases monotonically.
75 for sOld < sNew && !atomic.Cas((*uint32)(s), uint32(sOld), uint32(sNew)) {
78 // TODO(mknyszek): This isn't the only place we have
79 // an atomic monotonically increasing counter. It would
80 // be nice to have an "atomic max" which is just implemented
81 // as the above on most architectures. Some architectures
82 // like RISC-V however have native support for an atomic max.
85 func (s *sweepClass) clear() {
86 atomic.Store((*uint32)(s), 0)
89 // split returns the underlying span class as well as
90 // whether we're interested in the full or partial
91 // unswept lists for that class, indicated as a boolean
92 // (true means "full").
93 func (s sweepClass) split() (spc spanClass, full bool) {
94 return spanClass(s >> 1), s&1 == 0
97 // nextSpanForSweep finds and pops the next span for sweeping from the
98 // central sweep buffers. It returns ownership of the span to the caller.
99 // Returns nil if no such span exists.
100 func (h *mheap) nextSpanForSweep() *mspan {
102 for sc := sweep.centralIndex.load(); sc < numSweepClasses; sc++ {
103 spc, full := sc.split()
104 c := &h.central[spc].mcentral
107 s = c.fullUnswept(sg).pop()
109 s = c.partialUnswept(sg).pop()
112 // Write down that we found something so future sweepers
113 // can start from here.
114 sweep.centralIndex.update(sc)
118 // Write down that we found nothing.
119 sweep.centralIndex.update(sweepClassDone)
123 const sweepDrainedMask = 1 << 31
125 // activeSweep is a type that captures whether sweeping
126 // is done, and whether there are any outstanding sweepers.
128 // Every potential sweeper must call begin() before they look
129 // for work, and end() after they've finished sweeping.
130 type activeSweep struct {
131 // state is divided into two parts.
133 // The top bit (masked by sweepDrainedMask) is a boolean
134 // value indicating whether all the sweep work has been
135 // drained from the queue.
137 // The rest of the bits are a counter, indicating the
138 // number of outstanding concurrent sweepers.
142 // begin registers a new sweeper. Returns a sweepLocker
143 // for acquiring spans for sweeping. Any outstanding sweeper blocks
144 // sweep termination.
146 // If the sweepLocker is invalid, the caller can be sure that all
147 // outstanding sweep work has been drained, so there is nothing left
148 // to sweep. Note that there may be sweepers currently running, so
149 // this does not indicate that all sweeping has completed.
151 // Even if the sweepLocker is invalid, its sweepGen is always valid.
152 func (a *activeSweep) begin() sweepLocker {
154 state := a.state.Load()
155 if state&sweepDrainedMask != 0 {
156 return sweepLocker{mheap_.sweepgen, false}
158 if a.state.CompareAndSwap(state, state+1) {
159 return sweepLocker{mheap_.sweepgen, true}
164 // end deregisters a sweeper. Must be called once for each time
165 // begin is called if the sweepLocker is valid.
166 func (a *activeSweep) end(sl sweepLocker) {
167 if sl.sweepGen != mheap_.sweepgen {
168 throw("sweeper left outstanding across sweep generations")
171 state := a.state.Load()
172 if (state&^sweepDrainedMask)-1 >= sweepDrainedMask {
173 throw("mismatched begin/end of activeSweep")
175 if a.state.CompareAndSwap(state, state-1) {
176 if state != sweepDrainedMask {
179 if debug.gcpacertrace > 0 {
180 live := gcController.heapLive.Load()
181 print("pacer: sweep done at heap size ", live>>20, "MB; allocated ", (live-mheap_.sweepHeapLiveBasis)>>20, "MB during sweep; swept ", mheap_.pagesSwept.Load(), " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
188 // markDrained marks the active sweep cycle as having drained
189 // all remaining work. This is safe to be called concurrently
190 // with all other methods of activeSweep, though may race.
192 // Returns true if this call was the one that actually performed
194 func (a *activeSweep) markDrained() bool {
196 state := a.state.Load()
197 if state&sweepDrainedMask != 0 {
200 if a.state.CompareAndSwap(state, state|sweepDrainedMask) {
206 // sweepers returns the current number of active sweepers.
207 func (a *activeSweep) sweepers() uint32 {
208 return a.state.Load() &^ sweepDrainedMask
211 // isDone returns true if all sweep work has been drained and no more
212 // outstanding sweepers exist. That is, when the sweep phase is
214 func (a *activeSweep) isDone() bool {
215 return a.state.Load() == sweepDrainedMask
218 // reset sets up the activeSweep for the next sweep cycle.
