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 tracked in gcController.heapGoal variable). 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 mheap_.sweepDrained = 1
159 // Initialize GC pacer state.
160 // Use the environment variable GOGC for the initial gcPercent value.
161 gcController.init(readGOGC())
164 work.markDoneSema = 1
165 lockInit(&work.sweepWaiters.lock, lockRankSweepWaiters)
166 lockInit(&work.assistQueue.lock, lockRankAssistQueue)
167 lockInit(&work.wbufSpans.lock, lockRankWbufSpans)
170 // Temporary in order to enable register ABI work.
171 // TODO(register args): convert back to local chan in gcenabled, passed to "go" stmts.
172 var gcenable_setup chan int
174 // gcenable is called after the bulk of the runtime initialization,
175 // just before we're about to start letting user code run.
176 // It kicks off the background sweeper goroutine, the background
177 // scavenger goroutine, and enables GC.
179 // Kick off sweeping and scavenging.
180 gcenable_setup = make(chan int, 2)
186 memstats.enablegc = true // now that runtime is initialized, GC is okay
189 // Garbage collector phase.
190 // Indicates to write barrier and synchronization task to perform.
193 // The compiler knows about this variable.
194 // If you change it, you must change builtin/runtime.go, too.
195 // If you change the first four bytes, you must also change the write
196 // barrier insertion code.
197 var writeBarrier struct {
198 enabled bool // compiler emits a check of this before calling write barrier
199 pad [3]byte // compiler uses 32-bit load for "enabled" field
200 needed bool // whether we need a write barrier for current GC phase
201 cgo bool // whether we need a write barrier for a cgo check
202 alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load
205 // gcBlackenEnabled is 1 if mutator assists and background mark
206 // workers are allowed to blacken objects. This must only be set when
207 // gcphase == _GCmark.
208 var gcBlackenEnabled uint32
211 _GCoff = iota // GC not running; sweeping in background, write barrier disabled
212 _GCmark // GC marking roots and workbufs: allocate black, write barrier ENABLED
213 _GCmarktermination // GC mark termination: allocate black, P's help GC, write barrier ENABLED
217 func setGCPhase(x uint32) {
218 atomic.Store(&gcphase, x)
219 writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination
220 writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo
223 // gcMarkWorkerMode represents the mode that a concurrent mark worker
224 // should operate in.
226 // Concurrent marking happens through four different mechanisms. One
227 // is mutator assists, which happen in response to allocations and are
228 // not scheduled. The other three are variations in the per-P mark
229 // workers and are distinguished by gcMarkWorkerMode.
230 type gcMarkWorkerMode int
233 // gcMarkWorkerNotWorker indicates that the next scheduled G is not
234 // starting work and the mode should be ignored.
235 gcMarkWorkerNotWorker gcMarkWorkerMode = iota
237 // gcMarkWorkerDedicatedMode indicates that the P of a mark
238 // worker is dedicated to running that mark worker. The mark
239 // worker should run without preemption.
240 gcMarkWorkerDedicatedMode
242 // gcMarkWorkerFractionalMode indicates that a P is currently
243 // running the "fractional" mark worker. The fractional worker
244 // is necessary when GOMAXPROCS*gcBackgroundUtilization is not
245 // an integer and using only dedicated workers would result in
246 // utilization too far from the target of gcBackgroundUtilization.
247 // The fractional worker should run until it is preempted and
248 // will be scheduled to pick up the fractional part of
249 // GOMAXPROCS*gcBackgroundUtilization.
250 gcMarkWorkerFractionalMode
252 // gcMarkWorkerIdleMode indicates that a P is running the mark
253 // worker because it has nothing else to do. The idle worker
254 // should run until it is preempted and account its time
255 // against gcController.idleMarkTime.
259 // gcMarkWorkerModeStrings are the strings labels of gcMarkWorkerModes
260 // to use in execution traces.
261 var gcMarkWorkerModeStrings = [...]string{
268 // pollFractionalWorkerExit reports whether a fractional mark worker
269 // should self-preempt. It assumes it is called from the fractional
271 func pollFractionalWorkerExit() bool {
272 // This should be kept in sync with the fractional worker
273 // scheduler logic in findRunnableGCWorker.
275 delta := now - gcController.markStartTime
279 p := getg().m.p.ptr()
280 selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime)
281 // Add some slack to the utilization goal so that the
282 // fractional worker isn't behind again the instant it exits.
283 return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal
287 full lfstack // lock-free list of full blocks workbuf
288 empty lfstack // lock-free list of empty blocks workbuf
289 pad0 cpu.CacheLinePad // prevents false-sharing between full/empty and nproc/nwait
293 // free is a list of spans dedicated to workbufs, but
294 // that don't currently contain any workbufs.
296 // busy is a list of all spans containing workbufs on
297 // one of the workbuf lists.
301 // Restore 64-bit alignment on 32-bit.
304 // bytesMarked is the number of bytes marked this cycle. This
305 // includes bytes blackened in scanned objects, noscan objects
306 // that go straight to black, and permagrey objects scanned by
307 // markroot during the concurrent scan phase. This is updated
308 // atomically during the cycle. Updates may be batched
309 // arbitrarily, since the value is only read at the end of the
312 // Because of benign races during marking, this number may not
313 // be the exact number of marked bytes, but it should be very
316 // Put this field here because it needs 64-bit atomic access
317 // (and thus 8-byte alignment even on 32-bit architectures).
320 markrootNext uint32 // next markroot job
321 markrootJobs uint32 // number of markroot jobs
327 // Number of roots of various root types. Set by gcMarkRootPrepare.
328 nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int
330 // Base indexes of each root type. Set by gcMarkRootPrepare.
331 baseData, baseBSS, baseSpans, baseStacks, baseEnd uint32
333 // Each type of GC state transition is protected by a lock.
