// Copyright 2009 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. // Garbage collector (GC). // // The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple // GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is // non-generational and non-compacting. Allocation is done using size segregated per P allocation // areas to minimize fragmentation while eliminating locks in the common case. // // The algorithm decomposes into several steps. // This is a high level description of the algorithm being used. For an overview of GC a good // place to start is Richard Jones' gchandbook.org. // // The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see // Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978. // On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978), // 966-975. // For journal quality proofs that these steps are complete, correct, and terminate see // Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world. // Concurrency and Computation: Practice and Experience 15(3-5), 2003. // // 1. GC performs sweep termination. // // a. Stop the world. This causes all Ps to reach a GC safe-point. // // b. Sweep any unswept spans. There will only be unswept spans if // this GC cycle was forced before the expected time. // // 2. GC performs the "mark 1" sub-phase. In this sub-phase, Ps are // allowed to locally cache parts of the work queue. // // a. Prepare for the mark phase by setting gcphase to _GCmark // (from _GCoff), enabling the write barrier, enabling mutator // assists, and enqueueing root mark jobs. No objects may be // scanned until all Ps have enabled the write barrier, which is // accomplished using STW. // // b. Start the world. From this point, GC work is done by mark // workers started by the scheduler and by assists performed as // part of allocation. The write barrier shades both the // overwritten pointer and the new pointer value for any pointer // writes (see mbarrier.go for details). Newly allocated objects // are immediately marked black. // // c. GC performs root marking jobs. This includes scanning all // stacks, shading all globals, and shading any heap pointers in // off-heap runtime data structures. Scanning a stack stops a // goroutine, shades any pointers found on its stack, and then // resumes the goroutine. // // d. GC drains the work queue of grey objects, scanning each grey // object to black and shading all pointers found in the object // (which in turn may add those pointers to the work queue). // // 3. Once the global work queue is empty (but local work queue caches // may still contain work), GC performs the "mark 2" sub-phase. // // a. GC stops all workers, disables local work queue caches, // flushes each P's local work queue cache to the global work queue // cache, and reenables workers. // // b. GC again drains the work queue, as in 2d above. // // 4. Once the work queue is empty, GC performs mark termination. // // a. Stop the world. // // b. Set gcphase to _GCmarktermination, and disable workers and // assists. // // c. Drain any remaining work from the work queue (typically there // will be none). // // d. Perform other housekeeping like flushing mcaches. // // 5. GC performs the sweep phase. // // a. Prepare for the sweep phase by setting gcphase to _GCoff, // setting up sweep state and disabling the write barrier. // // b. Start the world. From this point on, newly allocated objects // are white, and allocating sweeps spans before use if necessary. // // c. GC does concurrent sweeping in the background and in response // to allocation. See description below. // // 6. When sufficient allocation has taken place, replay the sequence // starting with 1 above. See discussion of GC rate below. // Concurrent sweep. // // The sweep phase proceeds concurrently with normal program execution. // The heap is swept span-by-span both lazily (when a goroutine needs another span) // and concurrently in a background goroutine (this helps programs that are not CPU bound). // At the end of STW mark termination all spans are marked as "needs sweeping". // // The background sweeper goroutine simply sweeps spans one-by-one. // // To avoid requesting more OS memory while there are unswept spans, when a // goroutine needs another span, it first attempts to reclaim that much memory // by sweeping. When a goroutine needs to allocate a new small-object span, it // sweeps small-object spans for the same object size until it frees at least // one object. When a goroutine needs to allocate large-object span from heap, // it sweeps spans until it frees at least that many pages into heap. There is // one case where this may not suffice: if a goroutine sweeps and frees two // nonadjacent one-page spans to the heap, it will allocate a new two-page // span, but there can still be other one-page unswept spans which could be // combined into a two-page span. // // It's critical to ensure that no operations proceed on unswept spans (that would corrupt // mark bits in GC bitmap). During GC all mcaches are flushed into the central cache, // so they are empty. When a goroutine grabs a new span into mcache, it sweeps it. // When a goroutine explicitly frees an object or sets a finalizer, it ensures that // the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish). // The finalizer goroutine is kicked off only when all spans are swept. // When the next GC starts, it sweeps all not-yet-swept spans (if any). // GC rate. // Next GC is after we've allocated an extra amount of memory proportional to // the amount already in use. The proportion is controlled by GOGC environment variable // (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M // (this mark is tracked in next_gc variable). This keeps the GC cost in linear // proportion to the allocation cost. Adjusting GOGC just changes the linear constant // (and also the amount of extra memory used). // Oblets // // In order to prevent long pauses while scanning large objects and to // improve parallelism, the garbage collector breaks up scan jobs for // objects larger than maxObletBytes into "oblets" of at most // maxObletBytes. When scanning encounters the beginning of a large // object, it scans only the first oblet and enqueues the remaining // oblets as new scan jobs. package runtime import ( "runtime/internal/atomic" "runtime/internal/sys" "unsafe" ) const ( _DebugGC = 0 _ConcurrentSweep = true _FinBlockSize = 4 * 1024 // sweepMinHeapDistance is a lower bound on the heap distance // (in bytes) reserved for concurrent sweeping between GC // cycles. This will be scaled by gcpercent/100. sweepMinHeapDistance = 1024 * 1024 ) // heapminimum is the minimum heap size at which to trigger GC. // For small heaps, this overrides the usual GOGC*live set rule. // // When there is a very small live set but a lot of allocation, simply // collecting when the heap reaches GOGC*live results in many GC // cycles and high total per-GC overhead. This minimum amortizes this // per-GC overhead while keeping the heap reasonably small. // // During initialization this is set to 4MB*GOGC/100. In the case of // GOGC==0, this will set heapminimum to 0, resulting in constant // collection even when the heap size is small, which is useful for // debugging. var heapminimum uint64 = defaultHeapMinimum // defaultHeapMinimum is the value of heapminimum for GOGC==100. const defaultHeapMinimum = 4 << 20 // Initialized from $GOGC. GOGC=off means no GC. var gcpercent int32 func gcinit() { if unsafe.Sizeof(workbuf{}) != _WorkbufSize { throw("size of Workbuf is suboptimal") } // No sweep on the first cycle. mheap_.sweepdone = 1 // Set a reasonable initial GC trigger. memstats.triggerRatio = 7 / 8.0 // Fake a heap_marked value so it looks like a trigger at // heapminimum is the appropriate growth from heap_marked. // This will go into computing the initial GC goal. memstats.heap_marked = uint64(float64(heapminimum) / (1 + memstats.triggerRatio)) // Set gcpercent from the environment. This will also compute // and set the GC trigger and goal. _ = setGCPercent(readgogc()) work.startSema = 1 work.markDoneSema = 1 } func readgogc() int32 { p := gogetenv("GOGC") if p == "off" { return -1 } if n, ok := atoi32(p); ok { return n } return 100 } // gcenable is called after the bulk of the runtime initialization, // just before we're about to start letting user code run. // It kicks off the background sweeper goroutine and enables GC. func gcenable() { c := make(chan int, 1) go bgsweep(c) <-c memstats.enablegc = true // now that runtime is initialized, GC is okay } //go:linkname setGCPercent runtime/debug.setGCPercent func setGCPercent(in int32) (out int32) { lock(&mheap_.lock) out = gcpercent if in < 0 { in = -1 } gcpercent = in heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100 // Update pacing in response