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 (GC).
7 // The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple
8 // GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is
9 // non-generational and non-compacting. Allocation is done using size segregated per P allocation
10 // areas to minimize fragmentation while eliminating locks in the common case.
12 // The algorithm decomposes into several steps.
13 // This is a high level description of the algorithm being used. For an overview of GC a good
14 // place to start is Richard Jones' gchandbook.org.
16 // The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see
17 // Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978.
18 // On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978),
20 // For journal quality proofs that these steps are complete, correct, and terminate see
21 // Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world.
22 // Concurrency and Computation: Practice and Experience 15(3-5), 2003.
24 // 1. GC performs sweep termination.
26 // a. Stop the world. This causes all Ps to reach a GC safe-point.
28 // b. Sweep any unswept spans. There will only be unswept spans if
29 // this GC cycle was forced before the expected time.
31 // 2. GC performs the mark phase.
33 // a. Prepare for the mark phase by setting gcphase to _GCmark
34 // (from _GCoff), enabling the write barrier, enabling mutator
35 // assists, and enqueueing root mark jobs. No objects may be
36 // scanned until all Ps have enabled the write barrier, which is
37 // accomplished using STW.
39 // b. Start the world. From this point, GC work is done by mark
40 // workers started by the scheduler and by assists performed as
41 // part of allocation. The write barrier shades both the
42 // overwritten pointer and the new pointer value for any pointer
43 // writes (see mbarrier.go for details). Newly allocated objects
44 // are immediately marked black.
46 // c. GC performs root marking jobs. This includes scanning all
47 // stacks, shading all globals, and shading any heap pointers in
48 // off-heap runtime data structures. Scanning a stack stops a
49 // goroutine, shades any pointers found on its stack, and then
50 // resumes the goroutine.
52 // d. GC drains the work queue of grey objects, scanning each grey
53 // object to black and shading all pointers found in the object
54 // (which in turn may add those pointers to the work queue).
56 // e. Because GC work is spread across local caches, GC uses a
57 // distributed termination algorithm to detect when there are no
58 // more root marking jobs or grey objects (see gcMarkDone). At this
59 // point, GC transitions to mark termination.
61 // 3. GC performs mark termination.
65 // b. Set gcphase to _GCmarktermination, and disable workers and
68 // c. Perform housekeeping like flushing mcaches.
70 // 4. GC performs the sweep phase.
72 // a. Prepare for the sweep phase by setting gcphase to _GCoff,
73 // setting up sweep state and disabling the write barrier.
75 // b. Start the world. From this point on, newly allocated objects
76 // are white, and allocating sweeps spans before use if necessary.
78 // c. GC does concurrent sweeping in the background and in response
79 // to allocation. See description below.
81 // 5. When sufficient allocation has taken place, replay the sequence
82 // starting with 1 above. See discussion of GC rate below.
86 // The sweep phase proceeds concurrently with normal program execution.
87 // The heap is swept span-by-span both lazily (when a goroutine needs another span)
88 // and concurrently in a background goroutine (this helps programs that are not CPU bound).
89 // At the end of STW mark termination all spans are marked as "needs sweeping".
91 // The background sweeper goroutine simply sweeps spans one-by-one.
93 // To avoid requesting more OS memory while there are unswept spans, when a
94 // goroutine needs another span, it first attempts to reclaim that much memory
95 // by sweeping. When a goroutine needs to allocate a new small-object span, it
96 // sweeps small-object spans for the same object size until it frees at least
97 // one object. When a goroutine needs to allocate large-object span from heap,
98 // it sweeps spans until it frees at least that many pages into heap. There is
99 // one case where this may not suffice: if a goroutine sweeps and frees two
100 // nonadjacent one-page spans to the heap, it will allocate a new two-page
101 // span, but there can still be other one-page unswept spans which could be
102 // combined into a two-page span.
104 // It's critical to ensure that no operations proceed on unswept spans (that would corrupt
105 // mark bits in GC bitmap). During GC all mcaches are flushed into the central cache,
106 // so they are empty. When a goroutine grabs a new span into mcache, it sweeps it.
107 // When a goroutine explicitly frees an object or sets a finalizer, it ensures that
108 // the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish).
109 // The finalizer goroutine is kicked off only when all spans are swept.
110 // When the next GC starts, it sweeps all not-yet-swept spans (if any).
113 // Next GC is after we've allocated an extra amount of memory proportional to
114 // the amount already in use. The proportion is controlled by GOGC environment variable
115 // (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M
116 // (this mark is computed by the gcController.heapGoal method). This keeps the GC cost in
117 // linear proportion to the allocation cost. Adjusting GOGC just changes the linear constant
118 // (and also the amount of extra memory used).
122 // In order to prevent long pauses while scanning large objects and to
123 // improve parallelism, the garbage collector breaks up scan jobs for
124 // objects larger than maxObletBytes into "oblets" of at most
125 // maxObletBytes. When scanning encounters the beginning of a large
126 // object, it scans only the first oblet and enqueues the remaining
127 // oblets as new scan jobs.
133 "runtime/internal/atomic"
139 _ConcurrentSweep = true
140 _FinBlockSize = 4 * 1024
142 // debugScanConservative enables debug logging for stack
143 // frames that are scanned conservatively.
144 debugScanConservative = false
146 // sweepMinHeapDistance is a lower bound on the heap distance
147 // (in bytes) reserved for concurrent sweeping between GC
149 sweepMinHeapDistance = 1024 * 1024
153 if unsafe.Sizeof(workbuf{}) != _WorkbufSize {
154 throw("size of Workbuf is suboptimal")
156 // No sweep on the first cycle.
157 sweep.active.state.Store(sweepDrainedMask)
159 // Initialize GC pacer state.
160 // Use the environment variable GOGC for the initial gcPercent value.
161 // Use the environment variable GOMEMLIMIT for the initial memoryLimit value.
162 gcController.init(readGOGC(), readGOMEMLIMIT())
165 work.markDoneSema = 1
166 lockInit(&work.sweepWaiters.lock, lockRankSweepWaiters)
167 lockInit(&work.assistQueue.lock, lockRankAssistQueue)
168 lockInit(&work.wbufSpans.lock, lockRankWbufSpans)
171 // gcenable is called after the bulk of the runtime initialization,
172 // just before we're about to start letting user code run.
173 // It kicks off the background sweeper goroutine, the background
174 // scavenger goroutine, and enables GC.
176 // Kick off sweeping and scavenging.
177 c := make(chan int, 2)
182 memstats.enablegc = true // now that runtime is initialized, GC is okay
185 // Garbage collector phase.
186 // Indicates to write barrier and synchronization task to perform.
189 // The compiler knows about this variable.
190 // If you change it, you must change builtin/runtime.go, too.
191 // If you change the first four bytes, you must also change the write
192 // barrier insertion code.