220 // The world must be stopped.
221 func (a *activeSweep) reset() {
226 // finishsweep_m ensures that all spans are swept.
228 // The world must be stopped. This ensures there are no sweeps in
232 func finishsweep_m() {
235 // Sweeping must be complete before marking commences, so
236 // sweep any unswept spans. If this is a concurrent GC, there
237 // shouldn't be any spans left to sweep, so this should finish
238 // instantly. If GC was forced before the concurrent sweep
239 // finished, there may be spans to sweep.
240 for sweepone() != ^uintptr(0) {
244 // Make sure there aren't any outstanding sweepers left.
245 // At this point, with the world stopped, it means one of two
246 // things. Either we were able to preempt a sweeper, or that
247 // a sweeper didn't call sweep.active.end when it should have.
248 // Both cases indicate a bug, so throw.
249 if sweep.active.sweepers() != 0 {
250 throw("active sweepers found at start of mark phase")
253 // Reset all the unswept buffers, which should be empty.
254 // Do this in sweep termination as opposed to mark termination
255 // so that we can catch unswept spans and reclaim blocks as
257 sg := mheap_.sweepgen
258 for i := range mheap_.central {
259 c := &mheap_.central[i].mcentral
260 c.partialUnswept(sg).reset()
261 c.fullUnswept(sg).reset()
264 // Sweeping is done, so if the scavenger isn't already awake,
265 // wake it up. There's definitely work for it to do at this
269 nextMarkBitArenaEpoch()
272 func bgsweep(c chan int) {
275 lockInit(&sweep.lock, lockRankSweep)
279 goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
282 // bgsweep attempts to be a "low priority" goroutine by intentionally
283 // yielding time. It's OK if it doesn't run, because goroutines allocating
284 // memory will sweep and ensure that all spans are swept before the next
285 // GC cycle. We really only want to run when we're idle.
287 // However, calling Gosched after each span swept produces a tremendous
288 // amount of tracing events, sometimes up to 50% of events in a trace. It's
289 // also inefficient to call into the scheduler so much because sweeping a
290 // single span is in general a very fast operation, taking as little as 30 ns
291 // on modern hardware. (See #54767.)
293 // As a result, bgsweep sweeps in batches, and only calls into the scheduler
294 // at the end of every batch. Furthermore, it only yields its time if there
295 // isn't spare idle time available on other cores. If there's available idle
296 // time, helping to sweep can reduce allocation latencies by getting ahead of
297 // the proportional sweeper and having spans ready to go for allocation.
298 const sweepBatchSize = 10
300 for sweepone() != ^uintptr(0) {
303 if nSwept%sweepBatchSize == 0 {
307 for freeSomeWbufs(true) {
308 // N.B. freeSomeWbufs is already batched internally.
313 // This can happen if a GC runs between
314 // gosweepone returning ^0 above
315 // and the lock being acquired.
320 goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceEvGoBlock, 1)
324 // sweepLocker acquires sweep ownership of spans.
325 type sweepLocker struct {
326 // sweepGen is the sweep generation of the heap.
331 // sweepLocked represents sweep ownership of a span.
332 type sweepLocked struct {
336 // tryAcquire attempts to acquire sweep ownership of span s. If it
337 // successfully acquires ownership, it blocks sweep completion.
338 func (l *sweepLocker) tryAcquire(s *mspan) (sweepLocked, bool) {
340 throw("use of invalid sweepLocker")
342 // Check before attempting to CAS.
343 if atomic.Load(&s.sweepgen) != l.sweepGen-2 {
344 return sweepLocked{}, false
346 // Attempt to acquire sweep ownership of s.
347 if !atomic.Cas(&s.sweepgen, l.sweepGen-2, l.sweepGen-1) {
348 return sweepLocked{}, false
350 return sweepLocked{s}, true
353 // sweepone sweeps some unswept heap span and returns the number of pages returned
354 // to the heap, or ^uintptr(0) if there was nothing to sweep.
355 func sweepone() uintptr {
358 // Increment locks to ensure that the goroutine is not preempted
359 // in the middle of sweep thus leaving the span in an inconsistent state for next GC
362 // TODO(austin): sweepone is almost always called in a loop;
363 // lift the sweepLocker into its callers.
364 sl := sweep.active.begin()
370 // Find a span to sweep.
