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 {
40 // active tracks outstanding sweepers and the sweep
41 // termination condition.
44 // centralIndex is the current unswept span class.
45 // It represents an index into the mcentral span
46 // sets. Accessed and updated via its load and
47 // update methods. Not protected by a lock.
49 // Reset at mark termination.
50 // Used by mheap.nextSpanForSweep.
51 centralIndex sweepClass
54 // sweepClass is a spanClass and one bit to represent whether we're currently
55 // sweeping partial or full spans.
56 type sweepClass uint32
59 numSweepClasses = numSpanClasses * 2
60 sweepClassDone sweepClass = sweepClass(^uint32(0))
63 func (s *sweepClass) load() sweepClass {
64 return sweepClass(atomic.Load((*uint32)(s)))
67 func (s *sweepClass) update(sNew sweepClass) {
68 // Only update *s if its current value is less than sNew,
69 // since *s increases monotonically.
71 for sOld < sNew && !atomic.Cas((*uint32)(s), uint32(sOld), uint32(sNew)) {
74 // TODO(mknyszek): This isn't the only place we have
75 // an atomic monotonically increasing counter. It would
76 // be nice to have an "atomic max" which is just implemented
77 // as the above on most architectures. Some architectures
78 // like RISC-V however have native support for an atomic max.
81 func (s *sweepClass) clear() {
82 atomic.Store((*uint32)(s), 0)
85 // split returns the underlying span class as well as
86 // whether we're interested in the full or partial
87 // unswept lists for that class, indicated as a boolean
88 // (true means "full").
89 func (s sweepClass) split() (spc spanClass, full bool) {
90 return spanClass(s >> 1), s&1 == 0
93 // nextSpanForSweep finds and pops the next span for sweeping from the
94 // central sweep buffers. It returns ownership of the span to the caller.
95 // Returns nil if no such span exists.
96 func (h *mheap) nextSpanForSweep() *mspan {
98 for sc := sweep.centralIndex.load(); sc < numSweepClasses; sc++ {
99 spc, full := sc.split()
100 c := &h.central[spc].mcentral
103 s = c.fullUnswept(sg).pop()
105 s = c.partialUnswept(sg).pop()
108 // Write down that we found something so future sweepers
109 // can start from here.
110 sweep.centralIndex.update(sc)
114 // Write down that we found nothing.
115 sweep.centralIndex.update(sweepClassDone)
119 const sweepDrainedMask = 1 << 31
121 // activeSweep is a type that captures whether sweeping
122 // is done, and whether there are any outstanding sweepers.
124 // Every potential sweeper must call begin() before they look
125 // for work, and end() after they've finished sweeping.
126 type activeSweep struct {
127 // state is divided into two parts.
129 // The top bit (masked by sweepDrainedMask) is a boolean
130 // value indicating whether all the sweep work has been
131 // drained from the queue.
133 // The rest of the bits are a counter, indicating the
134 // number of outstanding concurrent sweepers.
138 // begin registers a new sweeper. Returns a sweepLocker
139 // for acquiring spans for sweeping. Any outstanding sweeper blocks
140 // sweep termination.
142 // If the sweepLocker is invalid, the caller can be sure that all
143 // outstanding sweep work has been drained, so there is nothing left
144 // to sweep. Note that there may be sweepers currently running, so
145 // this does not indicate that all sweeping has completed.
147 // Even if the sweepLocker is invalid, its sweepGen is always valid.
148 func (a *activeSweep) begin() sweepLocker {
150 state := a.state.Load()
151 if state&sweepDrainedMask != 0 {
152 return sweepLocker{mheap_.sweepgen, false}
154 if a.state.CompareAndSwap(state, state+1) {
155 return sweepLocker{mheap_.sweepgen, true}
160 // end deregisters a sweeper. Must be called once for each time
161 // begin is called if the sweepLocker is valid.
162 func (a *activeSweep) end(sl sweepLocker) {
163 if sl.sweepGen != mheap_.sweepgen {
164 throw("sweeper left outstanding across sweep generations")
167 state := a.state.Load()
168 if (state&^sweepDrainedMask)-1 >= sweepDrainedMask {
169 throw("mismatched begin/end of activeSweep")
171 if a.state.CompareAndSwap(state, state-1) {
172 if state != sweepDrainedMask {
175 if debug.gcpacertrace > 0 {
176 live := gcController.heapLive.Load()
177 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")
184 // markDrained marks the active sweep cycle as having drained
185 // all remaining work. This is safe to be called concurrently
186 // with all other methods of activeSweep, though may race.
