diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go index 25bf4a226be..0c92b2e7b63 100644 --- a/src/runtime/mgc.go +++ b/src/runtime/mgc.go @@ -149,26 +149,6 @@ const ( 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") @@ -196,17 +176,6 @@ func gcinit() { lockInit(&work.wbufSpans.lock, lockRankWbufSpans) } -func readgogc() int32 { - p := gogetenv("GOGC") - if p == "off" { - return -1 - } - if n, ok := atoi32(p); ok { - return n - } - return 100 -} - // Temporary in order to enable register ABI work. // TODO(register args): convert back to local chan in gcenabled, passed to "go" stmts. var gcenable_setup chan int @@ -226,31 +195,6 @@ func gcenable() { memstats.enablegc = true // now that runtime is initialized, GC is okay } -//go:linkname setGCPercent runtime/debug.setGCPercent -func setGCPercent(in int32) (out int32) { - // Run on the system stack since we grab the heap lock. - systemstack(func() { - 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 mark to - // finish so we always return with no GC running. - if in < 0 { - gcWaitOnMark(atomic.Load(&work.cycles)) - } - - return out -} - // Garbage collector phase. // Indicates to write barrier and synchronization task to perform. var gcphase uint32 @@ -330,473 +274,6 @@ var gcMarkWorkerModeStrings = [...]string{ "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. - // - // Stored as a uint64, but it's actually a float64. Use - // float64frombits to get the value. - // - // Read and written atomically. - assistWorkPerByte uint64 - - // assistBytesPerWork is 1/assistWorkPerByte. - // - // Stored as a uint64, but it's actually a float64. Use - // float64frombits to get the value. - // - // Read and written atomically. - // - // Note that because this is read and written independently - // from assistWorkPerByte users may notice a skew between - // the two values, and such a state should be safe. - assistBytesPerWork uint64 - - // 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 the 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 - - _ cpu.CacheLinePad -} - -// startCycle resets the GC controller's state and computes estimates -// for a new GC cycle. The caller must hold worldsema and the world -// must be stopped. -func (c *gcControllerState) startCycle() { - c.scanWork = 0 - c.bgScanCredit = 0 - c.assistTime = 0 - c.dedicatedMarkTime = 0 - c.fractionalMarkTime = 0 - c.idleMarkTime = 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 - } - - // In STW mode, we just want dedicated workers. - if debug.gcstoptheworld > 0 { - c.dedicatedMarkWorkersNeeded = int64(gomaxprocs) - 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 { - assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte)) - print("pacer: assist ratio=", assistRatio, - " (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 whenever memstats.heap_scan, -// memstats.heap_live, or memstats.next_gc is updated. It is safe to -// call concurrently, but it may race with other calls to revise. -// -// The result of this race is that the two assist ratio values may not line -// up or may be stale. In practice this is OK because the assist ratio -// moves slowly throughout a GC cycle, and the assist ratio is a best-effort -// heuristic anyway. Furthermore, no part of the heuristic depends on -// the two assist ratio values being exact reciprocals of one another, since -// the two values are used to convert values from different sources. -// -// The worst case result of this raciness is that we may miss a larger shift -// in the ratio (say, if we decide to pace more aggressively against the -// hard heap goal) but even this "hard goal" is best-effort (see #40460). -// The dedicated GC should ensure we don't exceed the hard goal by too much -// in the rare case we do exceed it. -// -// 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) - scan := atomic.Load64(&memstats.heap_scan) - work := atomic.Loadint64(&c.scanWork) - - // Assume we're under the soft goal. Pace GC to complete at - // next_gc assuming the heap is in steady-state. - heapGoal := int64(atomic.Load64(&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(scan) * 100 / float64(100+gcpercent)) - - if int64(live) > heapGoal || work > scanWorkExpected { - // We're past the soft goal, or we've already done more scan - // work than we expected. Pace GC so that in the worst case it - // will complete by the hard goal. - const maxOvershoot = 1.1 - heapGoal = int64(float64(heapGoal) * maxOvershoot) - - // Compute the upper bound on the scan work remaining. - scanWorkExpected = int64(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 - // slowly in the soft regime and not at all in the hard - // regime. - scanWorkRemaining := scanWorkExpected - work - 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. - // Note that the assist ratio values are updated atomically - // but not together. This means there may be some degree of - // skew between the two values. This is generally OK as the - // values shift relatively slowly over the course of a GC - // cycle. - assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining) - assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining) - atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte)) - atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork)) -} - -// 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 := gcEffectiveGrowthRatio() - 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 a 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 !