193 var writeBarrier struct {
194 enabled bool // compiler emits a check of this before calling write barrier
195 pad [3]byte // compiler uses 32-bit load for "enabled" field
196 needed bool // whether we need a write barrier for current GC phase
197 cgo bool // whether we need a write barrier for a cgo check
198 alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load
201 // gcBlackenEnabled is 1 if mutator assists and background mark
202 // workers are allowed to blacken objects. This must only be set when
203 // gcphase == _GCmark.
204 var gcBlackenEnabled uint32
207 _GCoff = iota // GC not running; sweeping in background, write barrier disabled
208 _GCmark // GC marking roots and workbufs: allocate black, write barrier ENABLED
209 _GCmarktermination // GC mark termination: allocate black, P's help GC, write barrier ENABLED
213 func setGCPhase(x uint32) {
214 atomic.Store(&gcphase, x)
215 writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination
216 writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo
219 // gcMarkWorkerMode represents the mode that a concurrent mark worker
220 // should operate in.
222 // Concurrent marking happens through four different mechanisms. One
223 // is mutator assists, which happen in response to allocations and are
224 // not scheduled. The other three are variations in the per-P mark
225 // workers and are distinguished by gcMarkWorkerMode.
226 type gcMarkWorkerMode int
229 // gcMarkWorkerNotWorker indicates that the next scheduled G is not
230 // starting work and the mode should be ignored.
231 gcMarkWorkerNotWorker gcMarkWorkerMode = iota
233 // gcMarkWorkerDedicatedMode indicates that the P of a mark
234 // worker is dedicated to running that mark worker. The mark
235 // worker should run without preemption.
236 gcMarkWorkerDedicatedMode
238 // gcMarkWorkerFractionalMode indicates that a P is currently
239 // running the "fractional" mark worker. The fractional worker
240 // is necessary when GOMAXPROCS*gcBackgroundUtilization is not
241 // an integer and using only dedicated workers would result in
242 // utilization too far from the target of gcBackgroundUtilization.
243 // The fractional worker should run until it is preempted and
244 // will be scheduled to pick up the fractional part of
245 // GOMAXPROCS*gcBackgroundUtilization.
246 gcMarkWorkerFractionalMode
248 // gcMarkWorkerIdleMode indicates that a P is running the mark
249 // worker because it has nothing else to do. The idle worker
250 // should run until it is preempted and account its time
251 // against gcController.idleMarkTime.
255 // gcMarkWorkerModeStrings are the strings labels of gcMarkWorkerModes
256 // to use in execution traces.
257 var gcMarkWorkerModeStrings = [...]string{
264 // pollFractionalWorkerExit reports whether a fractional mark worker
265 // should self-preempt. It assumes it is called from the fractional
267 func pollFractionalWorkerExit() bool {
268 // This should be kept in sync with the fractional worker
269 // scheduler logic in findRunnableGCWorker.
271 delta := now - gcController.markStartTime
275 p := getg().m.p.ptr()
276 selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime)
277 // Add some slack to the utilization goal so that the
278 // fractional worker isn't behind again the instant it exits.
279 return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal
284 type workType struct {
285 full lfstack // lock-free list of full blocks workbuf
286 empty lfstack // lock-free list of empty blocks workbuf
287 pad0 cpu.CacheLinePad // prevents false-sharing between full/empty and nproc/nwait
291 // free is a list of spans dedicated to workbufs, but
292 // that don't currently contain any workbufs.
294 // busy is a list of all spans containing workbufs on
295 // one of the workbuf lists.
299 // Restore 64-bit alignment on 32-bit.
302 // bytesMarked is the number of bytes marked this cycle. This
303 // includes bytes blackened in scanned objects, noscan objects
304 // that go straight to black, and permagrey objects scanned by
305 // markroot during the concurrent scan phase. This is updated
306 // atomically during the cycle. Updates may be batched
307 // arbitrarily, since the value is only read at the end of the
310 // Because of benign races during marking, this number may not
311 // be the exact number of marked bytes, but it should be very
314 // Put this field here because it needs 64-bit atomic access
315 // (and thus 8-byte alignment even on 32-bit architectures).
318 markrootNext uint32 // next markroot job
319 markrootJobs uint32 // number of markroot jobs
325 // Number of roots of various root types. Set by gcMarkRootPrepare.
327 // nStackRoots == len(stackRoots), but we have nStackRoots for
329 nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int
331 // Base indexes of each root type. Set by gcMarkRootPrepare.
332 baseData, baseBSS, baseSpans, baseStacks, baseEnd uint32
334 // stackRoots is a snapshot of all of the Gs that existed
335 // before the beginning of concurrent marking. The backing
336 // store of this must not be modified because it might be
337 // shared with allgs.
340 // Each type of GC state transition is protected by a lock.
341 // Since multiple threads can simultaneously detect the state
342 // transition condition, any thread that detects a transition
343 // condition must acquire the appropriate transition lock,
344 // re-check the transition condition and return if it no
345 // longer holds or perform the transition if it does.
346 // Likewise, any transition must invalidate the transition
347 // condition before releasing the lock. This ensures that each
348 // transition is performed by exactly one thread and threads
349 // that need the transition to happen block until it has
352 // startSema protects the transition from "off" to mark or
355 // markDoneSema protects transitions from mark to mark termination.
358 bgMarkReady note // signal background mark worker has started
359 bgMarkDone uint32 // cas to 1 when at a background mark completion point
360 // Background mark completion signaling
362 // mode is the concurrency mode of the current GC cycle.
365 // userForced indicates the current GC cycle was forced by an
366 // explicit user call.
369 // initialHeapLive is the value of gcController.heapLive at the
370 // beginning of this GC cycle.
371 initialHeapLive uint64
373 // assistQueue is a queue of assists that are blocked because
374 // there was neither enough credit to steal or enough work to
381 // sweepWaiters is a list of blocked goroutines to wake when
382 // we transition from mark termination to sweep.
383 sweepWaiters struct {
388 // cycles is the number of completed GC cycles, where a GC
389 // cycle is sweep termination, mark, mark termination, and
390 // sweep. This differs from memstats.numgc, which is
391 // incremented at mark termination.
394 // Timing/utilization stats for this cycle.
395 stwprocs, maxprocs int32
396 tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start
398 pauseNS int64 // total STW time this cycle
399 pauseStart int64 // nanotime() of last STW
401 // debug.gctrace heap sizes for this cycle.
402 heap0, heap1, heap2 uint64
404 // Cumulative estimated CPU usage.
408 // GC runs a garbage collection and blocks the caller until the
409 // garbage collection is complete. It may also block the entire
412 // We consider a cycle to be: sweep termination, mark, mark
413 // termination, and sweep. This function shouldn't return
414 // until a full cycle has been completed, from beginning to
415 // end. Hence, we always want to finish up the current cycle
416 // and start a new one. That means:
418 // 1. In sweep termination, mark, or mark termination of cycle
419 // N, wait until mark termination N completes and transitions
422 // 2. In sweep N, help with sweep N.
424 // At this point we can begin a full cycle N+1.