371 npages := ^uintptr(0)
374 s := mheap_.nextSpanForSweep()
376 noMoreWork = sweep.active.markDrained()
379 if state := s.state.get(); state != mSpanInUse {
380 // This can happen if direct sweeping already
381 // swept this span, but in that case the sweep
382 // generation should always be up-to-date.
383 if !(s.sweepgen == sl.sweepGen || s.sweepgen == sl.sweepGen+3) {
384 print("runtime: bad span s.state=", state, " s.sweepgen=", s.sweepgen, " sweepgen=", sl.sweepGen, "\n")
385 throw("non in-use span in unswept list")
389 if s, ok := sl.tryAcquire(s); ok {
390 // Sweep the span we found.
393 // Whole span was freed. Count it toward the
394 // page reclaimer credit since these pages can
395 // now be used for span allocation.
396 mheap_.reclaimCredit.Add(npages)
398 // Span is still in-use, so this returned no
399 // pages to the heap and the span needs to
400 // move to the swept in-use list.
409 // The sweep list is empty. There may still be
410 // concurrent sweeps running, but we're at least very
411 // close to done sweeping.
413 // Move the scavenge gen forward (signaling
414 // that there's new work to do) and wake the scavenger.
416 // The scavenger is signaled by the last sweeper because once
417 // sweeping is done, we will definitely have useful work for
418 // the scavenger to do, since the scavenger only runs over the
419 // heap once per GC cycle. This update is not done during sweep
420 // termination because in some cases there may be a long delay
421 // between sweep done and sweep termination (e.g. not enough
422 // allocations to trigger a GC) which would be nice to fill in
423 // with scavenging work.
424 if debug.scavtrace > 0 {
427 released := atomic.Loaduintptr(&mheap_.pages.scav.released)
428 printScavTrace(released, false)
429 atomic.Storeuintptr(&mheap_.pages.scav.released, 0)
440 // isSweepDone reports whether all spans are swept.
442 // Note that this condition may transition from false to true at any
443 // time as the sweeper runs. It may transition from true to false if a
444 // GC runs; to prevent that the caller must be non-preemptible or must
445 // somehow block GC progress.
446 func isSweepDone() bool {
447 return sweep.active.isDone()
450 // Returns only when span s has been swept.
453 func (s *mspan) ensureSwept() {
454 // Caller must disable preemption.
455 // Otherwise when this function returns the span can become unswept again
456 // (if GC is triggered on another goroutine).
458 if gp.m.locks == 0 && gp.m.mallocing == 0 && gp != gp.m.g0 {
459 throw("mspan.ensureSwept: m is not locked")
462 // If this operation fails, then that means that there are
463 // no more spans to be swept. In this case, either s has already
464 // been swept, or is about to be acquired for sweeping and swept.
465 sl := sweep.active.begin()
467 // The caller must be sure that the span is a mSpanInUse span.
468 if s, ok := sl.tryAcquire(s); ok {
476 // Unfortunately we can't sweep the span ourselves. Somebody else
477 // got to it first. We don't have efficient means to wait, but that's
478 // OK, it will be swept fairly soon.
480 spangen := atomic.Load(&s.sweepgen)
481 if spangen == sl.sweepGen || spangen == sl.sweepGen+3 {
488 // Sweep frees or collects finalizers for blocks not marked in the mark phase.
489 // It clears the mark bits in preparation for the next GC round.
490 // Returns true if the span was returned to heap.
491 // If preserve=true, don't return it to heap nor relink in mcentral lists;
492 // caller takes care of it.
493 func (sl *sweepLocked) sweep(preserve bool) bool {
494 // It's critical that we enter this function with preemption disabled,
495 // GC must not start while we are in the middle of this function.
497 if gp.m.locks == 0 && gp.m.mallocing == 0 && gp != gp.m.g0 {
498 throw("mspan.sweep: m is not locked")
503 // We'll release ownership of this span. Nil it out to
504 // prevent the caller from accidentally using it.
508 sweepgen := mheap_.sweepgen
509 if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
510 print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
511 throw("mspan.sweep: bad span state")
515 traceGCSweepSpan(s.npages * _PageSize)
518 mheap_.pagesSwept.Add(int64(s.npages))
523 // The allocBits indicate which unmarked objects don't need to be
524 // processed since they were free at the end of the last GC cycle
525 // and were not allocated since then.
526 // If the allocBits index is >= s.freeindex and the bit
527 // is not marked then the object remains unallocated
528 // since the last GC.
529 // This situation is analogous to being on a freelist.