188 // Returns true if this call was the one that actually performed
190 func (a *activeSweep) markDrained() bool {
192 state := a.state.Load()
193 if state&sweepDrainedMask != 0 {
196 if a.state.CompareAndSwap(state, state|sweepDrainedMask) {
202 // sweepers returns the current number of active sweepers.
203 func (a *activeSweep) sweepers() uint32 {
204 return a.state.Load() &^ sweepDrainedMask
207 // isDone returns true if all sweep work has been drained and no more
208 // outstanding sweepers exist. That is, when the sweep phase is
210 func (a *activeSweep) isDone() bool {
211 return a.state.Load() == sweepDrainedMask
214 // reset sets up the activeSweep for the next sweep cycle.
216 // The world must be stopped.
217 func (a *activeSweep) reset() {
222 // finishsweep_m ensures that all spans are swept.
224 // The world must be stopped. This ensures there are no sweeps in
228 func finishsweep_m() {
231 // Sweeping must be complete before marking commences, so
232 // sweep any unswept spans. If this is a concurrent GC, there
233 // shouldn't be any spans left to sweep, so this should finish
234 // instantly. If GC was forced before the concurrent sweep
235 // finished, there may be spans to sweep.
236 for sweepone() != ^uintptr(0) {
239 // Make sure there aren't any outstanding sweepers left.
240 // At this point, with the world stopped, it means one of two
241 // things. Either we were able to preempt a sweeper, or that
242 // a sweeper didn't call sweep.active.end when it should have.
243 // Both cases indicate a bug, so throw.
244 if sweep.active.sweepers() != 0 {
245 throw("active sweepers found at start of mark phase")
248 // Reset all the unswept buffers, which should be empty.
249 // Do this in sweep termination as opposed to mark termination
250 // so that we can catch unswept spans and reclaim blocks as
252 sg := mheap_.sweepgen
253 for i := range mheap_.central {
254 c := &mheap_.central[i].mcentral
255 c.partialUnswept(sg).reset()
256 c.fullUnswept(sg).reset()
259 // Sweeping is done, so there won't be any new memory to
260 // scavenge for a bit.
262 // If the scavenger isn't already awake, wake it up. There's
263 // definitely work for it to do at this point.
266 nextMarkBitArenaEpoch()
269 func bgsweep(c chan int) {
272 lockInit(&sweep.lock, lockRankSweep)
276 goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceBlockGCSweep, 1)
279 // bgsweep attempts to be a "low priority" goroutine by intentionally
280 // yielding time. It's OK if it doesn't run, because goroutines allocating
281 // memory will sweep and ensure that all spans are swept before the next
282 // GC cycle. We really only want to run when we're idle.
284 // However, calling Gosched after each span swept produces a tremendous
285 // amount of tracing events, sometimes up to 50% of events in a trace. It's
286 // also inefficient to call into the scheduler so much because sweeping a
287 // single span is in general a very fast operation, taking as little as 30 ns
288 // on modern hardware. (See #54767.)
290 // As a result, bgsweep sweeps in batches, and only calls into the scheduler
291 // at the end of every batch. Furthermore, it only yields its time if there
292 // isn't spare idle time available on other cores. If there's available idle
293 // time, helping to sweep can reduce allocation latencies by getting ahead of
294 // the proportional sweeper and having spans ready to go for allocation.
295 const sweepBatchSize = 10
297 for sweepone() != ^uintptr(0) {
299 if nSwept%sweepBatchSize == 0 {
303 for freeSomeWbufs(true) {
304 // N.B. freeSomeWbufs is already batched internally.
309 // This can happen if a GC runs between
310 // gosweepone returning ^0 above
311 // and the lock being acquired.
316 goparkunlock(&sweep.lock, waitReasonGCSweepWait, traceBlockGCSweep, 1)
320 // sweepLocker acquires sweep ownership of spans.