426 // 3. Trigger cycle N+1 by starting sweep termination N+1.
428 // 4. Wait for mark termination N+1 to complete.
430 // 5. Help with sweep N+1 until it's done.
432 // This all has to be written to deal with the fact that the
433 // GC may move ahead on its own. For example, when we block
434 // until mark termination N, we may wake up in cycle N+2.
436 // Wait until the current sweep termination, mark, and mark
437 // termination complete.
438 n := work.cycles.Load()
441 // We're now in sweep N or later. Trigger GC cycle N+1, which
442 // will first finish sweep N if necessary and then enter sweep
444 gcStart(gcTrigger{kind: gcTriggerCycle, n: n + 1})
446 // Wait for mark termination N+1 to complete.
449 // Finish sweep N+1 before returning. We do this both to
450 // complete the cycle and because runtime.GC() is often used
451 // as part of tests and benchmarks to get the system into a
452 // relatively stable and isolated state.
453 for work.cycles.Load() == n+1 && sweepone() != ^uintptr(0) {
458 // Callers may assume that the heap profile reflects the
459 // just-completed cycle when this returns (historically this
460 // happened because this was a STW GC), but right now the
461 // profile still reflects mark termination N, not N+1.
463 // As soon as all of the sweep frees from cycle N+1 are done,
464 // we can go ahead and publish the heap profile.
466 // First, wait for sweeping to finish. (We know there are no
467 // more spans on the sweep queue, but we may be concurrently
468 // sweeping spans, so we have to wait.)
469 for work.cycles.Load() == n+1 && !isSweepDone() {
473 // Now we're really done with sweeping, so we can publish the
474 // stable heap profile. Only do this if we haven't already hit
475 // another mark termination.
477 cycle := work.cycles.Load()
478 if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) {
484 // gcWaitOnMark blocks until GC finishes the Nth mark phase. If GC has
485 // already completed this mark phase, it returns immediately.
486 func gcWaitOnMark(n uint32) {
488 // Disable phase transitions.
489 lock(&work.sweepWaiters.lock)
490 nMarks := work.cycles.Load()
491 if gcphase != _GCmark {
492 // We've already completed this cycle's mark.
497 unlock(&work.sweepWaiters.lock)
501 // Wait until sweep termination, mark, and mark
502 // termination of cycle N complete.
503 work.sweepWaiters.list.push(getg())
504 goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceEvGoBlock, 1)
508 // gcMode indicates how concurrent a GC cycle should be.
512 gcBackgroundMode gcMode = iota // concurrent GC and sweep
513 gcForceMode // stop-the-world GC now, concurrent sweep
514 gcForceBlockMode // stop-the-world GC now and STW sweep (forced by user)
517 // A gcTrigger is a predicate for starting a GC cycle. Specifically,
518 // it is an exit condition for the _GCoff phase.
519 type gcTrigger struct {
521 now int64 // gcTriggerTime: current time
522 n uint32 // gcTriggerCycle: cycle number to start
525 type gcTriggerKind int
528 // gcTriggerHeap indicates that a cycle should be started when
529 // the heap size reaches the trigger heap size computed by the
531 gcTriggerHeap gcTriggerKind = iota
533 // gcTriggerTime indicates that a cycle should be started when
534 // it's been more than forcegcperiod nanoseconds since the
535 // previous GC cycle.
538 // gcTriggerCycle indicates that a cycle should be started if
539 // we have not yet started cycle number gcTrigger.n (relative
544 // test reports whether the trigger condition is satisfied, meaning
545 // that the exit condition for the _GCoff phase has been met. The exit
546 // condition should be tested when allocating.
547 func (t gcTrigger) test() bool {
548 if !memstats.enablegc || panicking.Load() != 0 || gcphase != _GCoff {
553 // Non-atomic access to gcController.heapLive for performance. If
554 // we are going to trigger on this, this thread just
555 // atomically wrote gcController.heapLive anyway and we'll see our
557 trigger, _ := gcController.trigger()
558 return gcController.heapLive.Load() >= trigger
560 if gcController.gcPercent.Load() < 0 {
563 lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
564 return lastgc != 0 && t.now-lastgc > forcegcperiod
566 // t.n > work.cycles, but accounting for wraparound.
567 return int32(t.n-work.cycles.Load()) > 0
572 // gcStart starts the GC. It transitions from _GCoff to _GCmark (if
573 // debug.gcstoptheworld == 0) or performs all of GC (if
574 // debug.gcstoptheworld != 0).
576 // This may return without performing this transition in some cases,
577 // such as when called on a system stack or with locks held.
578 func gcStart(trigger gcTrigger) {
579 // Since this is called from malloc and malloc is called in
580 // the guts of a number of libraries that might be holding
581 // locks, don't attempt to start GC in non-preemptible or
582 // potentially unstable situations.
584 if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" {
591 // Pick up the remaining unswept/not being swept spans concurrently
593 // This shouldn't happen if we're being invoked in background
594 // mode since proportional sweep should have just finished
595 // sweeping everything, but rounding errors, etc, may leave a
596 // few spans unswept. In forced mode, this is necessary since
597 // GC can be forced at any point in the sweeping cycle.
599 // We check the transition condition continuously here in case
600 // this G gets delayed in to the next GC cycle.
601 for trigger.test() && sweepone() != ^uintptr(0) {
605 // Perform GC initialization and the sweep termination
607 semacquire(&work.startSema)
608 // Re-check transition condition under transition lock.
610 semrelease(&work.startSema)
614 // In gcstoptheworld debug mode, upgrade the mode accordingly.
615 // We do this after re-checking the transition condition so
616 // that multiple goroutines that detect the heap trigger don't
617 // start multiple STW GCs.
618 mode := gcBackgroundMode
619 if debug.gcstoptheworld == 1 {
621 } else if debug.gcstoptheworld == 2 {
622 mode = gcForceBlockMode
625 // Ok, we're doing it! Stop everybody else
627 semacquire(&worldsema)
629 // For stats, check if this GC was forced by the user.
630 // Update it under gcsema to avoid gctrace getting wrong values.
631 work.userForced = trigger.kind == gcTriggerCycle
637 // Check that all Ps have finished deferred mcache flushes.
638 for _, p := range allp {
639 if fg := p.mcache.flushGen.Load(); fg != mheap_.sweepgen {
640 println("runtime: p", p.id, "flushGen", fg, "!= sweepgen", mheap_.sweepgen)
641 throw("p mcache not flushed")
645 gcBgMarkStartWorkers()
647 systemstack(gcResetMarkState)
649 work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs
650 if work.stwprocs > ncpu {
651 // This is used to compute CPU time of the STW phases,
652 // so it can't be more than ncpu, even if GOMAXPROCS is.
655 work.heap0 = gcController.heapLive.Load()
660 work.tSweepTerm = now
661 work.pauseStart = now
665 systemstack(stopTheWorldWithSema)
666 // Finish sweep before we start concurrent scan.