334 // Since multiple threads can simultaneously detect the state
335 // transition condition, any thread that detects a transition
336 // condition must acquire the appropriate transition lock,
337 // re-check the transition condition and return if it no
338 // longer holds or perform the transition if it does.
339 // Likewise, any transition must invalidate the transition
340 // condition before releasing the lock. This ensures that each
341 // transition is performed by exactly one thread and threads
342 // that need the transition to happen block until it has
345 // startSema protects the transition from "off" to mark or
348 // markDoneSema protects transitions from mark to mark termination.
351 bgMarkReady note // signal background mark worker has started
352 bgMarkDone uint32 // cas to 1 when at a background mark completion point
353 // Background mark completion signaling
355 // mode is the concurrency mode of the current GC cycle.
358 // userForced indicates the current GC cycle was forced by an
359 // explicit user call.
362 // totaltime is the CPU nanoseconds spent in GC since the
363 // program started if debug.gctrace > 0.
366 // initialHeapLive is the value of gcController.heapLive at the
367 // beginning of this GC cycle.
368 initialHeapLive uint64
370 // assistQueue is a queue of assists that are blocked because
371 // there was neither enough credit to steal or enough work to
378 // sweepWaiters is a list of blocked goroutines to wake when
379 // we transition from mark termination to sweep.
380 sweepWaiters struct {
385 // cycles is the number of completed GC cycles, where a GC
386 // cycle is sweep termination, mark, mark termination, and
387 // sweep. This differs from memstats.numgc, which is
388 // incremented at mark termination.
391 // Timing/utilization stats for this cycle.
392 stwprocs, maxprocs int32
393 tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start
395 pauseNS int64 // total STW time this cycle
396 pauseStart int64 // nanotime() of last STW
398 // debug.gctrace heap sizes for this cycle.
399 heap0, heap1, heap2, heapGoal uint64
402 // GC runs a garbage collection and blocks the caller until the
403 // garbage collection is complete. It may also block the entire
406 // We consider a cycle to be: sweep termination, mark, mark
407 // termination, and sweep. This function shouldn't return
408 // until a full cycle has been completed, from beginning to
409 // end. Hence, we always want to finish up the current cycle
410 // and start a new one. That means:
412 // 1. In sweep termination, mark, or mark termination of cycle
413 // N, wait until mark termination N completes and transitions
416 // 2. In sweep N, help with sweep N.
418 // At this point we can begin a full cycle N+1.
420 // 3. Trigger cycle N+1 by starting sweep termination N+1.
422 // 4. Wait for mark termination N+1 to complete.
424 // 5. Help with sweep N+1 until it's done.
426 // This all has to be written to deal with the fact that the
427 // GC may move ahead on its own. For example, when we block
428 // until mark termination N, we may wake up in cycle N+2.
430 // Wait until the current sweep termination, mark, and mark
431 // termination complete.
432 n := atomic.Load(&work.cycles)
435 // We're now in sweep N or later. Trigger GC cycle N+1, which
436 // will first finish sweep N if necessary and then enter sweep
438 gcStart(gcTrigger{kind: gcTriggerCycle, n: n + 1})
440 // Wait for mark termination N+1 to complete.
443 // Finish sweep N+1 before returning. We do this both to
444 // complete the cycle and because runtime.GC() is often used
445 // as part of tests and benchmarks to get the system into a
446 // relatively stable and isolated state.
447 for atomic.Load(&work.cycles) == n+1 && sweepone() != ^uintptr(0) {
452 // Callers may assume that the heap profile reflects the
453 // just-completed cycle when this returns (historically this
454 // happened because this was a STW GC), but right now the
455 // profile still reflects mark termination N, not N+1.
457 // As soon as all of the sweep frees from cycle N+1 are done,
458 // we can go ahead and publish the heap profile.
460 // First, wait for sweeping to finish. (We know there are no
461 // more spans on the sweep queue, but we may be concurrently
462 // sweeping spans, so we have to wait.)
463 for atomic.Load(&work.cycles) == n+1 && !isSweepDone() {
467 // Now we're really done with sweeping, so we can publish the
468 // stable heap profile. Only do this if we haven't already hit
469 // another mark termination.
471 cycle := atomic.Load(&work.cycles)
472 if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) {
478 // gcWaitOnMark blocks until GC finishes the Nth mark phase. If GC has
479 // already completed this mark phase, it returns immediately.
480 func gcWaitOnMark(n uint32) {
482 // Disable phase transitions.
483 lock(&work.sweepWaiters.lock)
484 nMarks := atomic.Load(&work.cycles)
485 if gcphase != _GCmark {
486 // We've already completed this cycle's mark.
491 unlock(&work.sweepWaiters.lock)
495 // Wait until sweep termination, mark, and mark
496 // termination of cycle N complete.
497 work.sweepWaiters.list.push(getg())
498 goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceEvGoBlock, 1)
502 // gcMode indicates how concurrent a GC cycle should be.
506 gcBackgroundMode gcMode = iota // concurrent GC and sweep
507 gcForceMode // stop-the-world GC now, concurrent sweep
508 gcForceBlockMode // stop-the-world GC now and STW sweep (forced by user)
511 // A gcTrigger is a predicate for starting a GC cycle. Specifically,
512 // it is an exit condition for the _GCoff phase.
513 type gcTrigger struct {
515 now int64 // gcTriggerTime: current time
516 n uint32 // gcTriggerCycle: cycle number to start
519 type gcTriggerKind int
522 // gcTriggerHeap indicates that a cycle should be started when
523 // the heap size reaches the trigger heap size computed by the
525 gcTriggerHeap gcTriggerKind = iota
527 // gcTriggerTime indicates that a cycle should be started when
528 // it's been more than forcegcperiod nanoseconds since the
529 // previous GC cycle.