321 type sweepLocker struct {
322 // sweepGen is the sweep generation of the heap.
327 // sweepLocked represents sweep ownership of a span.
328 type sweepLocked struct {
332 // tryAcquire attempts to acquire sweep ownership of span s. If it
333 // successfully acquires ownership, it blocks sweep completion.
334 func (l *sweepLocker) tryAcquire(s *mspan) (sweepLocked, bool) {
336 throw("use of invalid sweepLocker")
338 // Check before attempting to CAS.
339 if atomic.Load(&s.sweepgen) != l.sweepGen-2 {
340 return sweepLocked{}, false
342 // Attempt to acquire sweep ownership of s.
343 if !atomic.Cas(&s.sweepgen, l.sweepGen-2, l.sweepGen-1) {
344 return sweepLocked{}, false
346 return sweepLocked{s}, true
349 // sweepone sweeps some unswept heap span and returns the number of pages returned
350 // to the heap, or ^uintptr(0) if there was nothing to sweep.
351 func sweepone() uintptr {
354 // Increment locks to ensure that the goroutine is not preempted
355 // in the middle of sweep thus leaving the span in an inconsistent state for next GC
358 // TODO(austin): sweepone is almost always called in a loop;
359 // lift the sweepLocker into its callers.
360 sl := sweep.active.begin()
366 // Find a span to sweep.
367 npages := ^uintptr(0)
370 s := mheap_.nextSpanForSweep()
372 noMoreWork = sweep.active.markDrained()
375 if state := s.state.get(); state != mSpanInUse {
376 // This can happen if direct sweeping already
377 // swept this span, but in that case the sweep
378 // generation should always be up-to-date.
379 if !(s.sweepgen == sl.sweepGen || s.sweepgen == sl.sweepGen+3) {
380 print("runtime: bad span s.state=", state, " s.sweepgen=", s.sweepgen, " sweepgen=", sl.sweepGen, "\n")
381 throw("non in-use span in unswept list")
385 if s, ok := sl.tryAcquire(s); ok {
386 // Sweep the span we found.
389 // Whole span was freed. Count it toward the
390 // page reclaimer credit since these pages can
391 // now be used for span allocation.
392 mheap_.reclaimCredit.Add(npages)
394 // Span is still in-use, so this returned no
395 // pages to the heap and the span needs to
396 // move to the swept in-use list.
405 // The sweep list is empty. There may still be
406 // concurrent sweeps running, but we're at least very
407 // close to done sweeping.
409 // Move the scavenge gen forward (signaling
410 // that there's new work to do) and wake the scavenger.
412 // The scavenger is signaled by the last sweeper because once
413 // sweeping is done, we will definitely have useful work for
414 // the scavenger to do, since the scavenger only runs over the
415 // heap once per GC cycle. This update is not done during sweep
416 // termination because in some cases there may be a long delay
417 // between sweep done and sweep termination (e.g. not enough
418 // allocations to trigger a GC) which would be nice to fill in
419 // with scavenging work.
420 if debug.scavtrace > 0 {
424 // Get released stats.
425 releasedBg := mheap_.pages.scav.releasedBg.Load()
426 releasedEager := mheap_.pages.scav.releasedEager.Load()
429 printScavTrace(releasedBg, releasedEager, false)
432 mheap_.pages.scav.releasedBg.Add(-releasedBg)
433 mheap_.pages.scav.releasedEager.Add(-releasedEager)
444 // isSweepDone reports whether all spans are swept.
446 // Note that this condition may transition from false to true at any
447 // time as the sweeper runs. It may transition from true to false if a
448 // GC runs; to prevent that the caller must be non-preemptible or must
449 // somehow block GC progress.
450 func isSweepDone() bool {
451 return sweep.active.isDone()
454 // Returns only when span s has been swept.
457 func (s *mspan) ensureSwept() {
458 // Caller must disable preemption.
459 // Otherwise when this function returns the span can become unswept again
460 // (if GC is triggered on another goroutine).
462 if gp.m.locks == 0 && gp.m.mallocing == 0 && gp != gp.m.g0 {
463 throw("mspan.ensureSwept: m is not locked")
466 // If this operation fails, then that means that there are
467 // no more spans to be swept. In this case, either s has already
468 // been swept, or is about to be acquired for sweeping and swept.