to gcpercent change. gcSetTriggerRatio(memstats.triggerRatio) unlock(&mheap_.lock) // If we just disabled GC, wait for any concurrent GC to // finish so we always return with no GC running. if in < 0 { // Disable phase transitions. lock(&work.sweepWaiters.lock) if gcphase == _GCmark { // GC is active. Wait until we reach sweeping. gp := getg() gp.schedlink = work.sweepWaiters.head work.sweepWaiters.head.set(gp) goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1) } else { // GC isn't active. unlock(&work.sweepWaiters.lock) } } return out } // Garbage collector phase. // Indicates to write barrier and synchronization task to perform. var gcphase uint32 // The compiler knows about this variable. // If you change it, you must change builtin/runtime.go, too. // If you change the first four bytes, you must also change the write // barrier insertion code. var writeBarrier struct { enabled bool // compiler emits a check of this before calling write barrier pad [3]byte // compiler uses 32-bit load for "enabled" field needed bool // whether we need a write barrier for current GC phase cgo bool // whether we need a write barrier for a cgo check alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load } // gcBlackenEnabled is 1 if mutator assists and background mark // workers are allowed to blacken objects. This must only be set when // gcphase == _GCmark. var gcBlackenEnabled uint32 // gcBlackenPromptly indicates that optimizations that may // hide work from the global work queue should be disabled. // // If gcBlackenPromptly is true, per-P gcWork caches should // be flushed immediately and new objects should be allocated black. // // There is a tension between allocating objects white and // allocating them black. If white and the objects die before being // marked they can be collected during this GC cycle. On the other // hand allocating them black will reduce _GCmarktermination latency // since more work is done in the mark phase. This tension is resolved // by allocating white until the mark phase is approaching its end and // then allocating black for the remainder of the mark phase. var gcBlackenPromptly bool const ( _GCoff = iota // GC not running; sweeping in background, write barrier disabled _GCmark // GC marking roots and workbufs: allocate black, write barrier ENABLED _GCmarktermination // GC mark termination: allocate black, P's help GC, write barrier ENABLED ) //go:nosplit func setGCPhase(x uint32) { atomic.Store(&gcphase, x) writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo } // gcMarkWorkerMode represents the mode that a concurrent mark worker // should operate in. // // Concurrent marking happens through four different mechanisms. One // is mutator assists, which happen in response to allocations and are // not scheduled. The other three are variations in the per-P mark // workers and are distinguished by gcMarkWorkerMode. type gcMarkWorkerMode int const ( // gcMarkWorkerDedicatedMode indicates that the P of a mark // worker is dedicated to running that mark worker. The mark // worker should run without preemption. gcMarkWorkerDedicatedMode gcMarkWorkerMode = iota // gcMarkWorkerFractionalMode indicates that a P is currently // running the "fractional" mark worker. The fractional worker // is necessary when GOMAXPROCS*gcBackgroundUtilization is not // an integer. The fractional worker should run until it is // preempted and will be scheduled to pick up the fractional // part of GOMAXPROCS*gcBackgroundUtilization. gcMarkWorkerFractionalMode // gcMarkWorkerIdleMode indicates that a P is running the mark // worker because it has nothing else to do. The idle worker // should run until it is preempted and account its time // against gcController.idleMarkTime. gcMarkWorkerIdleMode ) // gcMarkWorkerModeStrings are the strings labels of gcMarkWorkerModes // to use in execution traces. var gcMarkWorkerModeStrings = [...]string{ "GC (dedicated)", "GC (fractional)", "GC (idle)", } // gcController implements the GC pacing controller that determines // when to trigger concurrent garbage collection and how much marking // work to do in mutator assists and background marking. // // It uses a feedback control algorithm to adjust the memstats.gc_trigger // trigger based on the heap growth and GC CPU utilization each cycle. // This algorithm optimizes for heap growth to match GOGC and for CPU // utilization between assist and background marking to be 25% of // GOMAXPROCS. The high-level design of this algorithm is documented // at https://golang.org/s/go15gcpacing. // // All fields of gcController are used only during a single mark // cycle. var gcController gcControllerState type gcControllerState struct { // scanWork is the total scan work performed this cycle. This // is updated atomically during the cycle. Updates occur in // bounded batches, since it is both written and read // throughout the cycle. At the end of the cycle, this is how // much of the retained heap is scannable. // // Currently this is the bytes of heap scanned. For most uses, // this is an opaque unit of work, but for estimation the // definition is important. scanWork int64 // bgScanCredit is the scan work credit accumulated by the // concurrent background scan. This credit is accumulated by // the background scan and stolen by mutator assists. This is // updated atomically. Updates occur in bounded batches, since // it is both written and read throughout the cycle. bgScanCredit int64 // assistTime is the nanoseconds spent in mutator assists // during this cycle. This is updated atomically. Updates // occur in bounded batches, since it is both written and read // throughout the cycle. assistTime int64 // dedicatedMarkTime is the nanoseconds spent in dedicated // mark workers during this cycle. This is updated atomically // at the end of the concurrent mark phase. dedicatedMarkTime int64 // fractionalMarkTime is the nanoseconds spent in the // fractional mark worker during this cycle. This is updated // atomically throughout the cycle and will be up-to-date if // the fractional mark worker is not currently running. fractionalMarkTime int64 // idleMarkTime is the nanoseconds spent in idle marking // during this cycle. This is updated atomically throughout // the cycle. idleMarkTime int64 // markStartTime is the absolute start time in nanoseconds // that assists and background mark workers started. markStartTime int64 // dedicatedMarkWorkersNeeded is the number of dedicated mark // workers that need to be started. This is computed at the // beginning of each cycle and decremented atomically as // dedicated mark workers get started. dedicatedMarkWorkersNeeded int64 // assistWorkPerByte is the ratio of scan work to allocated // bytes that should be performed by mutator assists. This is // computed at the beginning of each cycle and updated every // time heap_scan is updated. assistWorkPerByte float64 // assistBytesPerWork is 1/assistWorkPerByte. assistBytesPerWork float64 // fractionalUtilizationGoal is the fraction of wall clock // time that should be spent in the fractional mark worker on // each P that isn't running a dedicated worker. // // For example, if the utilization goal is 25% and there are // no dedicated workers, this will be 0.25. If there goal is // 25%, there is one dedicated worker, and GOMAXPROCS is 5, // this will be 0.05 to make up the missing 5%. // // If this is zero, no fractional workers are needed. fractionalUtilizationGoal float64 _ [sys.CacheLineSize]byte } // startCycle resets the GC controller's state and computes estimates // for a new GC cycle. The caller must hold worldsema. func (c *gcControllerState) startCycle() { c.scanWork = 0 c.bgScanCredit = 0 c.assistTime = 0 c.dedicatedMarkTime = 0 c.fractionalMarkTime = 0 c.idleMarkTime = 0 // If this is the first GC cycle or we're operating on a very // small heap, fake heap_marked so it looks like gc_trigger is // the appropriate growth from heap_marked, even though the // real heap_marked may not have a meaningful value (on the // first cycle) or may be much smaller (resulting in a large // error response). if memstats.gc_trigger <= heapminimum { memstats.heap_marked = uint64(float64(memstats.gc_trigger) / (1 + memstats.triggerRatio)) } // Re-compute the heap goal for this cycle in case something // changed. This is the same calculation we use elsewhere. memstats.next_gc = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100 if gcpercent < 0 { memstats.next_gc = ^uint64(0) } // Ensure that the heap goal is at least a little larger than // the current live heap size. This may not be the case if GC // start is delayed or if the allocation that pushed heap_live // over gc_trigger is large or if the trigger is really close to // GOGC. Assist is proportional to this distance, so enforce a // minimum distance, even if it means going over the GOGC goal // by a tiny bit. if memstats.next_gc < memstats.heap_live+1024*1024 { memstats.next_gc = memstats.heap_live + 1024*1024 } // Compute the background mark utilization goal. In general, // this may not come out exactly. We round the number of // dedicated workers so that the utilization is closest to // 25%. For small GOMAXPROCS, this would introduce too much // error, so we add fractional workers in that case. totalUtilizationGoal := float64(gomaxprocs) * gcBackgroundUtilization c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5) utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1 const maxUtilError = 0.3 if utilError < -maxUtilError || utilError > maxUtilError { // Rounding put us more than 30% off our goal. With // gcBackgroundUtilization of 25%, this happens for // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional // workers to compensate. if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal { // Too many dedicated workers. c.dedicatedMarkWorkersNeeded-- } c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(gomaxprocs) } else { c.fractionalUtilizationGoal = 0 } // Clear per-P state for _, p := range allp { p.gcAssistTime = 0 p.gcFractionalMarkTime = 0 } // Compute initial values for controls that are updated // throughout the cycle. c.revise() if debug.gcpacertrace > 0 { print("pacer: assist ratio=", c.assistWorkPerByte, " (scan ", memstats.heap_scan>>20, " MB in ", work.initialHeapLive>>20, "->", memstats.next_gc>>20, " MB)", " workers=", c.dedicatedMarkWorkersNeeded, "+", c.fractionalUtilizationGoal, "\n") } } // revise updates the assist ratio during the GC cycle to account for // improved estimates. This should be called either under STW or // whenever memstats.heap_scan, memstats.heap_live, or // memstats.next_gc is updated (with mheap_.lock held). // // It should only be called when gcBlackenEnabled != 0 (because this // is when assists are enabled and the necessary statistics are // available). func (c *gcControllerState) revise() { gcpercent := gcpercent if gcpercent < 0 { // If GC is disabled but we're running a forced GC, // act like GOGC is huge for the below calculations. gcpercent = 100000 } live := atomic.Load64(&memstats.heap_live) var heapGoal, scanWorkExpected int64 if live <= memstats.next_gc { // We're under the soft goal. Pace GC to complete at // next_gc assuming the heap is in steady-state. heapGoal = int64(memstats.next_gc) // Compute the expected scan work remaining. // // This is estimated based on the expected // steady-state scannable heap. For example, with // GOGC=100, only half of the scannable heap is // expected to be live, so that's what we target. // // (This is a float calculation to avoid overflowing on // 100*heap_scan.) scanWorkExpected = int64(float64(memstats.heap_scan) * 100 / float64(100+gcpercent)) } else { // We're past the soft goal. Pace GC so that in the // worst case it will complete by the hard goal. const maxOvershoot = 1.1 heapGoal = int64(float64(memstats.next_gc) * maxOvershoot) // Compute the upper bound on the scan work remaining. scanWorkExpected = int64(memstats.heap_scan) } // Compute the remaining scan work estimate. // // Note that we currently count allocations during GC as both // scannable heap (heap_scan) and scan work completed // (scanWork), so allocation will change this difference will // slowly in the soft regime and not at all in the hard // regime. scanWorkRemaining := scanWorkExpected - c.scanWork if scanWorkRemaining < 1000 { // We set a somewhat arbitrary lower bound on // remaining scan work since if we aim a little high, // we can miss by a little. // // We *do* need to enforce that this is at least 1, // since marking is racy and double-scanning objects // may legitimately make the remaining scan work // negative, even in the hard goal regime. scanWorkRemaining = 1000 } // Compute the heap distance remaining. heapRemaining := heapGoal - int64(live) if heapRemaining <= 0 { // This shouldn't happen, but if it does, avoid // dividing by zero or setting the assist negative. heapRemaining = 1 } // Compute the mutator assist ratio so by the time the mutator // allocates the remaining heap bytes up to next_gc, it will // have done (or stolen) the remaining amount of scan work. c.assistWorkPerByte = float64(scanWorkRemaining) / float64(heapRemaining) c.assistBytesPerWork = float64(heapRemaining) / float64(scanWorkRemaining) } // endCycle computes the trigger ratio for the next cycle. func (c *gcControllerState) endCycle() float64 { if work.userForced { // Forced GC means this cycle didn't start at the // trigger, so where it finished isn't good // information about how to adjust the trigger. // Just leave it where it is. return memstats.triggerRatio } // Proportional response gain for the trigger controller. Must // be in [0, 1]. Lower values smooth out transient effects but // take longer to respond to phase changes. Higher values // react to phase changes quickly, but are more affected by // transient changes. Values near 1 may be unstable. const triggerGain = 0.5 // Compute next cycle trigger ratio. First, this computes the // "error" for this cycle; that is, how far off the trigger // was from what it should have been, accounting for both heap // growth and GC CPU utilization. We compute the actual heap // growth during this cycle and scale that by how far off from // the goal CPU utilization we were (to estimate the heap // growth if we had the desired CPU utilization). The // difference between this estimate and the GOGC-based goal // heap growth is the error. goalGrowthRatio := float64(gcpercent) / 100 actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1 assistDuration := nanotime() - c.markStartTime // Assume background mark hit its utilization goal. utilization := gcBackgroundUtilization // Add assist utilization; avoid divide by zero. if assistDuration > 0 { utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs)) } triggerError := goalGrowthRatio - memstats.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-memstats.triggerRatio) // Finally, we adjust the trigger for next time by this error, // damped by the proportional gain. triggerRatio := memstats.triggerRatio + triggerGain*triggerError if debug.gcpacertrace > 0 { // Print controller state in terms of the design // document. H_m_prev := memstats.heap_marked h_t := memstats.triggerRatio H_T := memstats.gc_trigger h_a := actualGrowthRatio H_a := memstats.heap_live h_g := goalGrowthRatio H_g := int64(float64(H_m_prev) * (1 + h_g)) u_a := utilization u_g := gcGoalUtilization W_a := c.scanWork print("pacer: H_m_prev=", H_m_prev, " h_t=", h_t, " H_T=", H_T, " h_a=", h_a, " H_a=", H_a, " h_g=", h_g, " H_g=", H_g, " u_a=", u_a, " u_g=", u_g, " W_a=", W_a, " goalΔ=", goalGrowthRatio-h_t, " actualΔ=", h_a-h_t, " u_a/u_g=", u_a/u_g, "\n") } return triggerRatio } // enlistWorker encourages another dedicated mark worker to start on // another P if there are spare worker slots. It is used by putfull // when more work is made available. // //go:nowritebarrier func (c *gcControllerState) enlistWorker() { // If there are idle Ps, wake one so it will run an idle worker. // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112. // // if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 { // wakep() // return // } // There are no idle Ps. If we need more dedicated workers, // try to preempt a running P so it will switch to a worker. if c.dedicatedMarkWorkersNeeded <= 0 { return } // Pick a random other P to preempt. if gomaxprocs <= 1 { return } gp := getg() if gp == nil || gp.m == nil || gp.m.p == 0 { return } myID := gp.m.p.ptr().id for tries := 0; tries < 5; tries++ { id := int32(fastrandn(uint32(gomaxprocs - 1))) if id >= myID { id++ } p := allp[id] if p.status != _Prunning { continue } if preemptone(p) { return } } } // findRunnableGCWorker returns the background mark worker for _p_ if it // should be run. This must only be called when gcBlackenEnabled != 0. func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g { if gcBlackenEnabled == 0 { throw("gcControllerState.findRunnable: blackening not enabled") } if _p_.gcBgMarkWorker == 0 { // The mark worker associated with this P is blocked // performing a mark transition. We can't run it // because it may be on some other run or wait queue. return nil } if !gcMarkWorkAvailable(_p_) { // No work to be done right now. This can happen at // the end of the mark phase when there are still // assists tapering off. Don't bother running a worker // now because it'll just return immediately. return nil } decIfPositive := func(ptr *int64) bool { if *ptr > 0 { if atomic.Xaddint64(ptr, -1) >= 0 { return true } // We lost a race atomic.Xaddint64(ptr, +1) } return false } if