531 // Unlink & free special records for any objects we're about to free.
532 // Two complications here:
533 // 1. An object can have both finalizer and profile special records.
534 // In such case we need to queue finalizer for execution,
535 // mark the object as live and preserve the profile special.
536 // 2. A tiny object can have several finalizers setup for different offsets.
537 // If such object is not marked, we need to queue all finalizers at once.
538 // Both 1 and 2 are possible at the same time.
539 hadSpecials := s.specials != nil
540 siter := newSpecialsIter(s)
542 // A finalizer can be set for an inner byte of an object, find object beginning.
543 objIndex := uintptr(siter.s.offset) / size
544 p := s.base() + objIndex*size
545 mbits := s.markBitsForIndex(objIndex)
546 if !mbits.isMarked() {
547 // This object is not marked and has at least one special record.
548 // Pass 1: see if it has at least one finalizer.
550 endOffset := p - s.base() + size
551 for tmp := siter.s; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
552 if tmp.kind == _KindSpecialFinalizer {
553 // Stop freeing of object if it has a finalizer.
554 mbits.setMarkedNonAtomic()
559 // Pass 2: queue all finalizers _or_ handle profile record.
560 for siter.valid() && uintptr(siter.s.offset) < endOffset {
561 // Find the exact byte for which the special was setup
562 // (as opposed to object beginning).
564 p := s.base() + uintptr(special.offset)
565 if special.kind == _KindSpecialFinalizer || !hasFin {
566 siter.unlinkAndNext()
567 freeSpecial(special, unsafe.Pointer(p), size)
569 // The object has finalizers, so we're keeping it alive.
570 // All other specials only apply when an object is freed,
571 // so just keep the special record.
576 // object is still live
577 if siter.s.kind == _KindSpecialReachable {
578 special := siter.unlinkAndNext()
579 (*specialReachable)(unsafe.Pointer(special)).reachable = true
580 freeSpecial(special, unsafe.Pointer(p), size)
582 // keep special record
587 if hadSpecials && s.specials == nil {
591 if debug.allocfreetrace != 0 || debug.clobberfree != 0 || raceenabled || msanenabled || asanenabled {
592 // Find all newly freed objects. This doesn't have to
593 // efficient; allocfreetrace has massive overhead.
594 mbits := s.markBitsForBase()
595 abits := s.allocBitsForIndex(0)
596 for i := uintptr(0); i < s.nelems; i++ {
597 if !mbits.isMarked() && (abits.index < s.freeindex || abits.isMarked()) {
598 x := s.base() + i*s.elemsize
599 if debug.allocfreetrace != 0 {
600 tracefree(unsafe.Pointer(x), size)
602 if debug.clobberfree != 0 {
603 clobberfree(unsafe.Pointer(x), size)
605 // User arenas are handled on explicit free.
606 if raceenabled && !s.isUserArenaChunk {
607 racefree(unsafe.Pointer(x), size)
609 if msanenabled && !s.isUserArenaChunk {
610 msanfree(unsafe.Pointer(x), size)
612 if asanenabled && !s.isUserArenaChunk {
613 asanpoison(unsafe.Pointer(x), size)
621 // Check for zombie objects.
622 if s.freeindex < s.nelems {
623 // Everything < freeindex is allocated and hence
624 // cannot be zombies.
626 // Check the first bitmap byte, where we have to be
627 // careful with freeindex.
629 if (*s.gcmarkBits.bytep(obj / 8)&^*s.allocBits.bytep(obj / 8))>>(obj%8) != 0 {
632 // Check remaining bytes.
633 for i := obj/8 + 1; i < divRoundUp(s.nelems, 8); i++ {
634 if *s.gcmarkBits.bytep(i)&^*s.allocBits.bytep(i) != 0 {
640 // Count the number of free objects in this span.
641 nalloc := uint16(s.countAlloc())
642 nfreed := s.allocCount - nalloc
643 if nalloc > s.allocCount {
644 // The zombie check above should have caught this in
646 print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
647 throw("sweep increased allocation count")
650 s.allocCount = nalloc
651 s.freeindex = 0 // reset allocation index to start of span.
653 getg().m.p.ptr().traceReclaimed += uintptr(nfreed) * s.elemsize
656 // gcmarkBits becomes the allocBits.
657 // get a fresh cleared gcmarkBits in preparation for next GC
658 s.allocBits = s.gcmarkBits
659 s.gcmarkBits = newMarkBits(s.nelems)
661 // Initialize alloc bits cache.