469 sl := sweep.active.begin()
471 // The caller must be sure that the span is a mSpanInUse span.
472 if s, ok := sl.tryAcquire(s); ok {
480 // Unfortunately we can't sweep the span ourselves. Somebody else
481 // got to it first. We don't have efficient means to wait, but that's
482 // OK, it will be swept fairly soon.
484 spangen := atomic.Load(&s.sweepgen)
485 if spangen == sl.sweepGen || spangen == sl.sweepGen+3 {
492 // sweep frees or collects finalizers for blocks not marked in the mark phase.
493 // It clears the mark bits in preparation for the next GC round.
494 // Returns true if the span was returned to heap.
495 // If preserve=true, don't return it to heap nor relink in mcentral lists;
496 // caller takes care of it.
497 func (sl *sweepLocked) sweep(preserve bool) bool {
498 // It's critical that we enter this function with preemption disabled,
499 // GC must not start while we are in the middle of this function.
501 if gp.m.locks == 0 && gp.m.mallocing == 0 && gp != gp.m.g0 {
502 throw("mspan.sweep: m is not locked")
507 // We'll release ownership of this span. Nil it out to
508 // prevent the caller from accidentally using it.
512 sweepgen := mheap_.sweepgen
513 if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
514 print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
515 throw("mspan.sweep: bad span state")
519 traceGCSweepSpan(s.npages * _PageSize)
522 mheap_.pagesSwept.Add(int64(s.npages))
527 // The allocBits indicate which unmarked objects don't need to be
528 // processed since they were free at the end of the last GC cycle
529 // and were not allocated since then.
530 // If the allocBits index is >= s.freeindex and the bit
531 // is not marked then the object remains unallocated
532 // since the last GC.
533 // This situation is analogous to being on a freelist.
535 // Unlink & free special records for any objects we're about to free.
536 // Two complications here:
537 // 1. An object can have both finalizer and profile special records.
538 // In such case we need to queue finalizer for execution,
539 // mark the object as live and preserve the profile special.
540 // 2. A tiny object can have several finalizers setup for different offsets.
541 // If such object is not marked, we need to queue all finalizers at once.
542 // Both 1 and 2 are possible at the same time.
543 hadSpecials := s.specials != nil
544 siter := newSpecialsIter(s)
546 // A finalizer can be set for an inner byte of an object, find object beginning.
547 objIndex := uintptr(siter.s.offset) / size
548 p := s.base() + objIndex*size
549 mbits := s.markBitsForIndex(objIndex)
550 if !mbits.isMarked() {
551 // This object is not marked and has at least one special record.
552 // Pass 1: see if it has at least one finalizer.
554 endOffset := p - s.base() + size
555 for tmp := siter.s; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
556 if tmp.kind == _KindSpecialFinalizer {
557 // Stop freeing of object if it has a finalizer.
558 mbits.setMarkedNonAtomic()
563 // Pass 2: queue all finalizers _or_ handle profile record.
564 for siter.valid() && uintptr(siter.s.offset) < endOffset {
565 // Find the exact byte for which the special was setup
566 // (as opposed to object beginning).
568 p := s.base() + uintptr(special.offset)
569 if special.kind == _KindSpecialFinalizer || !hasFin {
570 siter.unlinkAndNext()
571 freeSpecial(special, unsafe.Pointer(p), size)
573 // The object has finalizers, so we're keeping it alive.
574 // All other specials only apply when an object is freed,
575 // so just keep the special record.
580 // object is still live
581 if siter.s.kind == _KindSpecialReachable {
582 special := siter.unlinkAndNext()
583 (*specialReachable)(unsafe.Pointer(special)).reachable = true
584 freeSpecial(special, unsafe.Pointer(p), size)
586 // keep special record
591 if hadSpecials && s.specials == nil {
595 if debug.allocfreetrace != 0 || debug.clobberfree != 0 || raceenabled || msanenabled || asanenabled {
596 // Find all newly freed objects. This doesn't have to
597 // efficient; allocfreetrace has massive overhead.