decIfPositive(&c.dedicatedMarkWorkersNeeded) { // This P is now dedicated to marking until the end of // the concurrent mark phase. _p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode } else if c.fractionalUtilizationGoal == 0 { // No need for fractional workers. return nil } else { // Is this P behind on the fractional utilization // goal? // // This should be kept in sync with pollFractionalWorkerExit. delta := nanotime() - gcController.markStartTime if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal { // Nope. No need to run a fractional worker. return nil } // Run a fractional worker. _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode } // Run the background mark worker gp := _p_.gcBgMarkWorker.ptr() casgstatus(gp, _Gwaiting, _Grunnable) if trace.enabled { traceGoUnpark(gp, 0) } return gp } // pollFractionalWorkerExit returns true if a fractional mark worker // should self-preempt. It assumes it is called from the fractional // worker. func pollFractionalWorkerExit() bool { // This should be kept in sync with the fractional worker // scheduler logic in findRunnableGCWorker. now := nanotime() delta := now - gcController.markStartTime if delta <= 0 { return true } p := getg().m.p.ptr() selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime) // Add some slack to the utilization goal so that the // fractional worker isn't behind again the instant it exits. return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal } // gcSetTriggerRatio sets the trigger ratio and updates everything // derived from it: the absolute trigger, the heap goal, mark pacing, // and sweep pacing. // // This can be called any time. If GC is the in the middle of a // concurrent phase, it will adjust the pacing of that phase. // // This depends on gcpercent, memstats.heap_marked, and // memstats.heap_live. These must be up to date. // // mheap_.lock must be held or the world must be stopped. func gcSetTriggerRatio(triggerRatio float64) { // Set the trigger ratio, capped to reasonable bounds. if triggerRatio < 0 { // This can happen if the mutator is allocating very // quickly or the GC is scanning very slowly. triggerRatio = 0 } else if gcpercent >= 0 { // Ensure there's always a little margin so that the // mutator assist ratio isn't infinity. maxTriggerRatio := 0.95 * float64(gcpercent) / 100 if triggerRatio > maxTriggerRatio { triggerRatio = maxTriggerRatio } } memstats.triggerRatio = triggerRatio // Compute the absolute GC trigger from the trigger ratio. // // We trigger the next GC cycle when the allocated heap has // grown by the trigger ratio over the marked heap size. trigger := ^uint64(0) if gcpercent >= 0 { trigger = uint64(float64(memstats.heap_marked) * (1 + triggerRatio)) // Don't trigger below the minimum heap size. minTrigger := heapminimum if !gosweepdone() { // Concurrent sweep happens in the heap growth // from heap_live to gc_trigger, so ensure // that concurrent sweep has some heap growth // in which to perform sweeping before we // start the next GC cycle. sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance*uint64(gcpercent)/100 if sweepMin > minTrigger { minTrigger = sweepMin } } if trigger < minTrigger { trigger = minTrigger } if int64(trigger) < 0 { print("runtime: next_gc=", memstats.next_gc, " heap_marked=", memstats.heap_marked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n") throw("gc_trigger underflow") } } memstats.gc_trigger = trigger // Compute the next GC goal, which is when the allocated heap // has grown by GOGC/100 over the heap marked by the last // cycle. goal := ^uint64(0) if gcpercent >= 0 { goal = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100 if goal < trigger { // The trigger ratio is always less than GOGC/100, but // other bounds on the trigger may have raised it. // Push up the goal, too. goal = trigger } } memstats.next_gc = goal if trace.enabled { traceNextGC() } // Update mark pacing. if gcphase != _GCoff { gcController.revise() } // Update sweep pacing. if gosweepdone() { mheap_.sweepPagesPerByte = 0 } else { // Concurrent sweep needs to sweep all of the in-use // pages by the time the allocated heap reaches the GC // trigger. Compute the ratio of in-use pages to sweep // per byte allocated, accounting for the fact that // some might already be swept. heapLiveBasis := atomic.Load64(&memstats.heap_live) heapDistance := int64(trigger) - int64(heapLiveBasis) // Add a little margin so rounding errors and // concurrent sweep are less likely to leave pages // unswept when GC starts. heapDistance -= 1024 * 1024 if heapDistance < _PageSize { // Avoid setting the sweep ratio extremely high heapDistance = _PageSize } pagesSwept := atomic.Load64(&mheap_.pagesSwept) sweepDistancePages := int64(mheap_.pagesInUse) - int64(pagesSwept) if sweepDistancePages <= 0 { mheap_.sweepPagesPerByte = 0 } else { mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance) mheap_.sweepHeapLiveBasis = heapLiveBasis // Write pagesSweptBasis last, since this // signals concurrent sweeps to recompute // their debt. atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept) } } } // gcGoalUtilization is the goal CPU utilization for // marking as a fraction of GOMAXPROCS. const gcGoalUtilization = 0.30 // gcBackgroundUtilization is the fixed CPU utilization for background // marking. It must be <= gcGoalUtilization. The difference between // gcGoalUtilization and gcBackgroundUtilization will be made up by // mark assists. The scheduler will aim to use within 50% of this // goal. // // Setting this to < gcGoalUtilization avoids saturating the trigger // feedback controller when there are no assists, which allows it to // better control CPU and heap growth. However, the larger the gap, // the more mutator assists are expected to happen, which impact // mutator latency. const gcBackgroundUtilization = 0.25 // gcCreditSlack is the amount of scan work credit that can can // accumulate locally before updating gcController.scanWork and, // optionally, gcController.bgScanCredit. Lower values give a more // accurate assist ratio and make it more likely that assists will // successfully steal background credit. Higher values reduce memory // contention. const gcCreditSlack = 2000 // gcAssistTimeSlack is the nanoseconds of mutator assist time that // can accumulate on a P before updating gcController.assistTime. const gcAssistTimeSlack = 5000 // gcOverAssistWork determines how many extra units of scan work a GC // assist does when an assist happens. This amortizes the cost of an // assist by pre-paying for this many bytes of future allocations. const gcOverAssistWork = 64 << 10 var work struct { full lfstack // lock-free list of full blocks workbuf empty lfstack // lock-free list of empty blocks workbuf pad0 [sys.CacheLineSize]uint8 // prevents false-sharing between full/empty and nproc/nwait wbufSpans struct { lock mutex // free is a list of spans dedicated to workbufs, but // that don't currently contain any workbufs. free mSpanList // busy is a list of all spans containing workbufs on // one of the workbuf lists. busy mSpanList } // Restore 64-bit alignment on 32-bit. _ uint32 // bytesMarked is the number of bytes marked this cycle. This // includes bytes blackened in scanned objects, noscan objects // that go straight to black, and permagrey objects scanned by // markroot during the concurrent scan phase. This is updated // atomically during the cycle. Updates may be batched // arbitrarily, since the value is only read at the end of the // cycle. // // Because of benign races during marking, this number may not // be the exact number of marked bytes, but it should be very // close. // // Put this field here because it needs 64-bit atomic access // (and thus 8-byte alignment even on 32-bit architectures). bytesMarked uint64 markrootNext uint32 // next markroot job markrootJobs uint32 // number of markroot jobs nproc uint32 tstart int64 nwait uint32 ndone uint32 alldone note // helperDrainBlock indicates that GC mark termination helpers // should pass gcDrainBlock to gcDrain to block in the // getfull() barrier. Otherwise, they should pass gcDrainNoBlock. // // TODO: This is a temporary fallback to work around races // that cause early mark termination. helperDrainBlock bool // Number of roots of various root types. Set by gcMarkRootPrepare. nFlushCacheRoots int nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int // markrootDone indicates that roots have been marked at least // once during the current GC cycle. This is checked by root // marking operations that have to happen only during the // first root marking pass, whether that's during the // concurrent mark phase in current GC or mark termination in // STW GC. markrootDone bool // Each type of GC state transition is protected by a lock. // Since multiple threads