662 s.refillAllocCache(0)
664 // The span must be in our exclusive ownership until we update sweepgen,
665 // check for potential races.
666 if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
667 print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
668 throw("mspan.sweep: bad span state after sweep")
670 if s.sweepgen == sweepgen+1 || s.sweepgen == sweepgen+3 {
671 throw("swept cached span")
674 // We need to set s.sweepgen = h.sweepgen only when all blocks are swept,
675 // because of the potential for a concurrent free/SetFinalizer.
677 // But we need to set it before we make the span available for allocation
678 // (return it to heap or mcentral), because allocation code assumes that a
679 // span is already swept if available for allocation.
681 // Serialization point.
682 // At this point the mark bits are cleared and allocation ready
683 // to go so release the span.
684 atomic.Store(&s.sweepgen, sweepgen)
686 if s.isUserArenaChunk {
688 // This is a case that should never be handled by a sweeper that
689 // preserves the span for reuse.
690 throw("sweep: tried to preserve a user arena span")
693 // There still exist pointers into the span or the span hasn't been
694 // freed yet. It's not ready to be reused. Put it back on the
695 // full swept list for the next cycle.
696 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
700 // It's only at this point that the sweeper doesn't actually need to look
701 // at this arena anymore, so subtract from pagesInUse now.
702 mheap_.pagesInUse.Add(-s.npages)
703 s.state.set(mSpanDead)
705 // The arena is ready to be recycled. Remove it from the quarantine list
706 // and place it on the ready list. Don't add it back to any sweep lists.
708 // It's the arena code's responsibility to get the chunk on the quarantine
709 // list by the time all references to the chunk are gone.
710 if s.list != &mheap_.userArena.quarantineList {
711 throw("user arena span is on the wrong list")
714 mheap_.userArena.quarantineList.remove(s)
715 mheap_.userArena.readyList.insert(s)
721 if spc.sizeclass() != 0 {
722 // Handle spans for small objects.
724 // Only mark the span as needing zeroing if we've freed any
725 // objects, because a fresh span that had been allocated into,
726 // wasn't totally filled, but then swept, still has all of its
727 // free slots zeroed.
729 stats := memstats.heapStats.acquire()
730 atomic.Xadd64(&stats.smallFreeCount[spc.sizeclass()], int64(nfreed))
731 memstats.heapStats.release()
733 // Count the frees in the inconsistent, internal stats.
734 gcController.totalFree.Add(int64(nfreed) * int64(s.elemsize))
737 // The caller may not have removed this span from whatever
738 // unswept set its on but taken ownership of the span for
739 // sweeping by updating sweepgen. If this span still is in
740 // an unswept set, then the mcentral will pop it off the
741 // set, check its sweepgen, and ignore it.
743 // Free totally free span directly back to the heap.
747 // Return span back to the right mcentral list.
748 if uintptr(nalloc) == s.nelems {
749 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
751 mheap_.central[spc].mcentral.partialSwept(sweepgen).push(s)
754 } else if !preserve {
755 // Handle spans for large objects.
757 // Free large object span to heap.
759 // NOTE(rsc,dvyukov): The original implementation of efence
760 // in CL 22060046 used sysFree instead of sysFault, so that
761 // the operating system would eventually give the memory
762 // back to us again, so that an efence program could run
763 // longer without running out of memory. Unfortunately,
764 // calling sysFree here without any kind of adjustment of the
765 // heap data structures means that when the memory does
766 // come back to us, we have the wrong metadata for it, either in
767 // the mspan structures or in the garbage collection bitmap.
768 // Using sysFault here means that the program will run out of
769 // memory fairly quickly in efence mode, but at least it won't
770 // have mysterious crashes due to confused memory reuse.
771 // It should be possible to switch back to sysFree if we also
772 // implement and then call some kind of mheap.deleteSpan.
773 if debug.efence > 0 {
774 s.limit = 0 // prevent mlookup from finding this span
775 sysFault(unsafe.Pointer(s.base()), size)
780 // Count the free in the consistent, external stats.
781 stats := memstats.heapStats.acquire()
782 atomic.Xadd64(&stats.largeFreeCount, 1)
783 atomic.Xadd64(&stats.largeFree, int64(size))
784 memstats.heapStats.release()
786 // Count the free in the inconsistent, internal stats.
787 gcController.totalFree.Add(int64(size))
792 // Add a large span directly onto the full+swept list.
793 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
798 // reportZombies reports any marked but free objects in s and throws.