598 mbits := s.markBitsForBase()
599 abits := s.allocBitsForIndex(0)
600 for i := uintptr(0); i < uintptr(s.nelems); i++ {
601 if !mbits.isMarked() && (abits.index < uintptr(s.freeindex) || abits.isMarked()) {
602 x := s.base() + i*s.elemsize
603 if debug.allocfreetrace != 0 {
604 tracefree(unsafe.Pointer(x), size)
606 if debug.clobberfree != 0 {
607 clobberfree(unsafe.Pointer(x), size)
609 // User arenas are handled on explicit free.
610 if raceenabled && !s.isUserArenaChunk {
611 racefree(unsafe.Pointer(x), size)
613 if msanenabled && !s.isUserArenaChunk {
614 msanfree(unsafe.Pointer(x), size)
616 if asanenabled && !s.isUserArenaChunk {
617 asanpoison(unsafe.Pointer(x), size)
625 // Check for zombie objects.
626 if s.freeindex < s.nelems {
627 // Everything < freeindex is allocated and hence
628 // cannot be zombies.
630 // Check the first bitmap byte, where we have to be
631 // careful with freeindex.
632 obj := uintptr(s.freeindex)
633 if (*s.gcmarkBits.bytep(obj / 8)&^*s.allocBits.bytep(obj / 8))>>(obj%8) != 0 {
636 // Check remaining bytes.
637 for i := obj/8 + 1; i < divRoundUp(uintptr(s.nelems), 8); i++ {
638 if *s.gcmarkBits.bytep(i)&^*s.allocBits.bytep(i) != 0 {
644 // Count the number of free objects in this span.
645 nalloc := uint16(s.countAlloc())
646 nfreed := s.allocCount - nalloc
647 if nalloc > s.allocCount {
648 // The zombie check above should have caught this in
650 print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
651 throw("sweep increased allocation count")
654 s.allocCount = nalloc
655 s.freeindex = 0 // reset allocation index to start of span.
656 s.freeIndexForScan = 0
658 getg().m.p.ptr().trace.reclaimed += uintptr(nfreed) * s.elemsize
661 // gcmarkBits becomes the allocBits.
662 // get a fresh cleared gcmarkBits in preparation for next GC
663 s.allocBits = s.gcmarkBits
664 s.gcmarkBits = newMarkBits(uintptr(s.nelems))
666 // refresh pinnerBits if they exists
667 if s.pinnerBits != nil {
668 s.refreshPinnerBits()
671 // Initialize alloc bits cache.
672 s.refillAllocCache(0)
674 // The span must be in our exclusive ownership until we update sweepgen,
675 // check for potential races.
676 if state := s.state.get(); state != mSpanInUse || s.sweepgen != sweepgen-1 {
677 print("mspan.sweep: state=", state, " sweepgen=", s.sweepgen, " mheap.sweepgen=", sweepgen, "\n")
678 throw("mspan.sweep: bad span state after sweep")
680 if s.sweepgen == sweepgen+1 || s.sweepgen == sweepgen+3 {
681 throw("swept cached span")
684 // We need to set s.sweepgen = h.sweepgen only when all blocks are swept,
685 // because of the potential for a concurrent free/SetFinalizer.
687 // But we need to set it before we make the span available for allocation
688 // (return it to heap or mcentral), because allocation code assumes that a
689 // span is already swept if available for allocation.
691 // Serialization point.
692 // At this point the mark bits are cleared and allocation ready
693 // to go so release the span.
694 atomic.Store(&s.sweepgen, sweepgen)
696 if s.isUserArenaChunk {
698 // This is a case that should never be handled by a sweeper that
699 // preserves the span for reuse.
700 throw("sweep: tried to preserve a user arena span")
703 // There still exist pointers into the span or the span hasn't been
704 // freed yet. It's not ready to be reused. Put it back on the
705 // full swept list for the next cycle.
706 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
710 // It's only at this point that the sweeper doesn't actually need to look
711 // at this arena anymore, so subtract from pagesInUse now.
712 mheap_.pagesInUse.Add(-s.npages)
713 s.state.set(mSpanDead)
715 // The arena is ready to be recycled. Remove it from the quarantine list
716 // and place it on the ready list. Don't add it back to any sweep lists.
718 // It's the arena code's responsibility to get the chunk on the quarantine
719 // list by the time all references to the chunk are gone.