can simultaneously detect the state // transition condition, any thread that detects a transition // condition must acquire the appropriate transition lock, // re-check the transition condition and return if it no // longer holds or perform the transition if it does. // Likewise, any transition must invalidate the transition // condition before releasing the lock. This ensures that each // transition is performed by exactly one thread and threads // that need the transition to happen block until it has // happened. // // startSema protects the transition from "off" to mark or // mark termination. startSema uint32 // markDoneSema protects transitions from mark 1 to mark 2 and // from mark 2 to mark termination. markDoneSema uint32 bgMarkReady note // signal background mark worker has started bgMarkDone uint32 // cas to 1 when at a background mark completion point // Background mark completion signaling // mode is the concurrency mode of the current GC cycle. mode gcMode // userForced indicates the current GC cycle was forced by an // explicit user call. userForced bool // totaltime is the CPU nanoseconds spent in GC since the // program started if debug.gctrace > 0. totaltime int64 // initialHeapLive is the value of memstats.heap_live at the // beginning of this GC cycle. initialHeapLive uint64 // assistQueue is a queue of assists that are blocked because // there was neither enough credit to steal or enough work to // do. assistQueue struct { lock mutex head, tail guintptr } // sweepWaiters is a list of blocked goroutines to wake when // we transition from mark termination to sweep. sweepWaiters struct { lock mutex head guintptr } // cycles is the number of completed GC cycles, where a GC // cycle is sweep termination, mark, mark termination, and // sweep. This differs from memstats.numgc, which is // incremented at mark termination. cycles uint32 // Timing/utilization stats for this cycle. stwprocs, maxprocs int32 tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start pauseNS int64 // total STW time this cycle pauseStart int64 // nanotime() of last STW // debug.gctrace heap sizes for this cycle. heap0, heap1, heap2, heapGoal uint64 } // GC runs a garbage collection and blocks the caller until the // garbage collection is complete. It may also block the entire // program. func GC() { // We consider a cycle to be: sweep termination, mark, mark // termination, and sweep. This function shouldn't return // until a full cycle has been completed, from beginning to // end. Hence, we always want to finish up the current cycle // and start a new one. That means: // // 1. In sweep termination, mark, or mark termination of cycle // N, wait until mark termination N completes and transitions // to sweep N. // // 2. In sweep N, help with sweep N. // // At this point we can begin a full cycle N+1. // // 3. Trigger cycle N+1 by starting sweep termination N+1. // // 4. Wait for mark termination N+1 to complete. // // 5. Help with sweep N+1 until it's done. // // This all has to be written to deal with the fact that the // GC may move ahead on its own. For example, when we block // until mark termination N, we may wake up in cycle N+2. gp := getg() // Prevent the GC phase or cycle count from changing. lock(&work.sweepWaiters.lock) n := atomic.Load(&work.cycles) if gcphase == _GCmark { // Wait until sweep termination, mark, and mark // termination of cycle N complete. gp.schedlink = work.sweepWaiters.head work.sweepWaiters.head.set(gp) goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1) } else { // We're in sweep N already. unlock(&work.sweepWaiters.lock) } // We're now in sweep N or later. Trigger GC cycle N+1, which // will first finish sweep N if necessary and then enter sweep // termination N+1. gcStart(gcBackgroundMode, gcTrigger{kind: gcTriggerCycle, n: n + 1}) // Wait for mark termination N+1 to complete. lock(&work.sweepWaiters.lock) if gcphase == _GCmark && atomic.Load(&work.cycles) == n+1 { gp.schedlink = work.sweepWaiters.head work.sweepWaiters.head.set(gp) goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1) } else { unlock(&work.sweepWaiters.lock) } // Finish sweep N+1 before returning. We do this both to // complete the cycle and because runtime.GC() is often used // as part of tests and benchmarks to get the system into a // relatively stable and isolated state. for atomic.Load(&work.cycles) == n+1 && gosweepone() != ^uintptr(0) { sweep.nbgsweep++ Gosched() } // Callers may assume that the heap profile reflects the // just-completed cycle when this returns (historically this // happened because this was a STW GC), but right now the // profile still reflects mark termination N, not N+1. // // As soon as all of the sweep frees from cycle N+1 are done, // we can go ahead and publish the heap profile. // // First, wait for sweeping to finish. (We know there are no // more spans on the sweep queue, but we may be concurrently // sweeping spans, so we have to wait.) for atomic.Load(&work.cycles) == n+1 && atomic.Load(&mheap_.sweepers) != 0 { Gosched() } // Now we're really done with sweeping, so we can publish the // stable heap profile. Only do this if we haven't already hit // another mark termination. mp := acquirem() cycle := atomic.Load(&work.cycles) if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) { mProf_PostSweep() } releasem(mp) } // gcMode indicates how concurrent a GC cycle should be. type gcMode int const ( gcBackgroundMode gcMode = iota // concurrent GC and sweep gcForceMode // stop-the-world GC now, concurrent sweep gcForceBlockMode // stop-the-world GC now and STW sweep (forced by user) ) // A gcTrigger is a predicate for starting a GC cycle. Specifically, // it is an exit condition for the _GCoff phase. type gcTrigger struct { kind gcTriggerKind now int64 // gcTriggerTime: current time n uint32 // gcTriggerCycle: cycle number to start } type gcTriggerKind int const ( // gcTriggerAlways indicates that a cycle should be started // unconditionally, even if GOGC is off or we're in a cycle // right now. This cannot be consolidated with other cycles. gcTriggerAlways gcTriggerKind = iota // gcTriggerHeap indicates that a cycle should be started when // the heap size reaches the trigger heap size computed by the // controller. gcTriggerHeap // gcTriggerTime indicates that a cycle should be started when // it's been more than forcegcperiod nanoseconds since the // previous GC cycle. gcTriggerTime // gcTriggerCycle indicates that a cycle should be started if // we have not yet started cycle number gcTrigger.n (relative // to work.cycles). gcTriggerCycle ) // test returns true if the trigger condition is satisfied, meaning // that the exit condition for the _GCoff phase has been met. The exit // condition should be tested when allocating. func (t gcTrigger) test() bool { if !memstats.enablegc || panicking != 0 { return false } if t.kind == gcTriggerAlways { return true } if gcphase != _GCoff { return false } switch t.kind { case gcTriggerHeap: // Non-atomic access to heap_live for performance. If // we are going to trigger on this, this thread just // atomically wrote heap_live anyway and we'll see our // own write. return memstats.heap_live >= memstats.gc_trigger case gcTriggerTime: if gcpercent < 0 { return false } lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime)) return lastgc != 0 && t.now-lastgc > forcegcperiod case gcTriggerCycle: // t.n > work.cycles, but accounting for wraparound. return int32(t.n-work.cycles) > 0 } return true } // gcStart transitions the GC from _GCoff to _GCmark (if // !mode.stwMark) or _GCmarktermination (if mode.stwMark) by // performing sweep termination and GC initialization. // // This may return without performing this transition in some cases, // such as when called on a system stack or with locks held. func gcStart(mode gcMode, trigger gcTrigger) { // Since this is called from malloc and malloc is called in // the guts of a number of libraries that might be holding // locks, don't attempt to start GC in non-preemptible or // potentially unstable situations. mp := acquirem() if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" { releasem(mp) return } releasem(mp) mp = nil // Pick up the remaining unswept/not being swept spans concurrently // // This shouldn't happen if we're being invoked in background // mode since proportional sweep should have just finished // sweeping everything, but rounding errors, etc, may leave a // few spans unswept. In forced mode, this is necessary since // GC can be forced at any point in the sweeping cycle. // // We check the transition condition continuously here in case // this G gets delayed in to the next GC cycle. for trigger.test() && gosweepone() != ^uintptr(0) { sweep.nbgsweep++ } // Perform GC initialization and the sweep termination // transition. semacquire(&work.startSema) // Re-check transition condition under transition lock. if !trigger.test() { semrelease(&work.startSema) return } // For stats, check if this GC was forced by the user. work.userForced = trigger.kind == gcTriggerAlways || trigger.kind == gcTriggerCycle // In gcstoptheworld debug mode, upgrade the mode accordingly. // We do this after re-checking the transition condition so // that multiple goroutines that detect the heap trigger don't // start multiple STW GCs. if mode == gcBackgroundMode { if debug.gcstoptheworld == 1 { mode = gcForceMode } else if debug.gcstoptheworld == 2 { mode = gcForceBlockMode } } // Ok, we're doing it! Stop everybody else semacquire(&worldsema) if trace.enabled { traceGCStart() } if mode == gcBackgroundMode { gcBgMarkStartWorkers() } gcResetMarkState() work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs if work.stwprocs > ncpu { // This is used to compute CPU time of the STW phases, // so it can't be more than ncpu, even if GOMAXPROCS is. work.stwprocs = ncpu } work.heap0 = atomic.Load64(&memstats.heap_live) work.pauseNS = 0 work.mode = mode now := nanotime() work.tSweepTerm = now work.pauseStart = now if trace.enabled { traceGCSTWStart(1) } systemstack(stopTheWorldWithSema) // Finish sweep before we start concurrent scan. systemstack(func() { finishsweep_m() }) // clearpools before we start the GC. If we wait they memory will not be // reclaimed until the next GC cycle. clearpools() work.cycles++ if mode == gcBackgroundMode { // Do as much work concurrently as possible gcController.startCycle() work.heapGoal = memstats.next_gc // Enter concurrent mark phase and enable // write barriers. // // Because the world is stopped, all Ps will // observe that write barriers are enabled by // the time we start the world and begin // scanning. // // Write barriers must be enabled before assists are // enabled because they must be enabled before // any non-leaf heap objects are marked. Since // allocations are blocked until assists can // happen, we want enable assists as early as // possible. setGCPhase(_GCmark) gcBgMarkPrepare() // Must happen before assist enable. gcMarkRootPrepare() // Mark all active tinyalloc blocks. Since we're // allocating from these, they need to be black like // other allocations. The alternative is to blacken // the tiny block on every allocation from it, which // would slow down the tiny allocator. gcMarkTinyAllocs() // At this point all Ps have enabled the write // barrier, thus maintaining the no white to // black invariant. Enable mutator assists to // put back-pressure on fast allocating // mutators. atomic.Store(&gcBlackenEnabled, 1) // Assists and workers can start the moment we start // the world. gcController.markStartTime = now // Concurrent mark. systemstack(func() { now = startTheWorldWithSema(trace.enabled) }) work.pauseNS += now - work.pauseStart work.tMark = now } else { if trace.enabled { // Switch to mark termination STW. traceGCSTWDone() traceGCSTWStart(0) } t := nanotime() work.tMark, work.tMarkTerm = t, t work.heapGoal = work.heap0 // Perform mark termination. This will restart the world. gcMarkTermination(memstats.triggerRatio) } semrelease(&work.startSema) } // gcMarkDone transitions the GC from mark 1 to mark 2 and from mark 2 // to mark termination. // // This should be called when all mark work has been drained. In mark // 1, this includes all root marking jobs, global work buffers, and // active work buffers in assists and background workers; however, // work may still be cached in per-P work buffers. In mark 2, per-P // caches are disabled. // // The calling context must be preemptible. // // Note that it is explicitly okay to have write barriers in this // function because completion of concurrent mark is best-effort // anyway. Any work created by write barriers here will be cleaned up // by mark termination. func gcMarkDone() { top: semacquire(&work.markDoneSema) // Re-check transition condition under transition lock. if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) { semrelease(&work.markDoneSema) return } // Disallow starting new workers so that any remaining workers // in the current mark phase will drain out. // // TODO(austin): Should dedicated workers keep an eye on this // and exit gcDrain promptly? atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, -0xffffffff) prevFractionalGoal := gcController.fractionalUtilizationGoal gcController.fractionalUtilizationGoal = 0 if !gcBlackenPromptly { // Transition from mark 1 to mark 2. // // The global work list is empty, but there can still be work // sitting in the per-P work caches. // Flush and disable work caches. // Disallow caching workbufs and indicate that we're in mark 2. gcBlackenPromptly = true // Prevent completion of mark 2 until we've flushed // cached workbufs. atomic.Xadd(&work.nwait, -1) // GC is set up for mark 2. Let Gs blocked on the // transition lock go while we flush caches. semrelease(&work.markDoneSema) systemstack(func() { // Flush all currently cached workbufs and // ensure all Ps see gcBlackenPromptly. This // also blocks until any remaining mark 1 // workers have exited their loop so we can // start new mark 2 workers. forEachP(func(_p_ *p) { wbBufFlush1(_p_) _p_.gcw.dispose() }) }) // Check that roots are marked. We should be able to // do this before the forEachP, but based on issue // #16083 there may be a (harmless) race where we can // enter mark 2 while some workers are still scanning // stacks. The forEachP ensures these scans are done. // // TODO(austin): Figure out the race and fix this // properly. gcMarkRootCheck() // Now we can start up mark 2 workers. atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 0xffffffff) gcController.fractionalUtilizationGoal = prevFractionalGoal incnwait := atomic.Xadd(&work.nwait, +1) if incnwait == work.nproc && !gcMarkWorkAvailable(nil) { // This loop will make progress because // gcBlackenPromptly is now true, so it won't // take this same "if" branch. goto top } } else { // Transition to mark termination. now := nanotime() work.tMarkTerm = now work.pauseStart = now getg().m.preemptoff = "gcing" if trace.enabled { traceGCSTWStart(0) } systemstack(stopTheWorldWithSema) // The gcphase is _GCmark, it will transition to _GCmarktermination // below. The important thing is that the wb remains active until // all marking is complete. This includes writes made by the GC. // Record that one root marking pass has completed. work.markrootDone = true // Disable assists and background workers. We must do // this before waking blocked assists. atomic.Store(&gcBlackenEnabled, 0) // Wake all blocked assists. These will run when we // start the world again. gcWakeAllAssists() // Likewise, release the transition lock. Blocked // workers and assists will run when we start the // world again. semrelease(&work.markDoneSema) // endCycle depends on all gcWork cache stats being // flushed. This is ensured by mark 2. nextTriggerRatio := gcController.endCycle() // Perform mark termination. This will restart the world. gcMarkTermination(nextTriggerRatio) } } func gcMarkTermination(nextTriggerRatio float64) { // World is stopped. // Start marktermination which includes enabling the write barrier. atomic.Store(&gcBlackenEnabled, 0) gcBlackenPromptly = false setGCPhase(_GCmarktermination) work.heap1 = memstats.heap_live startTime := nanotime() mp := acquirem() mp.preemptoff = "gcing" _g_ := getg() _g_.m.traceback = 2 gp := _g_.m.curg casgstatus(gp, _Grunning, _Gwaiting) gp.waitreason = "garbage collection" // Run gc on the g0 stack. We do this so that the g stack // we're currently running on will no longer change. Cuts // the root set down a bit (g0 stacks are not scanned, and // we don't need to scan gc's internal state). We also // need to switch to g0 so we can shrink the stack. systemstack(func() { gcMark(startTime) // Must return immediately. // The outer function's stack may have moved // during gcMark (it shrinks stacks, including the // outer function's stack), so we must not refer // to any of its variables. Return back to the // non-system stack to pick up the new addresses // before continuing. }) systemstack(func() { work.heap2 = work.bytesMarked if debug.gccheckmark > 0 { // Run a full stop-the-world mark using checkmark bits, // to check that we didn't forget to mark anything during // the concurrent mark process. gcResetMarkState() initCheckmarks() gcMark(startTime) clearCheckmarks() } // marking is complete so we can turn the write barrier off setGCPhase(_GCoff) gcSweep(work.mode) if debug.gctrace > 1 { startTime = nanotime() // The g stacks have been scanned so // they have gcscanvalid==true and gcworkdone==true. // Reset these so that all stacks will be rescanned. gcResetMarkState() finishsweep_m() // Still