671 // clearpools before we start the GC. If we wait they memory will not be
672 // reclaimed until the next GC cycle.
677 // Assists and workers can start the moment we start
679 gcController.startCycle(now, int(gomaxprocs), trigger)
681 // Notify the CPU limiter that assists may begin.
682 gcCPULimiter.startGCTransition(true, now)
684 // In STW mode, disable scheduling of user Gs. This may also
685 // disable scheduling of this goroutine, so it may block as
686 // soon as we start the world again.
687 if mode != gcBackgroundMode {
688 schedEnableUser(false)
691 // Enter concurrent mark phase and enable
694 // Because the world is stopped, all Ps will
695 // observe that write barriers are enabled by
696 // the time we start the world and begin
699 // Write barriers must be enabled before assists are
700 // enabled because they must be enabled before
701 // any non-leaf heap objects are marked. Since
702 // allocations are blocked until assists can
703 // happen, we want enable assists as early as
707 gcBgMarkPrepare() // Must happen before assist enable.
710 // Mark all active tinyalloc blocks. Since we're
711 // allocating from these, they need to be black like
712 // other allocations. The alternative is to blacken
713 // the tiny block on every allocation from it, which
714 // would slow down the tiny allocator.
717 // At this point all Ps have enabled the write
718 // barrier, thus maintaining the no white to
719 // black invariant. Enable mutator assists to
720 // put back-pressure on fast allocating
722 atomic.Store(&gcBlackenEnabled, 1)
724 // In STW mode, we could block the instant systemstack
725 // returns, so make sure we're not preemptible.
730 now = startTheWorldWithSema(trace.enabled)
731 work.pauseNS += now - work.pauseStart
733 memstats.gcPauseDist.record(now - work.pauseStart)
735 // Release the CPU limiter.
736 gcCPULimiter.finishGCTransition(now)
739 // Release the world sema before Gosched() in STW mode
740 // because we will need to reacquire it later but before
741 // this goroutine becomes runnable again, and we could
742 // self-deadlock otherwise.
743 semrelease(&worldsema)
746 // Make sure we block instead of returning to user code
748 if mode != gcBackgroundMode {
752 semrelease(&work.startSema)
755 // gcMarkDoneFlushed counts the number of P's with flushed work.
757 // Ideally this would be a captured local in gcMarkDone, but forEachP
758 // escapes its callback closure, so it can't capture anything.
760 // This is protected by markDoneSema.
761 var gcMarkDoneFlushed uint32
763 // gcMarkDone transitions the GC from mark to mark termination if all
764 // reachable objects have been marked (that is, there are no grey
765 // objects and can be no more in the future). Otherwise, it flushes
766 // all local work to the global queues where it can be discovered by
769 // This should be called when all local mark work has been drained and
770 // there are no remaining workers. Specifically, when
772 // work.nwait == work.nproc && !gcMarkWorkAvailable(p)
774 // The calling context must be preemptible.
776 // Flushing local work is important because idle Ps may have local
777 // work queued. This is the only way to make that work visible and
778 // drive GC to completion.
780 // It is explicitly okay to have write barriers in this function. If
781 // it does transition to mark termination, then all reachable objects
782 // have been marked, so the write barrier cannot shade any more
785 // Ensure only one thread is running the ragged barrier at a
787 semacquire(&work.markDoneSema)
790 // Re-check transition condition under transition lock.
792 // It's critical that this checks the global work queues are
793 // empty before performing the ragged barrier. Otherwise,
794 // there could be global work that a P could take after the P
795 // has passed the ragged barrier.
796 if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
797 semrelease(&work.markDoneSema)
801 // forEachP needs worldsema to execute, and we'll need it to
802 // stop the world later, so acquire worldsema now.
803 semacquire(&worldsema)
805 // Flush all local buffers and collect flushedWork flags.
806 gcMarkDoneFlushed = 0
809 // Mark the user stack as preemptible so that it may be scanned.
810 // Otherwise, our attempt to force all P's to a safepoint could
811 // result in a deadlock as we attempt to preempt a worker that's
812 // trying to preempt us (e.g. for a stack scan).
813 casGToWaiting(gp, _Grunning, waitReasonGCMarkTermination)
814 forEachP(func(pp *p) {
815 // Flush the write barrier buffer, since this may add
816 // work to the gcWork.
819 // Flush the gcWork, since this may create global work
820 // and set the flushedWork flag.
822 // TODO(austin): Break up these workbufs to
823 // better distribute work.
825 // Collect the flushedWork flag.
826 if pp.gcw.flushedWork {
827 atomic.Xadd(&gcMarkDoneFlushed, 1)
828 pp.gcw.flushedWork = false
831 casgstatus(gp, _Gwaiting, _Grunning)
834 if gcMarkDoneFlushed != 0 {
835 // More grey objects were discovered since the
836 // previous termination check, so there may be more
837 // work to do. Keep going. It's possible the
838 // transition condition became true again during the
839 // ragged barrier, so re-check it.
840 semrelease(&worldsema)
844 // There was no global work, no local work, and no Ps
845 // communicated work since we took markDoneSema. Therefore
846 // there are no grey objects and no more objects can be
847 // shaded. Transition to mark termination.
850 work.pauseStart = now
851 getg().m.preemptoff = "gcing"
855 systemstack(stopTheWorldWithSema)
856 // The gcphase is _GCmark, it will transition to _GCmarktermination
857 // below. The important thing is that the wb remains active until
858 // all marking is complete. This includes writes made by the GC.
860 // There is sometimes work left over when we enter mark termination due
861 // to write barriers performed after the completion barrier above.
862 // Detect this and resume concurrent mark. This is obviously
865 // See issue #27993 for details.
867 // Switch to the system stack to call wbBufFlush1, though in this case
868 // it doesn't matter because we're non-preemptible anyway.
871 for _, p := range allp {
880 getg().m.preemptoff = ""
882 now := startTheWorldWithSema(trace.enabled)
883 work.pauseNS += now - work.pauseStart
884 memstats.gcPauseDist.record(now - work.pauseStart)
886 semrelease(&worldsema)
890 gcComputeStartingStackSize()
892 // Disable assists and background workers. We must do
893 // this before waking blocked assists.
894 atomic.Store(&gcBlackenEnabled, 0)
896 // Notify the CPU limiter that GC assists will now cease.
897 gcCPULimiter.startGCTransition(false, now)
899 // Wake all blocked assists. These will run when we
900 // start the world again.
903 // Likewise, release the transition lock. Blocked
904 // workers and assists will run when we start the
906 semrelease(&work.markDoneSema)
908 // In STW mode, re-enable user goroutines. These will be
909 // queued to run after we start the world.
910 schedEnableUser(true)
912 // endCycle depends on all gcWork cache stats being flushed.
913 // The termination algorithm above ensured that up to
914 // allocations since the ragged barrier.
915 gcController.endCycle(now, int(gomaxprocs), work.userForced)
917 // Perform mark termination. This will restart the world.