532 // gcTriggerCycle indicates that a cycle should be started if
533 // we have not yet started cycle number gcTrigger.n (relative
538 // test reports whether the trigger condition is satisfied, meaning
539 // that the exit condition for the _GCoff phase has been met. The exit
540 // condition should be tested when allocating.
541 func (t gcTrigger) test() bool {
542 if !memstats.enablegc || panicking != 0 || gcphase != _GCoff {
547 // Non-atomic access to gcController.heapLive for performance. If
548 // we are going to trigger on this, this thread just
549 // atomically wrote gcController.heapLive anyway and we'll see our
551 return gcController.heapLive >= gcController.trigger
553 if gcController.gcPercent < 0 {
556 lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime))
557 return lastgc != 0 && t.now-lastgc > forcegcperiod
559 // t.n > work.cycles, but accounting for wraparound.
560 return int32(t.n-work.cycles) > 0
565 // gcStart starts the GC. It transitions from _GCoff to _GCmark (if
566 // debug.gcstoptheworld == 0) or performs all of GC (if
567 // debug.gcstoptheworld != 0).
569 // This may return without performing this transition in some cases,
570 // such as when called on a system stack or with locks held.
571 func gcStart(trigger gcTrigger) {
572 // Since this is called from malloc and malloc is called in
573 // the guts of a number of libraries that might be holding
574 // locks, don't attempt to start GC in non-preemptible or
575 // potentially unstable situations.
577 if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" {
584 // Pick up the remaining unswept/not being swept spans concurrently
586 // This shouldn't happen if we're being invoked in background
587 // mode since proportional sweep should have just finished
588 // sweeping everything, but rounding errors, etc, may leave a
589 // few spans unswept. In forced mode, this is necessary since
590 // GC can be forced at any point in the sweeping cycle.
592 // We check the transition condition continuously here in case
593 // this G gets delayed in to the next GC cycle.
594 for trigger.test() && sweepone() != ^uintptr(0) {
598 // Perform GC initialization and the sweep termination
600 semacquire(&work.startSema)
601 // Re-check transition condition under transition lock.
603 semrelease(&work.startSema)
607 // For stats, check if this GC was forced by the user.
608 work.userForced = trigger.kind == gcTriggerCycle
610 // In gcstoptheworld debug mode, upgrade the mode accordingly.
611 // We do this after re-checking the transition condition so
612 // that multiple goroutines that detect the heap trigger don't
613 // start multiple STW GCs.
614 mode := gcBackgroundMode
615 if debug.gcstoptheworld == 1 {
617 } else if debug.gcstoptheworld == 2 {
618 mode = gcForceBlockMode
621 // Ok, we're doing it! Stop everybody else
623 semacquire(&worldsema)
629 // Check that all Ps have finished deferred mcache flushes.
630 for _, p := range allp {
631 if fg := atomic.Load(&p.mcache.flushGen); fg != mheap_.sweepgen {
632 println("runtime: p", p.id, "flushGen", fg, "!= sweepgen", mheap_.sweepgen)
633 throw("p mcache not flushed")
637 gcBgMarkStartWorkers()
639 systemstack(gcResetMarkState)
641 work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs
642 if work.stwprocs > ncpu {
643 // This is used to compute CPU time of the STW phases,
644 // so it can't be more than ncpu, even if GOMAXPROCS is.
647 work.heap0 = atomic.Load64(&gcController.heapLive)
652 work.tSweepTerm = now
653 work.pauseStart = now
657 systemstack(stopTheWorldWithSema)
658 // Finish sweep before we start concurrent scan.
663 // clearpools before we start the GC. If we wait they memory will not be
664 // reclaimed until the next GC cycle.
669 gcController.startCycle()
670 work.heapGoal = gcController.heapGoal
672 // In STW mode, disable scheduling of user Gs. This may also
673 // disable scheduling of this goroutine, so it may block as
674 // soon as we start the world again.
675 if mode != gcBackgroundMode {
676 schedEnableUser(false)
679 // Enter concurrent mark phase and enable
682 // Because the world is stopped, all Ps will
683 // observe that write barriers are enabled by
684 // the time we start the world and begin
687 // Write barriers must be enabled before assists are
688 // enabled because they must be enabled before
689 // any non-leaf heap objects are marked. Since
690 // allocations are blocked until assists can
691 // happen, we want enable assists as early as
695 gcBgMarkPrepare() // Must happen before assist enable.
698 // Mark all active tinyalloc blocks. Since we're
699 // allocating from these, they need to be black like
700 // other allocations. The alternative is to blacken
701 // the tiny block on every allocation from it, which
702 // would slow down the tiny allocator.
705 // At this point all Ps have enabled the write
706 // barrier, thus maintaining the no white to
707 // black invariant. Enable mutator assists to
708 // put back-pressure on fast allocating
710 atomic.Store(&gcBlackenEnabled, 1)
712 // Assists and workers can start the moment we start
714 gcController.markStartTime = now
716 // In STW mode, we could block the instant systemstack
717 // returns, so make sure we're not preemptible.
722 now = startTheWorldWithSema(trace.enabled)
723 work.pauseNS += now - work.pauseStart
725 memstats.gcPauseDist.record(now - work.pauseStart)
728 // Release the world sema before Gosched() in STW mode
729 // because we will need to reacquire it later but before
730 // this goroutine becomes runnable again, and we could
731 // self-deadlock otherwise.
732 semrelease(&worldsema)
735 // Make sure we block instead of returning to user code
737 if mode != gcBackgroundMode {
741 semrelease(&work.startSema)
744 // gcMarkDoneFlushed counts the number of P's with flushed work.