in STW but gcphase is _GCoff, reset to _GCmarktermination // At this point all objects will be found during the gcMark which // does a complete STW mark and object scan. setGCPhase(_GCmarktermination) gcMark(startTime) setGCPhase(_GCoff) // marking is done, turn off wb. gcSweep(work.mode) } }) _g_.m.traceback = 0 casgstatus(gp, _Gwaiting, _Grunning) if trace.enabled { traceGCDone() } // all done mp.preemptoff = "" if gcphase != _GCoff { throw("gc done but gcphase != _GCoff") } // Update GC trigger and pacing for the next cycle. gcSetTriggerRatio(nextTriggerRatio) // Update timing memstats now := nanotime() sec, nsec, _ := time_now() unixNow := sec*1e9 + int64(nsec) work.pauseNS += now - work.pauseStart work.tEnd = now atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS) memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow) memstats.pause_total_ns += uint64(work.pauseNS) // Update work.totaltime. sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm) // We report idle marking time below, but omit it from the // overall utilization here since it's "free". markCpu := gcController.assistTime + gcController.dedicatedMarkTime + gcController.fractionalMarkTime markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm) cycleCpu := sweepTermCpu + markCpu + markTermCpu work.totaltime += cycleCpu // Compute overall GC CPU utilization. totalCpu := sched.totaltime + (now-sched.procresizetime)*int64(gomaxprocs) memstats.gc_cpu_fraction = float64(work.totaltime) / float64(totalCpu) // Reset sweep state. sweep.nbgsweep = 0 sweep.npausesweep = 0 if work.userForced { memstats.numforcedgc++ } // Bump GC cycle count and wake goroutines waiting on sweep. lock(&work.sweepWaiters.lock) memstats.numgc++ injectglist(work.sweepWaiters.head.ptr()) work.sweepWaiters.head = 0 unlock(&work.sweepWaiters.lock) // Finish the current heap profiling cycle and start a new // heap profiling cycle. We do this before starting the world // so events don't leak into the wrong cycle. mProf_NextCycle() systemstack(func() { startTheWorldWithSema(true) }) // Flush the heap profile so we can start a new cycle next GC. // This is relatively expensive, so we don't do it with the // world stopped. mProf_Flush() // Prepare workbufs for freeing by the sweeper. We do this // asynchronously because it can take non-trivial time. prepareFreeWorkbufs() // Free stack spans. This must be done between GC cycles. systemstack(freeStackSpans) // Print gctrace before dropping worldsema. As soon as we drop // worldsema another cycle could start and smash the stats // we're trying to print. if debug.gctrace > 0 { util := int(memstats.gc_cpu_fraction * 100) var sbuf [24]byte printlock() print("gc ", memstats.numgc, " @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ", util, "%: ") prev := work.tSweepTerm for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} { if i != 0 { print("+") } print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev)))) prev = ns } print(" ms clock, ") for i, ns := range []int64{sweepTermCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} { if i == 2 || i == 3 { // Separate mark time components with /. print("/") } else if i != 0 { print("+") } print(string(fmtNSAsMS(sbuf[:], uint64(ns)))) } print(" ms cpu, ", work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ", work.heapGoal>>20, " MB goal, ", work.maxprocs, " P") if work.userForced { print(" (forced)") } print("\n") printunlock() } semrelease(&worldsema) // Careful: another GC cycle may start now. releasem(mp) mp = nil // now that gc is done, kick off finalizer thread if needed if !concurrentSweep { // give the queued finalizers, if any, a chance to run Gosched() } } // gcBgMarkStartWorkers prepares background mark worker goroutines. // These goroutines will not run until the mark phase, but they must // be started while the work is not stopped and from a regular G // stack. The caller must hold worldsema. func gcBgMarkStartWorkers() { // Background marking is performed by per-P G's. Ensure that // each P has a background GC G. for _, p := range allp { if p.gcBgMarkWorker == 0 { go gcBgMarkWorker(p) notetsleepg(&work.bgMarkReady, -1) noteclear(&work.bgMarkReady) } } } // gcBgMarkPrepare sets up state for background marking. // Mutator assists must not yet be enabled. func gcBgMarkPrepare() { // Background marking will stop when the work queues are empty // and there are no more workers (note that, since this is // concurrent, this may be a transient state, but mark // termination will clean it up). Between background workers // and assists, we don't really know how many workers there // will be, so we pretend to have an arbitrarily large number // of workers, almost all of which are "waiting". While a // worker is working it decrements nwait. If nproc == nwait, // there are no workers. work.nproc = ^uint32(0) work.nwait = ^uint32(0) } func gcBgMarkWorker(_p_ *p) { gp := getg() type parkInfo struct { m muintptr // Release this m on park. attach puintptr // If non-nil, attach to this p on park. } // We pass park to a gopark unlock function, so it can't be on // the stack (see gopark). Prevent deadlock from recursively // starting GC by disabling preemption. gp.m.preemptoff = "GC worker init" park := new(parkInfo) gp.m.preemptoff = "" park.m.set(acquirem()) park.attach.set(_p_) // Inform gcBgMarkStartWorkers that this worker is ready. // After this point, the background mark worker is scheduled // cooperatively by gcController.findRunnable. Hence, it must // never be preempted, as this would put it into _Grunnable // and put it on a run queue. Instead, when the preempt flag // is set, this puts itself into _Gwaiting to be woken up by // gcController.findRunnable at the appropriate time. notewakeup(&work.bgMarkReady) for { // Go to sleep until woken by gcController.findRunnable. // We can't releasem yet since even the call to gopark // may be preempted. gopark(func(g *g, parkp unsafe.Pointer) bool { park := (*parkInfo)(parkp) // The worker G is no longer running, so it's // now safe to allow preemption. releasem(park.m.ptr()) // If the worker isn't attached to its P, // attach now. During initialization and after // a phase change, the worker may have been // running on a different P. As soon as we // attach, the owner P may schedule the // worker, so this must be done after the G is // stopped. if park.attach != 0 { p := park.attach.ptr() park.attach.set(nil) // cas the worker because we may be // racing with a new worker starting // on this P. if !p.gcBgMarkWorker.cas(0, guintptr(unsafe.Pointer(g))) { // The P got a new worker. // Exit this worker. return false } } return true }, unsafe.Pointer(park), "GC worker (idle)", traceEvGoBlock, 0) // Loop until the P dies and disassociates this // worker (the P may later be reused, in which case // it will get a new worker) or we failed to associate. if _p_.gcBgMarkWorker.ptr() != gp { break } // Disable preemption so we can use the gcw. If the // scheduler wants to preempt us, we'll stop draining, // dispose the gcw, and then preempt. park.m.set(acquirem()) if gcBlackenEnabled == 0 { throw("gcBgMarkWorker: blackening not enabled") } startTime := nanotime() _p_.gcMarkWorkerStartTime = startTime decnwait := atomic.Xadd(&work.nwait, -1) if decnwait == work.nproc { println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc) throw("work.nwait was > work.nproc") } systemstack(func() { // Mark our goroutine preemptible so its stack // can be scanned. This lets two mark workers // scan each other (otherwise, they would // deadlock). We must not modify anything on // the G stack. However, stack shrinking is // disabled for mark workers, so it is safe to // read from the G stack. casgstatus(gp, _Grunning, _Gwaiting) switch _p_.gcMarkWorkerMode { default: throw("gcBgMarkWorker: unexpected gcMarkWorkerMode") case gcMarkWorkerDedicatedMode: gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit) if gp.preempt { // We were preempted. This is // a useful signal to kick // everything out of the run // queue so it can run // somewhere else. lock(&sched.lock) for { gp, _ := runqget(_p_) if gp == nil { break } globrunqput(gp) } unlock(&sched.lock) } // Go back to draining, this time // without preemption. gcDrain(&_p_.gcw, gcDrainNoBlock|gcDrainFlushBgCredit) case gcMarkWorkerFractionalMode: gcDrain(&_p_.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit) case gcMarkWorkerIdleMode: gcDrain(&_p_.