720 if s.list != &mheap_.userArena.quarantineList {
721 throw("user arena span is on the wrong list")
724 mheap_.userArena.quarantineList.remove(s)
725 mheap_.userArena.readyList.insert(s)
731 if spc.sizeclass() != 0 {
732 // Handle spans for small objects.
734 // Only mark the span as needing zeroing if we've freed any
735 // objects, because a fresh span that had been allocated into,
736 // wasn't totally filled, but then swept, still has all of its
737 // free slots zeroed.
739 stats := memstats.heapStats.acquire()
740 atomic.Xadd64(&stats.smallFreeCount[spc.sizeclass()], int64(nfreed))
741 memstats.heapStats.release()
743 // Count the frees in the inconsistent, internal stats.
744 gcController.totalFree.Add(int64(nfreed) * int64(s.elemsize))
747 // The caller may not have removed this span from whatever
748 // unswept set its on but taken ownership of the span for
749 // sweeping by updating sweepgen. If this span still is in
750 // an unswept set, then the mcentral will pop it off the
751 // set, check its sweepgen, and ignore it.
753 // Free totally free span directly back to the heap.
757 // Return span back to the right mcentral list.
758 if nalloc == s.nelems {
759 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
761 mheap_.central[spc].mcentral.partialSwept(sweepgen).push(s)
764 } else if !preserve {
765 // Handle spans for large objects.
767 // Free large object span to heap.
769 // NOTE(rsc,dvyukov): The original implementation of efence
770 // in CL 22060046 used sysFree instead of sysFault, so that
771 // the operating system would eventually give the memory
772 // back to us again, so that an efence program could run
773 // longer without running out of memory. Unfortunately,
774 // calling sysFree here without any kind of adjustment of the
775 // heap data structures means that when the memory does
776 // come back to us, we have the wrong metadata for it, either in
777 // the mspan structures or in the garbage collection bitmap.
778 // Using sysFault here means that the program will run out of
779 // memory fairly quickly in efence mode, but at least it won't
780 // have mysterious crashes due to confused memory reuse.
781 // It should be possible to switch back to sysFree if we also
782 // implement and then call some kind of mheap.deleteSpan.
783 if debug.efence > 0 {
784 s.limit = 0 // prevent mlookup from finding this span
785 sysFault(unsafe.Pointer(s.base()), size)
790 // Count the free in the consistent, external stats.
791 stats := memstats.heapStats.acquire()
792 atomic.Xadd64(&stats.largeFreeCount, 1)
793 atomic.Xadd64(&stats.largeFree, int64(size))
794 memstats.heapStats.release()
796 // Count the free in the inconsistent, internal stats.
797 gcController.totalFree.Add(int64(size))
802 // Add a large span directly onto the full+swept list.
803 mheap_.central[spc].mcentral.fullSwept(sweepgen).push(s)
808 // reportZombies reports any marked but free objects in s and throws.
810 // This generally means one of the following:
812 // 1. User code converted a pointer to a uintptr and then back
813 // unsafely, and a GC ran while the uintptr was the only reference to
816 // 2. User code (or a compiler bug) constructed a bad pointer that
817 // points to a free slot, often a past-the-end pointer.
819 // 3. The GC two cycles ago missed a pointer and freed a live object,
820 // but it was still live in the last cycle, so this GC cycle found a
821 // pointer to that object and marked it.
822 func (s *mspan) reportZombies() {
824 print("runtime: marked free object in span ", s, ", elemsize=", s.elemsize, " freeindex=", s.freeindex, " (bad use of unsafe.Pointer? try -d=checkptr)\n")
825 mbits := s.markBitsForBase()
826 abits := s.allocBitsForIndex(0)
827 for i := uintptr(0); i < uintptr(s.nelems); i++ {
828 addr := s.base() + i*s.elemsize
830 alloc := i < uintptr(s.freeindex) || abits.isMarked()
836 if mbits.isMarked() {
841 zombie := mbits.isMarked() && !alloc
851 hexdumpWords(addr, addr+length, nil)
856 throw("found pointer to free object")
859 // deductSweepCredit deducts sweep credit for allocating a span of
860 // size spanBytes. This must be performed *before* the span is
861 // allocated to ensure the system has enough credit. If necessary, it
862 // performs sweeping to prevent going in to debt. If the caller will
863 // also sweep pages (e.g., for a large allocation), it can pass a
864 // non-zero callerSweepPages to leave that many pages unswept.