746 // Ideally this would be a captured local in gcMarkDone, but forEachP
747 // escapes its callback closure, so it can't capture anything.
749 // This is protected by markDoneSema.
750 var gcMarkDoneFlushed uint32
752 // gcMarkDone transitions the GC from mark to mark termination if all
753 // reachable objects have been marked (that is, there are no grey
754 // objects and can be no more in the future). Otherwise, it flushes
755 // all local work to the global queues where it can be discovered by
758 // This should be called when all local mark work has been drained and
759 // there are no remaining workers. Specifically, when
761 // work.nwait == work.nproc && !gcMarkWorkAvailable(p)
763 // The calling context must be preemptible.
765 // Flushing local work is important because idle Ps may have local
766 // work queued. This is the only way to make that work visible and
767 // drive GC to completion.
769 // It is explicitly okay to have write barriers in this function. If
770 // it does transition to mark termination, then all reachable objects
771 // have been marked, so the write barrier cannot shade any more
774 // Ensure only one thread is running the ragged barrier at a
776 semacquire(&work.markDoneSema)
779 // Re-check transition condition under transition lock.
781 // It's critical that this checks the global work queues are
782 // empty before performing the ragged barrier. Otherwise,
783 // there could be global work that a P could take after the P
784 // has passed the ragged barrier.
785 if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) {
786 semrelease(&work.markDoneSema)
790 // forEachP needs worldsema to execute, and we'll need it to
791 // stop the world later, so acquire worldsema now.
792 semacquire(&worldsema)
794 // Flush all local buffers and collect flushedWork flags.
795 gcMarkDoneFlushed = 0
798 // Mark the user stack as preemptible so that it may be scanned.
799 // Otherwise, our attempt to force all P's to a safepoint could
800 // result in a deadlock as we attempt to preempt a worker that's
801 // trying to preempt us (e.g. for a stack scan).
802 casgstatus(gp, _Grunning, _Gwaiting)
803 forEachP(func(_p_ *p) {
804 // Flush the write barrier buffer, since this may add
805 // work to the gcWork.
808 // Flush the gcWork, since this may create global work
809 // and set the flushedWork flag.
811 // TODO(austin): Break up these workbufs to
812 // better distribute work.
814 // Collect the flushedWork flag.
815 if _p_.gcw.flushedWork {
816 atomic.Xadd(&gcMarkDoneFlushed, 1)
817 _p_.gcw.flushedWork = false
820 casgstatus(gp, _Gwaiting, _Grunning)
823 if gcMarkDoneFlushed != 0 {
824 // More grey objects were discovered since the
825 // previous termination check, so there may be more
826 // work to do. Keep going. It's possible the
827 // transition condition became true again during the
828 // ragged barrier, so re-check it.
829 semrelease(&worldsema)
833 // There was no global work, no local work, and no Ps
834 // communicated work since we took markDoneSema. Therefore
835 // there are no grey objects and no more objects can be
836 // shaded. Transition to mark termination.
839 work.pauseStart = now
840 getg().m.preemptoff = "gcing"
844 systemstack(stopTheWorldWithSema)
845 // The gcphase is _GCmark, it will transition to _GCmarktermination
846 // below. The important thing is that the wb remains active until
847 // all marking is complete. This includes writes made by the GC.
849 // There is sometimes work left over when we enter mark termination due
850 // to write barriers performed after the completion barrier above.
851 // Detect this and resume concurrent mark. This is obviously
854 // See issue #27993 for details.
856 // Switch to the system stack to call wbBufFlush1, though in this case
857 // it doesn't matter because we're non-preemptible anyway.
860 for _, p := range allp {
869 getg().m.preemptoff = ""
871 now := startTheWorldWithSema(true)
872 work.pauseNS += now - work.pauseStart
873 memstats.gcPauseDist.record(now - work.pauseStart)
875 semrelease(&worldsema)
879 // Disable assists and background workers. We must do
880 // this before waking blocked assists.
881 atomic.Store(&gcBlackenEnabled, 0)
883 // Wake all blocked assists. These will run when we
884 // start the world again.
887 // Likewise, release the transition lock. Blocked
888 // workers and assists will run when we start the
890 semrelease(&work.markDoneSema)
892 // In STW mode, re-enable user goroutines. These will be
893 // queued to run after we start the world.
894 schedEnableUser(true)
896 // endCycle depends on all gcWork cache stats being flushed.
897 // The termination algorithm above ensured that up to
898 // allocations since the ragged barrier.
899 nextTriggerRatio := gcController.endCycle(work.userForced)
901 // Perform mark termination. This will restart the world.
902 gcMarkTermination(nextTriggerRatio)
905 // World must be stopped and mark assists and background workers must be
907 func gcMarkTermination(nextTriggerRatio float64) {
908 // Start marktermination (write barrier remains enabled for now).
909 setGCPhase(_GCmarktermination)
911 work.heap1 = gcController.heapLive
912 startTime := nanotime()
915 mp.preemptoff = "gcing"
919 casgstatus(gp, _Grunning, _Gwaiting)
920 gp.waitreason = waitReasonGarbageCollection
922 // Run gc on the g0 stack. We do this so that the g stack
923 // we're currently running on will no longer change. Cuts
924 // the root set down a bit (g0 stacks are not scanned, and
925 // we don't need to scan gc's internal state). We also
926 // need to switch to g0 so we can shrink the stack.
929 // Must return immediately.
930 // The outer function's stack may have moved
931 // during gcMark (it shrinks stacks, including the
932 // outer function's stack), so we must not refer
933 // to any of its variables. Return back to the
934 // non-system stack to pick up the new addresses
935 // before continuing.
939 work.heap2 = work.bytesMarked
940 if debug.gccheckmark > 0 {
941 // Run a full non-parallel, stop-the-world
942 // mark using checkmark bits, to check that we
943 // didn't forget to mark anything during the
944 // concurrent mark process.