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit) } casgstatus(gp, _Gwaiting, _Grunning) }) // If we are nearing the end of mark, dispose // of the cache promptly. We must do this // before signaling that we're no longer // working so that other workers can't observe // no workers and no work while we have this // cached, and before we compute done. if gcBlackenPromptly { _p_.gcw.dispose() } // Account for time. duration := nanotime() - startTime switch _p_.gcMarkWorkerMode { case gcMarkWorkerDedicatedMode: atomic.Xaddint64(&gcController.dedicatedMarkTime, duration) atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1) case gcMarkWorkerFractionalMode: atomic.Xaddint64(&gcController.fractionalMarkTime, duration) atomic.Xaddint64(&_p_.gcFractionalMarkTime, duration) case gcMarkWorkerIdleMode: atomic.Xaddint64(&gcController.idleMarkTime, duration) } // Was this the last worker and did we run out // of work? incnwait := atomic.Xadd(&work.nwait, +1) if incnwait > work.nproc { println("runtime: p.gcMarkWorkerMode=", _p_.gcMarkWorkerMode, "work.nwait=", incnwait, "work.nproc=", work.nproc) throw("work.nwait > work.nproc") } // If this worker reached a background mark completion // point, signal the main GC goroutine. if incnwait == work.nproc && !gcMarkWorkAvailable(nil) { // Make this G preemptible and disassociate it // as the worker for this P so // findRunnableGCWorker doesn't try to // schedule it. _p_.gcBgMarkWorker.set(nil) releasem(park.m.ptr()) gcMarkDone() // Disable preemption and prepare to reattach // to the P. // // We may be running on a different P at this // point, so we can't reattach until this G is // parked. park.m.set(acquirem()) park.attach.set(_p_) } } } // gcMarkWorkAvailable returns true if executing a mark worker // on p is potentially useful. p may be nil, in which case it only // checks the global sources of work. func gcMarkWorkAvailable(p *p) bool { if p != nil && !p.gcw.empty() { return true } if !work.full.empty() { return true // global work available } if work.markrootNext < work.markrootJobs { return true // root scan work available } return false } // gcMark runs the mark (or, for concurrent GC, mark termination) // All gcWork caches must be empty. // STW is in effect at this point. //TODO go:nowritebarrier func gcMark(start_time int64) { if debug.allocfreetrace > 0 { tracegc() } if gcphase != _GCmarktermination { throw("in gcMark expecting to see gcphase as _GCmarktermination") } work.tstart = start_time // Queue root marking jobs. gcMarkRootPrepare() work.nwait = 0 work.ndone = 0 work.nproc = uint32(gcprocs()) if work.full == 0 && work.nDataRoots+work.nBSSRoots+work.nSpanRoots+work.nStackRoots == 0 { // There's no work on the work queue and no root jobs // that can produce work, so don't bother entering the // getfull() barrier. // // This will be the situation the vast majority of the // time after concurrent mark. However, we still need // a fallback for STW GC and because there are some // known races that occasionally leave work around for // mark termination. // // We're still hedging our bets here: if we do // accidentally produce some work, we'll still process // it, just not necessarily in parallel. // // TODO(austin): Fix the races and and remove // work draining from mark termination so we don't // need the fallback path. work.helperDrainBlock = false } else { work.helperDrainBlock = true } if work.nproc > 1 { noteclear(&work.alldone) helpgc(int32(work.nproc)) } gchelperstart() gcw := &getg().m.p.ptr().gcw if work.helperDrainBlock { gcDrain(gcw, gcDrainBlock) } else { gcDrain(gcw, gcDrainNoBlock) } gcw.dispose() if debug.gccheckmark > 0 { // This is expensive when there's a large number of // Gs, so only do it if checkmark is also enabled. gcMarkRootCheck() } if work.full != 0 { throw("work.full != 0") } if work.nproc > 1 { notesleep(&work.alldone) } // Record that at least one root marking pass has completed. work.markrootDone = true // Double-check that all gcWork caches are empty. This should // be ensured by mark 2 before we enter mark termination. for _, p := range allp { gcw := &p.gcw if !gcw.empty() { throw("P has cached GC work at end of mark termination") } if gcw.scanWork != 0 || gcw.bytesMarked != 0 { throw("P has unflushed stats at end of mark termination") } } cachestats() // Update the marked heap stat. memstats.heap_marked = work.bytesMarked // Update other GC heap size stats. This must happen after // cachestats (which flushes local statistics to these) and // flushallmcaches (which modifies heap_live). memstats.heap_live = work.bytesMarked memstats.heap_scan = uint64(gcController.scanWork) if trace.enabled { traceHeapAlloc() } } func gcSweep(mode gcMode) { if gcphase != _GCoff { throw("gcSweep being done but phase is not GCoff") } lock(&mheap_.lock) mheap_.sweepgen += 2 mheap_.sweepdone = 0 if mheap_.sweepSpans[mheap_.sweepgen/2%2].index != 0 { // We should have drained this list during the last // sweep phase. We certainly need to start this phase // with an empty swept list. throw("non-empty swept list") } mheap_.pagesSwept = 0 unlock(&mheap_.lock) if !_ConcurrentSweep || mode == gcForceBlockMode { // Special case synchronous sweep. // Record that no proportional sweeping has to happen. lock(&mheap_.lock) mheap_.sweepPagesPerByte = 0 unlock(&mheap_.lock) // Sweep all spans eagerly. for sweepone() != ^uintptr(0) { sweep.npausesweep++ } // Free workbufs eagerly. prepareFreeWorkbufs() for freeSomeWbufs(false) { } // All "free" events for this mark/sweep cycle have // now happened, so we can make this profile cycle // available immediately. mProf_NextCycle() mProf_Flush() return } // Background sweep. lock(&sweep.lock) if sweep.parked { sweep.parked = false ready(sweep.g, 0, true) } unlock(&sweep.lock) } // gcResetMarkState resets global state prior to marking (concurrent // or STW) and resets the stack scan state of all Gs. // // This is safe to do without the world stopped because any Gs created // during or after this will start out in the reset state. func gcResetMarkState() { // This may be called during a concurrent phase, so make sure // allgs doesn't change. lock(&allglock) for _, gp := range allgs { gp.gcscandone = false // set to true in gcphasework gp.gcscanvalid = false // stack has not been scanned gp.gcAssistBytes = 0 } unlock(&allglock) work.bytesMarked = 0 work.initialHeapLive = atomic.Load64(&memstats.heap_live) work.markrootDone = false } // Hooks for other packages var poolcleanup func() //go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup func sync_runtime_registerPoolCleanup(f func()) { poolcleanup = f } func clearpools() { // clear sync.Pools if poolcleanup != nil { poolcleanup() } // Clear central sudog cache. // Leave per-P caches alone, they have strictly bounded size. // Disconnect cached list before dropping it on the floor, // so that a dangling ref to one entry does not pin all of them. lock(&sched.sudoglock) var sg, sgnext *sudog for sg = sched.sudogcache; sg != nil; sg = sgnext { sgnext = sg.next sg.next = nil } sched.sudogcache = nil unlock(&sched.sudoglock) // Clear central defer pools. // Leave per-P pools alone, they have strictly bounded size. lock(&sched.deferlock) for i := range sched.deferpool { // disconnect cached list before dropping it on the floor, // so that a dangling ref to one entry does not pin all of them. var d, dlink *_defer for d = sched.deferpool[i]; d != nil; d = dlink { dlink = d.link d.link = nil } sched.deferpool[i] = nil } unlock(&sched.deferlock) } // gchelper runs mark termination tasks on Ps other than the P // coordinating mark termination. // // The caller is responsible for ensuring that this has a P to run on, // even though it's running during STW. Because of this, it's allowed // to have write barriers. // //go:yeswritebarrierrec func gchelper() { _g_ := getg() _g_.m.traceback = 2 gchelperstart() // Parallel mark over GC roots and heap if gcphase == _GCmarktermination { gcw := &_g_.m.p.ptr().gcw if work.helperDrainBlock { gcDrain(gcw, gcDrainBlock) // blocks in getfull } else { gcDrain(gcw, gcDrainNoBlock) } gcw.dispose() } nproc := atomic.Load(&work.nproc) // work.nproc can change right after we increment work.ndone if atomic.Xadd(&work.ndone, +1) == nproc-1 { notewakeup(&work.alldone) } _g_.m.traceback = 0 } func gchelperstart() { _g_ := getg() if _g_.m.helpgc < 0 || _g_.m.helpgc >= _MaxGcproc { throw("gchelperstart: bad m->helpgc") } if _g_ != _g_.m.g0 { throw("gchelper not running on g0 stack") } } // Timing // itoaDiv formats val/(10**dec) into buf. func itoaDiv(buf []byte, val uint64, dec int) []byte { i := len(buf) - 1 idec := i - dec for val >= 10 || i >= idec { buf[i] = byte(val%10 + '0') i-- if i == idec { buf[i] = '.' i-- } val /= 10 } buf[i] = byte(val + '0') return buf[i:] } // fmtNSAsMS nicely formats ns nanoseconds as milliseconds. func fmtNSAsMS(buf []byte, ns uint64) []byte { if ns >= 10e6 { // Format as whole milliseconds. return itoaDiv(buf, ns/1e6, 0) } // Format two digits of precision, with at most three decimal places. x := ns / 1e3 if x == 0 { buf[0] = '0' return buf[:1] } dec := 3 for x >= 100 { x /= 10 dec-- } return itoaDiv(buf, x, dec) }