800 // This generally means one of the following:
802 // 1. User code converted a pointer to a uintptr and then back
803 // unsafely, and a GC ran while the uintptr was the only reference to
806 // 2. User code (or a compiler bug) constructed a bad pointer that
807 // points to a free slot, often a past-the-end pointer.
809 // 3. The GC two cycles ago missed a pointer and freed a live object,
810 // but it was still live in the last cycle, so this GC cycle found a
811 // pointer to that object and marked it.
812 func (s *mspan) reportZombies() {
814 print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer? try -d=checkptr)\n")
815 mbits := s.markBitsForBase()
816 abits := s.allocBitsForIndex(0)
817 for i := uintptr(0); i < s.nelems; i++ {
818 addr := s.base() + i*s.elemsize
820 alloc := i < s.freeindex || abits.isMarked()
826 if mbits.isMarked() {
831 zombie := mbits.isMarked() && !alloc
841 hexdumpWords(addr, addr+length, nil)
846 throw("found pointer to free object")
849 // deductSweepCredit deducts sweep credit for allocating a span of
850 // size spanBytes. This must be performed *before* the span is
851 // allocated to ensure the system has enough credit. If necessary, it
852 // performs sweeping to prevent going in to debt. If the caller will
853 // also sweep pages (e.g., for a large allocation), it can pass a
854 // non-zero callerSweepPages to leave that many pages unswept.
856 // deductSweepCredit makes a worst-case assumption that all spanBytes
857 // bytes of the ultimately allocated span will be available for object
860 // deductSweepCredit is the core of the "proportional sweep" system.
861 // It uses statistics gathered by the garbage collector to perform
862 // enough sweeping so that all pages are swept during the concurrent
863 // sweep phase between GC cycles.
865 // mheap_ must NOT be locked.
866 func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
867 if mheap_.sweepPagesPerByte == 0 {
868 // Proportional sweep is done or disabled.
877 sweptBasis := mheap_.pagesSweptBasis.Load()
879 // Fix debt if necessary.
880 newHeapLive := uintptr(gcController.heapLive.Load()-mheap_.sweepHeapLiveBasis) + spanBytes
881 pagesTarget := int64(mheap_.sweepPagesPerByte*float64(newHeapLive)) - int64(callerSweepPages)
882 for pagesTarget > int64(mheap_.pagesSwept.Load()-sweptBasis) {
883 if sweepone() == ^uintptr(0) {
884 mheap_.sweepPagesPerByte = 0
887 if mheap_.pagesSweptBasis.Load() != sweptBasis {
888 // Sweep pacing changed. Recompute debt.
898 // clobberfree sets the memory content at x to bad content, for debugging
900 func clobberfree(x unsafe.Pointer, size uintptr) {
901 // size (span.elemsize) is always a multiple of 4.
902 for i := uintptr(0); i < size; i += 4 {
903 *(*uint32)(add(x, i)) = 0xdeadbeef
907 // gcPaceSweeper updates the sweeper's pacing parameters.
909 // Must be called whenever the GC's pacing is updated.
911 // The world must be stopped, or mheap_.lock must be held.
912 func gcPaceSweeper(trigger uint64) {
913 assertWorldStoppedOrLockHeld(&mheap_.lock)
915 // Update sweep pacing.
917 mheap_.sweepPagesPerByte = 0
919 // Concurrent sweep needs to sweep all of the in-use
920 // pages by the time the allocated heap reaches the GC
921 // trigger. Compute the ratio of in-use pages to sweep
922 // per byte allocated, accounting for the fact that
923 // some might already be swept.
924 heapLiveBasis := gcController.heapLive.Load()
925 heapDistance := int64(trigger) - int64(heapLiveBasis)
926 // Add a little margin so rounding errors and
927 // concurrent sweep are less likely to leave pages
928 // unswept when GC starts.
929 heapDistance -= 1024 * 1024
930 if heapDistance < _PageSize {
931 // Avoid setting the sweep ratio extremely high
932 heapDistance = _PageSize
934 pagesSwept := mheap_.pagesSwept.Load()
935 pagesInUse := mheap_.pagesInUse.Load()
936 sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
937 if sweepDistancePages <= 0 {
938 mheap_.sweepPagesPerByte = 0
940 mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
941 mheap_.sweepHeapLiveBasis = heapLiveBasis
942 // Write pagesSweptBasis last, since this
943 // signals concurrent sweeps to recompute
945 mheap_.pagesSweptBasis.Store(pagesSwept)