947 gcw := &getg().m.p.ptr().gcw
949 wbBufFlush1(getg().m.p.ptr())
954 // marking is complete so we can turn the write barrier off
960 casgstatus(gp, _Gwaiting, _Grunning)
969 if gcphase != _GCoff {
970 throw("gc done but gcphase != _GCoff")
973 // Record heapGoal and heap_inuse for scavenger.
974 gcController.lastHeapGoal = gcController.heapGoal
975 memstats.last_heap_inuse = memstats.heap_inuse
977 // Update GC trigger and pacing for the next cycle.
978 gcController.commit(nextTriggerRatio)
980 // Update timing memstats
982 sec, nsec, _ := time_now()
983 unixNow := sec*1e9 + int64(nsec)
984 work.pauseNS += now - work.pauseStart
986 memstats.gcPauseDist.record(now - work.pauseStart)
987 atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user
988 atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us
989 memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS)
990 memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow)
991 memstats.pause_total_ns += uint64(work.pauseNS)
993 // Update work.totaltime.
994 sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm)
995 // We report idle marking time below, but omit it from the
996 // overall utilization here since it's "free".
997 markCpu := gcController.assistTime + gcController.dedicatedMarkTime + gcController.fractionalMarkTime
998 markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm)
999 cycleCpu := sweepTermCpu + markCpu + markTermCpu
1000 work.totaltime += cycleCpu
1002 // Compute overall GC CPU utilization.
1003 totalCpu := sched.totaltime + (now-sched.procresizetime)*int64(gomaxprocs)
1004 memstats.gc_cpu_fraction = float64(work.totaltime) / float64(totalCpu)
1006 // Reset sweep state.
1008 sweep.npausesweep = 0
1010 if work.userForced {
1011 memstats.numforcedgc++
1014 // Bump GC cycle count and wake goroutines waiting on sweep.
1015 lock(&work.sweepWaiters.lock)
1017 injectglist(&work.sweepWaiters.list)
1018 unlock(&work.sweepWaiters.lock)
1020 // Finish the current heap profiling cycle and start a new
1021 // heap profiling cycle. We do this before starting the world
1022 // so events don't leak into the wrong cycle.
1025 // There may be stale spans in mcaches that need to be swept.
1026 // Those aren't tracked in any sweep lists, so we need to
1027 // count them against sweep completion until we ensure all
1028 // those spans have been forced out.
1029 sl := newSweepLocker()
1030 sl.blockCompletion()
1032 systemstack(func() { startTheWorldWithSema(true) })
1034 // Flush the heap profile so we can start a new cycle next GC.
1035 // This is relatively expensive, so we don't do it with the
1039 // Prepare workbufs for freeing by the sweeper. We do this
1040 // asynchronously because it can take non-trivial time.
1041 prepareFreeWorkbufs()
1043 // Free stack spans. This must be done between GC cycles.
1044 systemstack(freeStackSpans)
1046 // Ensure all mcaches are flushed. Each P will flush its own
1047 // mcache before allocating, but idle Ps may not. Since this
1048 // is necessary to sweep all spans, we need to ensure all
1049 // mcaches are flushed before we start the next GC cycle.
1050 systemstack(func() {
1051 forEachP(func(_p_ *p) {
1052 _p_.mcache.prepareForSweep()
1055 // Now that we've swept stale spans in mcaches, they don't
1056 // count against unswept spans.
1059 // Print gctrace before dropping worldsema. As soon as we drop
1060 // worldsema another cycle could start and smash the stats
1061 // we're trying to print.
1062 if debug.gctrace > 0 {
1063 util := int(memstats.gc_cpu_fraction * 100)
1067 print("gc ", memstats.numgc,
1068 " @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
1070 prev := work.tSweepTerm
1071 for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} {
1075 print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev))))
1078 print(" ms clock, ")
1079 for i, ns := range []int64{sweepTermCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} {
1080 if i == 2 || i == 3 {
1081 // Separate mark time components with /.
1086 print(string(fmtNSAsMS(sbuf[:], uint64(ns))))
1089 work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ",
1090 work.heapGoal>>20, " MB goal, ",
1091 work.maxprocs, " P")
1092 if work.userForced {
1099 semrelease(&worldsema)
1101 // Careful: another GC cycle may start now.
1106 // now that gc is done, kick off finalizer thread if needed
1107 if !concurrentSweep {
1108 // give the queued finalizers, if any, a chance to run
1113 // gcBgMarkStartWorkers prepares background mark worker goroutines. These
1114 // goroutines will not run until the mark phase, but they must be started while
1115 // the work is not stopped and from a regular G stack. The caller must hold
1117 func gcBgMarkStartWorkers() {
1118 // Background marking is performed by per-P G's. Ensure that each P has
1119 // a background GC G.
1121 // Worker Gs don't exit if gomaxprocs is reduced. If it is raised
1122 // again, we can reuse the old workers; no need to create new workers.
1123 for gcBgMarkWorkerCount < gomaxprocs {
1126 notetsleepg(&work.bgMarkReady, -1)
1127 noteclear(&work.bgMarkReady)
1128 // The worker is now guaranteed to be added to the pool before
1129 // its P's next findRunnableGCWorker.
1131 gcBgMarkWorkerCount++
1135 // gcBgMarkPrepare sets up state for background marking.
1136 // Mutator assists must not yet be enabled.
1137 func gcBgMarkPrepare() {
1138 // Background marking will stop when the work queues are empty
1139 // and there are no more workers (note that, since this is
1140 // concurrent, this may be a transient state, but mark
1141 // termination will clean it up). Between background workers
1142 // and assists, we don't really know how many workers there
1143 // will be, so we pretend to have an arbitrarily large number
1144 // of workers, almost all of which are "waiting". While a
1145 // worker is working it decrements nwait. If nproc == nwait,
1146 // there are no workers.