866 // deductSweepCredit makes a worst-case assumption that all spanBytes
867 // bytes of the ultimately allocated span will be available for object
870 // deductSweepCredit is the core of the "proportional sweep" system.
871 // It uses statistics gathered by the garbage collector to perform
872 // enough sweeping so that all pages are swept during the concurrent
873 // sweep phase between GC cycles.
875 // mheap_ must NOT be locked.
876 func deductSweepCredit(spanBytes uintptr, callerSweepPages uintptr) {
877 if mheap_.sweepPagesPerByte == 0 {
878 // Proportional sweep is done or disabled.
886 // Fix debt if necessary.
888 sweptBasis := mheap_.pagesSweptBasis.Load()
889 live := gcController.heapLive.Load()
890 liveBasis := mheap_.sweepHeapLiveBasis
891 newHeapLive := spanBytes
892 if liveBasis < live {
893 // Only do this subtraction when we don't overflow. Otherwise, pagesTarget
894 // might be computed as something really huge, causing us to get stuck
895 // sweeping here until the next mark phase.
897 // Overflow can happen here if gcPaceSweeper is called concurrently with
898 // sweeping (i.e. not during a STW, like it usually is) because this code
899 // is intentionally racy. A concurrent call to gcPaceSweeper can happen
900 // if a GC tuning parameter is modified and we read an older value of
901 // heapLive than what was used to set the basis.
903 // This state should be transient, so it's fine to just let newHeapLive
904 // be a relatively small number. We'll probably just skip this attempt to
908 newHeapLive += uintptr(live - liveBasis)
910 pagesTarget := int64(mheap_.sweepPagesPerByte*float64(newHeapLive)) - int64(callerSweepPages)
911 for pagesTarget > int64(mheap_.pagesSwept.Load()-sweptBasis) {
912 if sweepone() == ^uintptr(0) {
913 mheap_.sweepPagesPerByte = 0
916 if mheap_.pagesSweptBasis.Load() != sweptBasis {
917 // Sweep pacing changed. Recompute debt.
927 // clobberfree sets the memory content at x to bad content, for debugging
929 func clobberfree(x unsafe.Pointer, size uintptr) {
930 // size (span.elemsize) is always a multiple of 4.
931 for i := uintptr(0); i < size; i += 4 {
932 *(*uint32)(add(x, i)) = 0xdeadbeef
936 // gcPaceSweeper updates the sweeper's pacing parameters.
938 // Must be called whenever the GC's pacing is updated.
940 // The world must be stopped, or mheap_.lock must be held.
941 func gcPaceSweeper(trigger uint64) {
942 assertWorldStoppedOrLockHeld(&mheap_.lock)
944 // Update sweep pacing.
946 mheap_.sweepPagesPerByte = 0
948 // Concurrent sweep needs to sweep all of the in-use
949 // pages by the time the allocated heap reaches the GC
950 // trigger. Compute the ratio of in-use pages to sweep
951 // per byte allocated, accounting for the fact that
952 // some might already be swept.
953 heapLiveBasis := gcController.heapLive.Load()
954 heapDistance := int64(trigger) - int64(heapLiveBasis)
955 // Add a little margin so rounding errors and
956 // concurrent sweep are less likely to leave pages
957 // unswept when GC starts.
958 heapDistance -= 1024 * 1024
959 if heapDistance < _PageSize {
960 // Avoid setting the sweep ratio extremely high
961 heapDistance = _PageSize
963 pagesSwept := mheap_.pagesSwept.Load()
964 pagesInUse := mheap_.pagesInUse.Load()
965 sweepDistancePages := int64(pagesInUse) - int64(pagesSwept)
966 if sweepDistancePages <= 0 {
967 mheap_.sweepPagesPerByte = 0
969 mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance)
970 mheap_.sweepHeapLiveBasis = heapLiveBasis
971 // Write pagesSweptBasis last, since this
972 // signals concurrent sweeps to recompute
974 mheap_.pagesSweptBasis.Store(pagesSwept)