921 // World must be stopped and mark assists and background workers must be
923 func gcMarkTermination() {
924 // Start marktermination (write barrier remains enabled for now).
925 setGCPhase(_GCmarktermination)
927 work.heap1 = gcController.heapLive.Load()
928 startTime := nanotime()
931 mp.preemptoff = "gcing"
934 casGToWaiting(curgp, _Grunning, waitReasonGarbageCollection)
936 // Run gc on the g0 stack. We do this so that the g stack
937 // we're currently running on will no longer change. Cuts
938 // the root set down a bit (g0 stacks are not scanned, and
939 // we don't need to scan gc's internal state). We also
940 // need to switch to g0 so we can shrink the stack.
943 // Must return immediately.
944 // The outer function's stack may have moved
945 // during gcMark (it shrinks stacks, including the
946 // outer function's stack), so we must not refer
947 // to any of its variables. Return back to the
948 // non-system stack to pick up the new addresses
949 // before continuing.
953 work.heap2 = work.bytesMarked
954 if debug.gccheckmark > 0 {
955 // Run a full non-parallel, stop-the-world
956 // mark using checkmark bits, to check that we
957 // didn't forget to mark anything during the
958 // concurrent mark process.
961 gcw := &getg().m.p.ptr().gcw
963 wbBufFlush1(getg().m.p.ptr())
968 // marking is complete so we can turn the write barrier off
974 casgstatus(curgp, _Gwaiting, _Grunning)
983 if gcphase != _GCoff {
984 throw("gc done but gcphase != _GCoff")
987 // Record heapInUse for scavenger.
988 memstats.lastHeapInUse = gcController.heapInUse.load()
990 // Update GC trigger and pacing, as well as downstream consumers
991 // of this pacing information, for the next cycle.
992 systemstack(gcControllerCommit)
994 // Update timing memstats
996 sec, nsec, _ := time_now()
997 unixNow := sec*1e9 + int64(nsec)
998 work.pauseNS += now - work.pauseStart
1000 memstats.gcPauseDist.record(now - work.pauseStart)
1001 atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user
1002 atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us
1003 memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS)
1004 memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow)
1005 memstats.pause_total_ns += uint64(work.pauseNS)
1007 sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm)
1008 // We report idle marking time below, but omit it from the
1009 // overall utilization here since it's "free".
1010 markAssistCpu := gcController.assistTime.Load()
1011 markDedicatedCpu := gcController.dedicatedMarkTime.Load()
1012 markFractionalCpu := gcController.fractionalMarkTime.Load()
1013 markIdleCpu := gcController.idleMarkTime.Load()
1014 markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm)
1015 scavAssistCpu := scavenge.assistTime.Load()
1016 scavBgCpu := scavenge.backgroundTime.Load()
1018 // Update cumulative GC CPU stats.
1019 work.cpuStats.gcAssistTime += markAssistCpu
1020 work.cpuStats.gcDedicatedTime += markDedicatedCpu + markFractionalCpu
1021 work.cpuStats.gcIdleTime += markIdleCpu
1022 work.cpuStats.gcPauseTime += sweepTermCpu + markTermCpu
1023 work.cpuStats.gcTotalTime += sweepTermCpu + markAssistCpu + markDedicatedCpu + markFractionalCpu + markIdleCpu + markTermCpu
1025 // Update cumulative scavenge CPU stats.
1026 work.cpuStats.scavengeAssistTime += scavAssistCpu
1027 work.cpuStats.scavengeBgTime += scavBgCpu
1028 work.cpuStats.scavengeTotalTime += scavAssistCpu + scavBgCpu
1030 // Update total CPU.
1031 work.cpuStats.totalTime = sched.totaltime + (now-sched.procresizetime)*int64(gomaxprocs)
1032 work.cpuStats.idleTime += sched.idleTime.Load()
1034 // Compute userTime. We compute this indirectly as everything that's not the above.
1036 // Since time spent in _Pgcstop is covered by gcPauseTime, and time spent in _Pidle
1037 // is covered by idleTime, what we're left with is time spent in _Prunning and _Psyscall,
1038 // the latter of which is fine because the P will either go idle or get used for something
1039 // else via sysmon. Meanwhile if we subtract GC time from whatever's left, we get non-GC
1040 // _Prunning time. Note that this still leaves time spent in sweeping and in the scheduler,
1041 // but that's fine. The overwhelming majority of this time will be actual user time.
1042 work.cpuStats.userTime = work.cpuStats.totalTime - (work.cpuStats.gcTotalTime +
1043 work.cpuStats.scavengeTotalTime + work.cpuStats.idleTime)
1045 // Compute overall GC CPU utilization.
1046 // Omit idle marking time from the overall utilization here since it's "free".
1047 memstats.gc_cpu_fraction = float64(work.cpuStats.gcTotalTime-work.cpuStats.gcIdleTime) / float64(work.cpuStats.totalTime)
1049 // Reset assist time and background time stats.
1051 // Do this now, instead of at the start of the next GC cycle, because
1052 // these two may keep accumulating even if the GC is not active.
1053 scavenge.assistTime.Store(0)
1054 scavenge.backgroundTime.Store(0)
1056 // Reset idle time stat.
1057 sched.idleTime.Store(0)
1059 // Reset sweep state.
1061 sweep.npausesweep = 0
1063 if work.userForced {
1064 memstats.numforcedgc++
1067 // Bump GC cycle count and wake goroutines waiting on sweep.
1068 lock(&work.sweepWaiters.lock)
1070 injectglist(&work.sweepWaiters.list)
1071 unlock(&work.sweepWaiters.lock)
1073 // Release the CPU limiter.
1074 gcCPULimiter.finishGCTransition(now)
1076 // Finish the current heap profiling cycle and start a new
1077 // heap profiling cycle. We do this before starting the world
1078 // so events don't leak into the wrong cycle.
1081 // There may be stale spans in mcaches that need to be swept.
1082 // Those aren't tracked in any sweep lists, so we need to
1083 // count them against sweep completion until we ensure all
1084 // those spans have been forced out.
1085 sl := sweep.active.begin()
1087 throw("failed to set sweep barrier")
1090 systemstack(func() { startTheWorldWithSema(trace.enabled) })
1092 // Flush the heap profile so we can start a new cycle next GC.
1093 // This is relatively expensive, so we don't do it with the
1097 // Prepare workbufs for freeing by the sweeper. We do this
1098 // asynchronously because it can take non-trivial time.
1099 prepareFreeWorkbufs()
1101 // Free stack spans. This must be done between GC cycles.
1102 systemstack(freeStackSpans)
1104 // Ensure all mcaches are flushed. Each P will flush its own
1105 // mcache before allocating, but idle Ps may not. Since this
1106 // is necessary to sweep all spans, we need to ensure all
1107 // mcaches are flushed before we start the next GC cycle.
1108 systemstack(func() {
1109 forEachP(func(pp *p) {
1110 pp.mcache.prepareForSweep()
1113 // Now that we've swept stale spans in mcaches, they don't
1114 // count against unswept spans.