1147 work.nproc = ^uint32(0)
1148 work.nwait = ^uint32(0)
1151 // gcBgMarkWorker is an entry in the gcBgMarkWorkerPool. It points to a single
1152 // gcBgMarkWorker goroutine.
1153 type gcBgMarkWorkerNode struct {
1154 // Unused workers are managed in a lock-free stack. This field must be first.
1157 // The g of this worker.
1160 // Release this m on park. This is used to communicate with the unlock
1161 // function, which cannot access the G's stack. It is unused outside of
1162 // gcBgMarkWorker().
1166 func gcBgMarkWorker() {
1169 // We pass node to a gopark unlock function, so it can't be on
1170 // the stack (see gopark). Prevent deadlock from recursively
1171 // starting GC by disabling preemption.
1172 gp.m.preemptoff = "GC worker init"
1173 node := new(gcBgMarkWorkerNode)
1174 gp.m.preemptoff = ""
1178 node.m.set(acquirem())
1179 notewakeup(&work.bgMarkReady)
1180 // After this point, the background mark worker is generally scheduled
1181 // cooperatively by gcController.findRunnableGCWorker. While performing
1182 // work on the P, preemption is disabled because we are working on
1183 // P-local work buffers. When the preempt flag is set, this puts itself
1184 // into _Gwaiting to be woken up by gcController.findRunnableGCWorker
1185 // at the appropriate time.
1187 // When preemption is enabled (e.g., while in gcMarkDone), this worker
1188 // may be preempted and schedule as a _Grunnable G from a runq. That is
1189 // fine; it will eventually gopark again for further scheduling via
1190 // findRunnableGCWorker.
1192 // Since we disable preemption before notifying bgMarkReady, we
1193 // guarantee that this G will be in the worker pool for the next
1194 // findRunnableGCWorker. This isn't strictly necessary, but it reduces
1195 // latency between _GCmark starting and the workers starting.
1198 // Go to sleep until woken by
1199 // gcController.findRunnableGCWorker.
1200 gopark(func(g *g, nodep unsafe.Pointer) bool {
1201 node := (*gcBgMarkWorkerNode)(nodep)
1203 if mp := node.m.ptr(); mp != nil {
1204 // The worker G is no longer running; release
1207 // N.B. it is _safe_ to release the M as soon
1208 // as we are no longer performing P-local mark
1211 // However, since we cooperatively stop work
1212 // when gp.preempt is set, if we releasem in
1213 // the loop then the following call to gopark
1214 // would immediately preempt the G. This is
1215 // also safe, but inefficient: the G must
1216 // schedule again only to enter gopark and park
1217 // again. Thus, we defer the release until
1218 // after parking the G.
1222 // Release this G to the pool.
1223 gcBgMarkWorkerPool.push(&node.node)
1224 // Note that at this point, the G may immediately be
1225 // rescheduled and may be running.
1227 }, unsafe.Pointer(node), waitReasonGCWorkerIdle, traceEvGoBlock, 0)
1229 // Preemption must not occur here, or another G might see
1230 // p.gcMarkWorkerMode.
1232 // Disable preemption so we can use the gcw. If the
1233 // scheduler wants to preempt us, we'll stop draining,
1234 // dispose the gcw, and then preempt.
1235 node.m.set(acquirem())
1236 pp := gp.m.p.ptr() // P can't change with preemption disabled.
1238 if gcBlackenEnabled == 0 {
1239 println("worker mode", pp.gcMarkWorkerMode)
1240 throw("gcBgMarkWorker: blackening not enabled")
1243 if pp.gcMarkWorkerMode == gcMarkWorkerNotWorker {
1244 throw("gcBgMarkWorker: mode not set")
1247 startTime := nanotime()
1248 pp.gcMarkWorkerStartTime = startTime
1250 decnwait := atomic.Xadd(&work.nwait, -1)
1251 if decnwait == work.nproc {
1252 println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
1253 throw("work.nwait was > work.nproc")
1256 systemstack(func() {
1257 // Mark our goroutine preemptible so its stack
1258 // can be scanned. This lets two mark workers
1259 // scan each other (otherwise, they would
1260 // deadlock). We must not modify anything on
1261 // the G stack. However, stack shrinking is
1262 // disabled for mark workers, so it is safe to
1263 // read from the G stack.
1264 casgstatus(gp, _Grunning, _Gwaiting)
1265 switch pp.gcMarkWorkerMode {
1267 throw("gcBgMarkWorker: unexpected gcMarkWorkerMode")
1268 case gcMarkWorkerDedicatedMode:
1269 gcDrain(&pp.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
1271 // We were preempted. This is
1272 // a useful signal to kick
1273 // everything out of the run
1274 // queue so it can run
1276 if drainQ, n := runqdrain(pp); n > 0 {
1278 globrunqputbatch(&drainQ, int32(n))
1282 // Go back to draining, this time
1283 // without preemption.
1284 gcDrain(&pp.gcw, gcDrainFlushBgCredit)
1285 case gcMarkWorkerFractionalMode:
1286 gcDrain(&pp.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
1287 case gcMarkWorkerIdleMode:
1288 gcDrain(&pp.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
1290 casgstatus(gp, _Gwaiting, _Grunning)
1293 // Account for time.
1294 duration := nanotime() - startTime
1295 switch pp.gcMarkWorkerMode {
1296 case gcMarkWorkerDedicatedMode:
1297 atomic.Xaddint64(&gcController.dedicatedMarkTime, duration)
1298 atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1)
1299 case gcMarkWorkerFractionalMode:
1300 atomic.Xaddint64(&gcController.fractionalMarkTime, duration)
1301 atomic.Xaddint64(&pp.gcFractionalMarkTime, duration)
1302 case gcMarkWorkerIdleMode:
1303 atomic.Xaddint64(&gcController.idleMarkTime, duration)
1306 // Was this the last worker and did we run out
1308 incnwait := atomic.Xadd(&work.nwait, +1)
1309 if incnwait > work.nproc {
1310 println("runtime: p.gcMarkWorkerMode=", pp.gcMarkWorkerMode,
1311 "work.nwait=", incnwait, "work.nproc=", work.nproc)
1312 throw("work.nwait > work.nproc")
1315 // We'll releasem after this point and thus this P may run
1316 // something else. We must clear the worker mode to avoid
1317 // attributing the mode to a different (non-worker) G in
1319 pp.gcMarkWorkerMode = gcMarkWorkerNotWorker
1321 // If this worker reached a background mark completion
1322 // point, signal the main GC goroutine.
1323 if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
1324 // We don't need the P-local buffers here, allow
1325 // preemption becuse we may schedule like a regular
1326 // goroutine in gcMarkDone (block on locks, etc).
1327 releasem(node.m.ptr())
1335 // gcMarkWorkAvailable reports whether executing a mark worker
1336 // on p is potentially useful. p may be nil, in which case it only
1337 // checks the global sources of work.
1338 func gcMarkWorkAvailable(p *p) bool {
1339 if p != nil && !p.gcw.empty() {
1342 if !work.full.empty() {
1343 return true // global work available
1345 if work.markrootNext < work.markrootJobs {
1346 return true // root scan work available
1351 // gcMark runs the mark (or, for concurrent GC, mark termination)
1352 // All gcWork caches must be empty.
1353 // STW is in effect at this point.
1354 func gcMark(startTime int64) {
1355 if debug.allocfreetrace > 0 {
1359 if gcphase != _GCmarktermination {
1360 throw("in gcMark expecting to see gcphase as _GCmarktermination")
1362 work.tstart = startTime
1364 // Check that there's no marking work remaining.
1365 if work.full != 0 || work.markrootNext < work.markrootJobs {
1366 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")
1367 panic("non-empty mark queue after concurrent mark")
1370 if debug.gccheckmark > 0 {
1371 // This is expensive when there's a large number of
1372 // Gs, so only do it if checkmark is also enabled.
1376 throw("work.full != 0")
1379 // Clear out buffers and double-check that all gcWork caches
1380 // are empty. This should be ensured by gcMarkDone before we
1381 // enter mark termination.
1383 // TODO: We could clear out buffers just before mark if this
1384 // has a non-negligible impact on STW time.
1385 for _, p := range allp {
1386 // The write barrier may have buffered pointers since
1387 // the gcMarkDone barrier. However, since the barrier
1388 // ensured all reachable objects were marked, all of
1389 // these must be pointers to black objects. Hence we
1390 // can just discard the write barrier buffer.
1391 if debug.gccheckmark > 0 {
1392 // For debugging, flush the buffer and make
1393 // sure it really was all marked.
1402 print("runtime: P ", p.id, " flushedWork ", gcw.flushedWork)
1403 if gcw.wbuf1 == nil {
1404 print(" wbuf1=<nil>")
1406 print(" wbuf1.n=", gcw.wbuf1.nobj)
1408 if gcw.wbuf2 == nil {
1409 print(" wbuf2=<nil>")
1411 print(" wbuf2.n=", gcw.wbuf2.nobj)
1414 throw("P has cached GC work at end of mark termination")
1416 // There may still be cached empty buffers, which we
1417 // need to flush since we're going to free them. Also,
1418 // there may be non-zero stats because we allocated
1419 // black after the gcMarkDone barrier.
1423 // Update the marked heap stat.
1424 gcController.heapMarked = work.bytesMarked
1426 // Flush scanAlloc from each mcache since we're about to modify
1427 // heapScan directly. If we were to flush this later, then scanAlloc
1428 // might have incorrect information.
1429 for _, p := range allp {
1434 gcController.heapScan += uint64(c.scanAlloc)
1438 // Update other GC heap size stats. This must happen after
1439 // cachestats (which flushes local statistics to these) and
1440 // flushallmcaches (which modifies gcController.heapLive).
1441 gcController.heapLive = work.bytesMarked
1442 gcController.heapScan = uint64(gcController.scanWork)
1449 // gcSweep must be called on the system stack because it acquires the heap
1450 // lock. See mheap for details.
1452 // The world must be stopped.
1455 func gcSweep(mode gcMode) {
1456 assertWorldStopped()
1458 if gcphase != _GCoff {
1459 throw("gcSweep being done but phase is not GCoff")
1463 mheap_.sweepgen += 2
1464 mheap_.sweepDrained = 0
1465 mheap_.pagesSwept = 0
1466 mheap_.sweepArenas = mheap_.allArenas
1467 mheap_.reclaimIndex = 0
1468 mheap_.reclaimCredit = 0
1469 unlock(&mheap_.lock)
1471 sweep.centralIndex.clear()
1473 if !_ConcurrentSweep || mode == gcForceBlockMode {
1474 // Special case synchronous sweep.
1475 // Record that no proportional sweeping has to happen.
1477 mheap_.sweepPagesPerByte = 0
1478 unlock(&mheap_.lock)
1479 // Sweep all spans eagerly.
1480 for sweepone() != ^uintptr(0) {
1483 // Free workbufs eagerly.
1484 prepareFreeWorkbufs()
1485 for freeSomeWbufs(false) {
1487 // All "free" events for this mark/sweep cycle have
1488 // now happened, so we can make this profile cycle
1489 // available immediately.
1495 // Background sweep.