1115 sweep.active.end(sl)
1117 // Print gctrace before dropping worldsema. As soon as we drop
1118 // worldsema another cycle could start and smash the stats
1119 // we're trying to print.
1120 if debug.gctrace > 0 {
1121 util := int(memstats.gc_cpu_fraction * 100)
1125 print("gc ", memstats.numgc,
1126 " @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
1128 prev := work.tSweepTerm
1129 for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} {
1133 print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev))))
1136 print(" ms clock, ")
1137 for i, ns := range []int64{
1139 gcController.assistTime.Load(),
1140 gcController.dedicatedMarkTime.Load() + gcController.fractionalMarkTime.Load(),
1141 gcController.idleMarkTime.Load(),
1144 if i == 2 || i == 3 {
1145 // Separate mark time components with /.
1150 print(string(fmtNSAsMS(sbuf[:], uint64(ns))))
1153 work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ",
1154 gcController.lastHeapGoal>>20, " MB goal, ",
1155 gcController.lastStackScan.Load()>>20, " MB stacks, ",
1156 gcController.globalsScan.Load()>>20, " MB globals, ",
1157 work.maxprocs, " P")
1158 if work.userForced {
1165 // Set any arena chunks that were deferred to fault.
1166 lock(&userArenaState.lock)
1167 faultList := userArenaState.fault
1168 userArenaState.fault = nil
1169 unlock(&userArenaState.lock)
1170 for _, lc := range faultList {
1171 lc.mspan.setUserArenaChunkToFault()
1174 semrelease(&worldsema)
1176 // Careful: another GC cycle may start now.
1181 // now that gc is done, kick off finalizer thread if needed
1182 if !concurrentSweep {
1183 // give the queued finalizers, if any, a chance to run
1188 // gcBgMarkStartWorkers prepares background mark worker goroutines. These
1189 // goroutines will not run until the mark phase, but they must be started while
1190 // the work is not stopped and from a regular G stack. The caller must hold
1192 func gcBgMarkStartWorkers() {
1193 // Background marking is performed by per-P G's. Ensure that each P has
1194 // a background GC G.
1196 // Worker Gs don't exit if gomaxprocs is reduced. If it is raised
1197 // again, we can reuse the old workers; no need to create new workers.
1198 for gcBgMarkWorkerCount < gomaxprocs {
1201 notetsleepg(&work.bgMarkReady, -1)
1202 noteclear(&work.bgMarkReady)
1203 // The worker is now guaranteed to be added to the pool before
1204 // its P's next findRunnableGCWorker.
1206 gcBgMarkWorkerCount++
1210 // gcBgMarkPrepare sets up state for background marking.
1211 // Mutator assists must not yet be enabled.
1212 func gcBgMarkPrepare() {
1213 // Background marking will stop when the work queues are empty
1214 // and there are no more workers (note that, since this is
1215 // concurrent, this may be a transient state, but mark
1216 // termination will clean it up). Between background workers
1217 // and assists, we don't really know how many workers there
1218 // will be, so we pretend to have an arbitrarily large number
1219 // of workers, almost all of which are "waiting". While a
1220 // worker is working it decrements nwait. If nproc == nwait,
1221 // there are no workers.
1222 work.nproc = ^uint32(0)
1223 work.nwait = ^uint32(0)
1226 // gcBgMarkWorker is an entry in the gcBgMarkWorkerPool. It points to a single
1227 // gcBgMarkWorker goroutine.
1228 type gcBgMarkWorkerNode struct {
1229 // Unused workers are managed in a lock-free stack. This field must be first.
1232 // The g of this worker.
1235 // Release this m on park. This is used to communicate with the unlock
1236 // function, which cannot access the G's stack. It is unused outside of
1237 // gcBgMarkWorker().
1241 func gcBgMarkWorker() {
1244 // We pass node to a gopark unlock function, so it can't be on
1245 // the stack (see gopark). Prevent deadlock from recursively
1246 // starting GC by disabling preemption.
1247 gp.m.preemptoff = "GC worker init"
1248 node := new(gcBgMarkWorkerNode)
1249 gp.m.preemptoff = ""
1253 node.m.set(acquirem())
1254 notewakeup(&work.bgMarkReady)
1255 // After this point, the background mark worker is generally scheduled
1256 // cooperatively by gcController.findRunnableGCWorker. While performing
1257 // work on the P, preemption is disabled because we are working on
1258 // P-local work buffers. When the preempt flag is set, this puts itself
1259 // into _Gwaiting to be woken up by gcController.findRunnableGCWorker
1260 // at the appropriate time.
1262 // When preemption is enabled (e.g., while in gcMarkDone), this worker
1263 // may be preempted and schedule as a _Grunnable G from a runq. That is
1264 // fine; it will eventually gopark again for further scheduling via
1265 // findRunnableGCWorker.
1267 // Since we disable preemption before notifying bgMarkReady, we
1268 // guarantee that this G will be in the worker pool for the next
1269 // findRunnableGCWorker. This isn't strictly necessary, but it reduces
1270 // latency between _GCmark starting and the workers starting.
1273 // Go to sleep until woken by
1274 // gcController.findRunnableGCWorker.
1275 gopark(func(g *g, nodep unsafe.Pointer) bool {
1276 node := (*gcBgMarkWorkerNode)(nodep)
1278 if mp := node.m.ptr(); mp != nil {
1279 // The worker G is no longer running; release
1282 // N.B. it is _safe_ to release the M as soon
1283 // as we are no longer performing P-local mark
1286 // However, since we cooperatively stop work
1287 // when gp.preempt is set, if we releasem in
1288 // the loop then the following call to gopark
1289 // would immediately preempt the G. This is
1290 // also safe, but inefficient: the G must
1291 // schedule again only to enter gopark and park
1292 // again. Thus, we defer the release until
1293 // after parking the G.
1297 // Release this G to the pool.
1298 gcBgMarkWorkerPool.push(&node.node)
1299 // Note that at this point, the G may immediately be
1300 // rescheduled and may be running.
1302 }, unsafe.Pointer(node), waitReasonGCWorkerIdle, traceEvGoBlock, 0)
1304 // Preemption must not occur here, or another G might see
1305 // p.gcMarkWorkerMode.
1307 // Disable preemption so we can use the gcw. If the
1308 // scheduler wants to preempt us, we'll stop draining,
1309 // dispose the gcw, and then preempt.
1310 node.m.set(acquirem())
1311 pp := gp.m.p.ptr() // P can't change with preemption disabled.