1498 sweep.parked = false
1499 ready(sweep.g, 0, true)
1504 // gcResetMarkState resets global state prior to marking (concurrent
1505 // or STW) and resets the stack scan state of all Gs.
1507 // This is safe to do without the world stopped because any Gs created
1508 // during or after this will start out in the reset state.
1510 // gcResetMarkState must be called on the system stack because it acquires
1511 // the heap lock. See mheap for details.
1514 func gcResetMarkState() {
1515 // This may be called during a concurrent phase, so lock to make sure
1516 // allgs doesn't change.
1517 forEachG(func(gp *g) {
1518 gp.gcscandone = false // set to true in gcphasework
1519 gp.gcAssistBytes = 0
1522 // Clear page marks. This is just 1MB per 64GB of heap, so the
1523 // time here is pretty trivial.
1525 arenas := mheap_.allArenas
1526 unlock(&mheap_.lock)
1527 for _, ai := range arenas {
1528 ha := mheap_.arenas[ai.l1()][ai.l2()]
1529 for i := range ha.pageMarks {
1534 work.bytesMarked = 0
1535 work.initialHeapLive = atomic.Load64(&gcController.heapLive)
1538 // Hooks for other packages
1540 var poolcleanup func()
1542 //go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup
1543 func sync_runtime_registerPoolCleanup(f func()) {
1549 if poolcleanup != nil {
1553 // Clear central sudog cache.
1554 // Leave per-P caches alone, they have strictly bounded size.
1555 // Disconnect cached list before dropping it on the floor,
1556 // so that a dangling ref to one entry does not pin all of them.
1557 lock(&sched.sudoglock)
1558 var sg, sgnext *sudog
1559 for sg = sched.sudogcache; sg != nil; sg = sgnext {
1563 sched.sudogcache = nil
1564 unlock(&sched.sudoglock)
1566 // Clear central defer pools.
1567 // Leave per-P pools alone, they have strictly bounded size.
1568 lock(&sched.deferlock)
1569 for i := range sched.deferpool {
1570 // disconnect cached list before dropping it on the floor,
1571 // so that a dangling ref to one entry does not pin all of them.
1572 var d, dlink *_defer
1573 for d = sched.deferpool[i]; d != nil; d = dlink {
1577 sched.deferpool[i] = nil
1579 unlock(&sched.deferlock)
1584 // itoaDiv formats val/(10**dec) into buf.
1585 func itoaDiv(buf []byte, val uint64, dec int) []byte {
1588 for val >= 10 || i >= idec {
1589 buf[i] = byte(val%10 + '0')
1597 buf[i] = byte(val + '0')
1601 // fmtNSAsMS nicely formats ns nanoseconds as milliseconds.
1602 func fmtNSAsMS(buf []byte, ns uint64) []byte {
1604 // Format as whole milliseconds.
1605 return itoaDiv(buf, ns/1e6, 0)
1607 // Format two digits of precision, with at most three decimal places.
1618 return itoaDiv(buf, x, dec)
1621 // Helpers for testing GC.
1623 // gcTestMoveStackOnNextCall causes the stack to be moved on a call
1624 // immediately following the call to this. It may not work correctly
1625 // if any other work appears after this call (such as returning).
1626 // Typically the following call should be marked go:noinline so it
1627 // performs a stack check.
1629 // In rare cases this may not cause the stack to move, specifically if
1630 // there's a preemption between this call and the next.
1631 func gcTestMoveStackOnNextCall() {
1633 gp.stackguard0 = stackForceMove
1636 // gcTestIsReachable performs a GC and returns a bit set where bit i
1637 // is set if ptrs[i] is reachable.
1638 func gcTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) {
1639 // This takes the pointers as unsafe.Pointers in order to keep
1640 // them live long enough for us to attach specials. After
1641 // that, we drop our references to them.
1644 panic("too many pointers for uint64 mask")
1647 // Block GC while we attach specials and drop our references
1648 // to ptrs. Otherwise, if a GC is in progress, it could mark
1649 // them reachable via this function before we have a chance to
1653 // Create reachability specials for ptrs.
1654 specials := make([]*specialReachable, len(ptrs))
1655 for i, p := range ptrs {
1656 lock(&mheap_.speciallock)
1657 s := (*specialReachable)(mheap_.specialReachableAlloc.alloc())
1658 unlock(&mheap_.speciallock)
1659 s.special.kind = _KindSpecialReachable
1660 if !addspecial(p, &s.special) {
1661 throw("already have a reachable special (duplicate pointer?)")
1664 // Make sure we don't retain ptrs.
1670 // Force a full GC and sweep.
1673 // Process specials.
1674 for i, s := range specials {
1677 println("runtime: object", i, "was not swept")
1678 throw("IsReachable failed")
1683 lock(&mheap_.speciallock)
1684 mheap_.specialReachableAlloc.free(unsafe.Pointer(s))
1685 unlock(&mheap_.speciallock)
1691 // gcTestPointerClass returns the category of what p points to, one of:
1692 // "heap", "stack", "data", "bss", "other". This is useful for checking
1693 // that a test is doing what it's intended to do.
1695 // This is nosplit simply to avoid extra pointer shuffling that may
1696 // complicate a test.
1699 func gcTestPointerClass(p unsafe.Pointer) string {
1700 p2 := uintptr(noescape(p))
1702 if gp.stack.lo <= p2 && p2 < gp.stack.hi {
1705 if base, _, _ := findObject(p2, 0, 0); base != 0 {
1708 for _, datap := range activeModules() {
1709 if datap.data <= p2 && p2 < datap.edata || datap.noptrdata <= p2 && p2 < datap.enoptrdata {
1712 if datap.bss <= p2 && p2 < datap.ebss || datap.noptrbss <= p2 && p2 <= datap.enoptrbss {