1313 if gcBlackenEnabled == 0 {
1314 println("worker mode", pp.gcMarkWorkerMode)
1315 throw("gcBgMarkWorker: blackening not enabled")
1318 if pp.gcMarkWorkerMode == gcMarkWorkerNotWorker {
1319 throw("gcBgMarkWorker: mode not set")
1322 startTime := nanotime()
1323 pp.gcMarkWorkerStartTime = startTime
1324 var trackLimiterEvent bool
1325 if pp.gcMarkWorkerMode == gcMarkWorkerIdleMode {
1326 trackLimiterEvent = pp.limiterEvent.start(limiterEventIdleMarkWork, startTime)
1329 decnwait := atomic.Xadd(&work.nwait, -1)
1330 if decnwait == work.nproc {
1331 println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
1332 throw("work.nwait was > work.nproc")
1335 systemstack(func() {
1336 // Mark our goroutine preemptible so its stack
1337 // can be scanned. This lets two mark workers
1338 // scan each other (otherwise, they would
1339 // deadlock). We must not modify anything on
1340 // the G stack. However, stack shrinking is
1341 // disabled for mark workers, so it is safe to
1342 // read from the G stack.
1343 casGToWaiting(gp, _Grunning, waitReasonGCWorkerActive)
1344 switch pp.gcMarkWorkerMode {
1346 throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
1347 case gcMarkWorkerDedicatedMode:
1348 gcDrain(&pp.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
1350 // We were preempted. This is
1351 // a useful signal to kick
1352 // everything out of the run
1353 // queue so it can run
1355 if drainQ, n := runqdrain(pp); n > 0 {
1357 globrunqputbatch(&drainQ, int32(n))
1361 // Go back to draining, this time
1362 // without preemption.
1363 gcDrain(&pp.gcw, gcDrainFlushBgCredit)
1364 case gcMarkWorkerFractionalMode:
1365 gcDrain(&pp.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
1366 case gcMarkWorkerIdleMode:
1367 gcDrain(&pp.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
1369 casgstatus(gp, _Gwaiting, _Grunning)
1372 // Account for time and mark us as stopped.
1374 duration := now - startTime
1375 gcController.markWorkerStop(pp.gcMarkWorkerMode, duration)
1376 if trackLimiterEvent {
1377 pp.limiterEvent.stop(limiterEventIdleMarkWork, now)
1379 if pp.gcMarkWorkerMode == gcMarkWorkerFractionalMode {
1380 atomic.Xaddint64(&pp.gcFractionalMarkTime, duration)
1383 // Was this the last worker and did we run out
1385 incnwait := atomic.Xadd(&work.nwait, +1)
1386 if incnwait > work.nproc {
1387 println("runtime: p.gcMarkWorkerMode=", pp.gcMarkWorkerMode,
1388 "work.nwait=", incnwait, "work.nproc=", work.nproc)
1389 throw("work.nwait > work.nproc")
1392 // We'll releasem after this point and thus this P may run
1393 // something else. We must clear the worker mode to avoid
1394 // attributing the mode to a different (non-worker) G in
1396 pp.gcMarkWorkerMode = gcMarkWorkerNotWorker
1398 // If this worker reached a background mark completion
1399 // point, signal the main GC goroutine.
1400 if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
1401 // We don't need the P-local buffers here, allow
1402 // preemption because we may schedule like a regular
1403 // goroutine in gcMarkDone (block on locks, etc).
1404 releasem(node.m.ptr())
1412 // gcMarkWorkAvailable reports whether executing a mark worker
1413 // on p is potentially useful. p may be nil, in which case it only
1414 // checks the global sources of work.
1415 func gcMarkWorkAvailable(p *p) bool {
1416 if p != nil && !p.gcw.empty() {
1419 if !work.full.empty() {
1420 return true // global work available
1422 if work.markrootNext < work.markrootJobs {
1423 return true // root scan work available
1428 // gcMark runs the mark (or, for concurrent GC, mark termination)
1429 // All gcWork caches must be empty.
1430 // STW is in effect at this point.
1431 func gcMark(startTime int64) {
1432 if debug.allocfreetrace > 0 {
1436 if gcphase != _GCmarktermination {
1437 throw("in gcMark expecting to see gcphase as _GCmarktermination")
1439 work.tstart = startTime
1441 // Check that there's no marking work remaining.
1442 if work.full != 0 || work.markrootNext < work.markrootJobs {
1443 print("runtime: full=", hex(work.full), " next=", work.markrootNext, " jobs=", work.markrootJobs, " nDataRoots=", work.nDataRoots, " nBSSRoots=", work.nBSSRoots, " nSpanRoots=", work.nSpanRoots, " nStackRoots=", work.nStackRoots, "\n")
1444 panic("non-empty mark queue after concurrent mark")
1447 if debug.gccheckmark > 0 {
1448 // This is expensive when there's a large number of
1449 // Gs, so only do it if checkmark is also enabled.
1453 throw("work.full != 0")
1456 // Drop allg snapshot. allgs may have grown, in which case
1457 // this is the only reference to the old backing store and
1458 // there's no need to keep it around.
1459 work.stackRoots = nil
1461 // Clear out buffers and double-check that all gcWork caches
1462 // are empty. This should be ensured by gcMarkDone before we
1463 // enter mark termination.
1465 // TODO: We could clear out buffers just before mark if this
1466 // has a non-negligible impact on STW time.
1467 for _, p := range allp {
1468 // The write barrier may have buffered pointers since
1469 // the gcMarkDone barrier. However, since the barrier
1470 // ensured all reachable objects were marked, all of
1471 // these must be pointers to black objects. Hence we
1472 // can just discard the write barrier buffer.
1473 if debug.gccheckmark > 0 {
1474 // For debugging, flush the buffer and make
1475 // sure it really was all marked.
1484 print("runtime: P ", p.id, " flushedWork ", gcw.flushedWork)
1485 if gcw.wbuf1 == nil {
1486 print(" wbuf1=<nil>")
1488 print(" wbuf1.n=", gcw.wbuf1.nobj)
1490 if gcw.wbuf2 == nil {
1491 print(" wbuf2=<nil>")
1493 print(" wbuf2.n=", gcw.wbuf2.nobj)
1496 throw("P has cached GC work at end of mark termination")
1498 // There may still be cached empty buffers, which we
1499 // need to flush since we're going to free them. Also,
1500 // there may be non-zero stats because we allocated
1501 // black after the gcMarkDone barrier.
1505 // Flush scanAlloc from each mcache since we're about to modify
1506 // heapScan directly. If we were to flush this later, then scanAlloc
1507 // might have incorrect information.
1509 // Note that it's not important to retain this information; we know
1510 // exactly what heapScan is at this point via scanWork.
1511 for _, p := range allp {
1519 // Reset controller state.
1520 gcController.resetLive(work.bytesMarked)
1523 // gcSweep must be called on the system stack because it acquires the heap
1524 // lock. See mheap for details.
1526 // The world must be stopped.
1529 func gcSweep(mode gcMode) {
1530 assertWorldStopped()
1532 if gcphase != _GCoff {
1533 throw("gcSweep being done but phase is not GCoff")
1537 mheap_.sweepgen += 2
1538 sweep.active.reset()
1539 mheap_.pagesSwept.Store(0)
1540 mheap_.sweepArenas = mheap_.allArenas
1541 mheap_.reclaimIndex.Store(0)
1542 mheap_.reclaimCredit.Store(0)
1543 unlock(&mheap_.lock)
1545 sweep.centralIndex.clear()
1547 if !_ConcurrentSweep || mode == gcForceBlockMode {
1548 // Special case synchronous sweep.
1549 // Record that no proportional sweeping has to happen.
1551 mheap_.sweepPagesPerByte = 0
1552 unlock(&mheap_.lock)
1553 // Sweep all spans eagerly.
1554 for sweepone() != ^uintptr(0) {
1557 // Free workbufs eagerly.
1558 prepareFreeWorkbufs()
1559 for freeSomeWbufs(false) {
1561 // All "free" events for this mark/sweep cycle have
1562 // now happened, so we can make this profile cycle
1563 // available immediately.
1569 // Background sweep.
1572 sweep.parked = false
1573 ready(sweep.g, 0, true)
1578 // gcResetMarkState resets global state prior to marking (concurrent
1579 // or STW) and resets the stack scan state of all Gs.
1581 // This is safe to do without the world stopped because any Gs created
1582 // during or after this will start out in the reset state.
1584 // gcResetMarkState must be called on the system stack because it acquires
1585 // the heap lock. See mheap for details.
1588 func gcResetMarkState() {
1589 // This may be called during a concurrent phase, so lock to make sure
1590 // allgs doesn't change.
1591 forEachG(func(gp *g) {
1592 gp.gcscandone = false // set to true in gcphasework
1593 gp.gcAssistBytes = 0
1596 // Clear page marks. This is just 1MB per 64GB of heap, so the
1597 // time here is pretty trivial.
1599 arenas := mheap_.allArenas
1600 unlock(&mheap_.lock)
1601 for _, ai := range arenas {
1602 ha := mheap_.arenas[ai.l1()][ai.l2()]
1603 for i := range ha.pageMarks {
1608 work.bytesMarked = 0
1609 work.initialHeapLive = gcController.heapLive.Load()
1612 // Hooks for other packages
1614 var poolcleanup func()
1615 var boringCaches []unsafe.Pointer // for crypto/internal/boring
1617 //go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup
1618 func sync_runtime_registerPoolCleanup(f func()) {
1622 //go:linkname boring_registerCache crypto/internal/boring/bcache.registerCache
1623 func boring_registerCache(p unsafe.Pointer) {
1624 boringCaches = append(boringCaches, p)
1629 if poolcleanup != nil {
1633 // clear boringcrypto caches
1634 for _, p := range boringCaches {
1635 atomicstorep(p, nil)
1638 // Clear central sudog cache.
1639 // Leave per-P caches alone, they have strictly bounded size.
1640 // Disconnect cached list before dropping it on the floor,
1641 // so that a dangling ref to one entry does not pin all of them.
1642 lock(&sched.sudoglock)
1643 var sg, sgnext *sudog
1644 for sg = sched.sudogcache; sg != nil; sg = sgnext {
1648 sched.sudogcache = nil
1649 unlock(&sched.sudoglock)
1651 // Clear central defer pool.
1652 // Leave per-P pools alone, they have strictly bounded size.
1653 lock(&sched.deferlock)
1654 // disconnect cached list before dropping it on the floor,
1655 // so that a dangling ref to one entry does not pin all of them.
1656 var d, dlink *_defer
1657 for d = sched.deferpool; d != nil; d = dlink {
1661 sched.deferpool = nil
1662 unlock(&sched.deferlock)
1667 // itoaDiv formats val/(10**dec) into buf.
1668 func itoaDiv(buf []byte, val uint64, dec int) []byte {
1671 for val >= 10 || i >= idec {
1672 buf[i] = byte(val%10 + '0')
1680 buf[i] = byte(val + '0')
1684 // fmtNSAsMS nicely formats ns nanoseconds as milliseconds.
1685 func fmtNSAsMS(buf []byte, ns uint64) []byte {
1687 // Format as whole milliseconds.
1688 return itoaDiv(buf, ns/1e6, 0)
1690 // Format two digits of precision, with at most three decimal places.
1701 return itoaDiv(buf, x, dec)
1704 // Helpers for testing GC.
1706 // gcTestMoveStackOnNextCall causes the stack to be moved on a call
1707 // immediately following the call to this. It may not work correctly
1708 // if any other work appears after this call (such as returning).
1709 // Typically the following call should be marked go:noinline so it
1710 // performs a stack check.
1712 // In rare cases this may not cause the stack to move, specifically if
1713 // there's a preemption between this call and the next.
1714 func gcTestMoveStackOnNextCall() {
1716 gp.stackguard0 = stackForceMove
1719 // gcTestIsReachable performs a GC and returns a bit set where bit i
1720 // is set if ptrs[i] is reachable.
1721 func gcTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) {
1722 // This takes the pointers as unsafe.Pointers in order to keep
1723 // them live long enough for us to attach specials. After
1724 // that, we drop our references to them.
1727 panic("too many pointers for uint64 mask")
1730 // Block GC while we attach specials and drop our references
1731 // to ptrs. Otherwise, if a GC is in progress, it could mark
1732 // them reachable via this function before we have a chance to
1736 // Create reachability specials for ptrs.
1737 specials := make([]*specialReachable, len(ptrs))
1738 for i, p := range ptrs {
1739 lock(&mheap_.speciallock)
1740 s := (*specialReachable)(mheap_.specialReachableAlloc.alloc())
1741 unlock(&mheap_.speciallock)
1742 s.special.kind = _KindSpecialReachable
1743 if !addspecial(p, &s.special) {
1744 throw("already have a reachable special (duplicate pointer?)")
1747 // Make sure we don't retain ptrs.
1753 // Force a full GC and sweep.
1756 // Process specials.
1757 for i, s := range specials {
1760 println("runtime: object", i, "was not swept")
1761 throw("IsReachable failed")
1766 lock(&mheap_.speciallock)
1767 mheap_.specialReachableAlloc.free(unsafe.Pointer(s))
1768 unlock(&mheap_.speciallock)
1774 // gcTestPointerClass returns the category of what p points to, one of:
1775 // "heap", "stack", "data", "bss", "other". This is useful for checking
1776 // that a test is doing what it's intended to do.
1778 // This is nosplit simply to avoid extra pointer shuffling that may
1779 // complicate a test.
1782 func gcTestPointerClass(p unsafe.Pointer) string {
1783 p2 := uintptr(noescape(p))
1785 if gp.stack.lo <= p2 && p2 < gp.stack.hi {
1788 if base, _, _ := findObject(p2, 0, 0); base != 0 {
1791 for _, datap := range activeModules() {
1792 if datap.data <= p2 && p2 < datap.edata || datap.noptrdata <= p2 && p2 < datap.enoptrdata {
1795 if datap.bss <= p2 && p2 < datap.ebss || datap.noptrbss <= p2 && p2 <= datap.enoptrbss {