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 - } - - // Grab a worker before we commit to running below. - node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop()) - if node == nil { - // There is at least one worker per P, so normally there are - // enough workers to run on all Ps, if necessary. However, once - // a worker enters gcMarkDone it may park without rejoining the - // pool, thus freeing a P with no corresponding worker. - // gcMarkDone never depends on another worker doing work, so it - // is safe to simply do nothing here. - // - // If gcMarkDone bails out without completing the mark phase, - // it will always do so with queued global work. Thus, that P - // will be immediately eligible to re-run the worker G it was - // just using, ensuring work can complete. - return nil - } - - decIfPositive := func(ptr *int64) bool { - for { - v := atomic.Loadint64(ptr) - if v <= 0 { - return false - } - - if atomic.Casint64(ptr, v, v-1) { - return true - } - } - } - - 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. - gcBgMarkWorkerPool.push(&node.node) - 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. - gcBgMarkWorkerPool.push(&node.node) - return nil - } - // Run a fractional worker. - _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode - } - - // Run the background mark worker. - gp := node.gp.ptr() - casgstatus(gp, _Gwaiting, _Grunnable) - if trace.enabled { - traceGoUnpark(gp, 0) - } - return gp -} - // pollFractionalWorkerExit reports whether a fractional mark worker // should self-preempt. It assumes it is called from the fractional // worker. @@ -815,203 +292,6 @@ func pollFractionalWorkerExit() bool { 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) { - assertWorldStoppedOrLockHeld(&mheap_.lock) - - // 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 - } - - // Set the trigger ratio, capped to reasonable bounds. - if gcpercent >= 0 { - scalingFactor := float64(gcpercent) / 100 - // Ensure there's always a little margin so that the - // mutator assist ratio isn't infinity. - maxTriggerRatio := 0.95 * scalingFactor - if triggerRatio > maxTriggerRatio { - triggerRatio = maxTriggerRatio - } - - // If we let triggerRatio go too low, then if the application - // is allocating very rapidly we might end up in a situation - // where we're allocating black during a nearly always-on GC. - // The result of this is a growing heap and ultimately an - // increase in RSS. By capping us at a point >0, we're essentially - // saying that we're OK using more CPU during the GC to prevent - // this growth in RSS. - // - // The current constant was chosen empirically: given a sufficiently - // fast/scalable allocator with 48 Ps that could drive the trigger ratio - // to <0.05, this constant causes applications to retain the same peak - // RSS compared to not having this allocator. - minTriggerRatio := 0.6 * scalingFactor - if triggerRatio < minTriggerRatio { - triggerRatio = minTriggerRatio - } - } else if triggerRatio < 0 { - // gcpercent < 0, so just make sure we're not getting a negative - // triggerRatio. This case isn't expected to happen in practice, - // and doesn't really matter because if gcpercent < 0 then we won't - // ever consume triggerRatio further on in this function, but let's - // just be defensive here; the triggerRatio being negative is almost - // certainly undesirable. - triggerRatio = 0 - } - 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 !isSweepDone() { - // 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 - 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") - } - if trigger > goal { - // 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 - } - } - - // Commit to the trigger and goal. - memstats.gc_trigger = trigger - atomic.Store64(&memstats.next_gc, goal) - if trace.enabled { - traceNextGC() - } - - // Update mark pacing. - if gcphase != _GCoff { - gcController.revise() - } - - // Update sweep pacing. - if isSweepDone() { - 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) - pagesInUse := atomic.Load64(&mheap_.pagesInUse) - sweepDistancePages := int64(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) - } - } - - gcPaceScavenger() -} - -// gcEffectiveGrowthRatio returns the current effective heap growth -// ratio (GOGC/100) based on heap_marked from the previous GC and -// next_gc for the current GC. -// -// This may differ from gcpercent/100 because of various upper and -// lower bounds on gcpercent. For example, if the heap is smaller than -// heapminimum, this can be higher than gcpercent/100. -// -// mheap_.lock must be held or the world must be stopped. -func gcEffectiveGrowthRatio() float64 { - assertWorldStoppedOrLockHeld(&mheap_.lock) - - egogc := float64(atomic.Load64(&memstats.next_gc)-memstats.heap_marked) / float64(memstats.heap_marked) - if egogc < 0 { - // Shouldn't happen, but just in case. - egogc = 0 - } - return egogc -} - -// 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 -// 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 diff --git a/src/runtime/mgcpacer.go b/src/runtime/mgcpacer.go new file mode 100644 index 00000000000..b5184b91825 --- /dev/null +++ b/src/runtime/mgcpacer.go @@ -0,0 +1,735 @@ +// Copyright 2021 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. + +package runtime + +import ( + "internal/cpu" + "runtime/internal/atomic" + _ "unsafe" // for linkname +) + +const ( + // gcGoalUtilization is the goal CPU utilization for + // marking as a fraction of GOMAXPROCS. + 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. + gcBackgroundUtilization = 0.25 + + // gcCreditSlack is the amount of scan work credit that 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. + gcCreditSlack = 2000 + + // gcAssistTimeSlack is the nanoseconds of mutator assist time that + // can accumulate on a P before updating gcController.assistTime. + 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. + gcOverAssistWork = 64 << 10 + + // defaultHeapMinimum is the value of heapminimum for GOGC==100. + defaultHeapMinimum = 4 << 20 +) + +var ( + // 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. + heapminimum uint64 = defaultHeapMinimum + + // Initialized from $GOGC. GOGC=off means no GC. + gcpercent int32 +) + +// 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. + // + // Stored as a uint64, but it's actually a float64. Use + // float64frombits to get the value. + // + // Read and written atomically. + assistWorkPerByte uint64 + + // assistBytesPerWork is 1/assistWorkPerByte. + // + // Stored as a uint64, but it's actually a float64. Use + // float64frombits to get the value. + // + // Read and written atomically. + // + // Note that because this is read and written independently + // from assistWorkPerByte users may notice a skew between + // the two values, and such a state should be safe. + assistBytesPerWork uint64 + + // 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 the 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 + + _ cpu.CacheLinePad +} + +// startCycle resets the GC controller's state and computes estimates +// for a new GC cycle. The caller must hold worldsema and the world +// must be stopped. +func (c *gcControllerState) startCycle() { + c.scanWork = 0 + c.bgScanCredit = 0 + c.assistTime = 0 + c.dedicatedMarkTime = 0 + c.fractionalMarkTime = 0 + c.idleMarkTime = 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 + } + + // In STW mode, we just want dedicated workers. + if debug.gcstoptheworld > 0 { + c.dedicatedMarkWorkersNeeded = int64(gomaxprocs) + 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 { + assistRatio := float64frombits(atomic.Load64(&c.assistWorkPerByte)) + print("pacer: assist ratio=", assistRatio, + " (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 whenever memstats.heap_scan, +// memstats.heap_live, or memstats.next_gc is updated. It is safe to +// call concurrently, but it may race with other calls to revise. +// +// The result of this race is that the two assist ratio values may not line +// up or may be stale. In practice this is OK because the assist ratio +// moves slowly throughout a GC cycle, and the assist ratio is a best-effort +// heuristic anyway. Furthermore, no part of the heuristic depends on +// the two assist ratio values being exact reciprocals of one another, since +// the two values are used to convert values from different sources. +// +// The worst case result of this raciness is that we may miss a larger shift +// in the ratio (say, if we decide to pace more aggressively against the +// hard heap goal) but even this "hard goal" is best-effort (see #40460). +// The dedicated GC should ensure we don't exceed the hard goal by too much +// in the rare case we do exceed it. +// +// 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) + scan := atomic.Load64(&memstats.heap_scan) + work := atomic.Loadint64(&c.scanWork) + + // Assume we're under the soft goal. Pace GC to complete at + // next_gc assuming the heap is in steady-state. + heapGoal := int64(atomic.Load64(&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(scan) * 100 / float64(100+gcpercent)) + + if int64(live) > heapGoal || work > scanWorkExpected { + // We're past the soft goal, or we've already done more scan + // work than we expected. Pace GC so that in the worst case it + // will complete by the hard goal. + const maxOvershoot = 1.1 + heapGoal = int64(float64(heapGoal) * maxOvershoot) + + // Compute the upper bound on the scan work remaining. + scanWorkExpected = int64(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 + // slowly in the soft regime and not at all in the hard + // regime. + scanWorkRemaining := scanWorkExpected - work + 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. + // Note that the assist ratio values are updated atomically + // but not together. This means there may be some degree of + // skew between the two values. This is generally OK as the + // values shift relatively slowly over the course of a GC + // cycle. + assistWorkPerByte := float64(scanWorkRemaining) / float64(heapRemaining) + assistBytesPerWork := float64(heapRemaining) / float64(scanWorkRemaining) + atomic.Store64(&c.assistWorkPerByte, float64bits(assistWorkPerByte)) + atomic.Store64(&c.assistBytesPerWork, float64bits(assistBytesPerWork)) +} + +// 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 := gcEffectiveGrowthRatio() + 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 a 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 !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 + } + + // Grab a worker before we commit to running below. + node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop()) + if node == nil { + // There is at least one worker per P, so normally there are + // enough workers to run on all Ps, if necessary. However, once + // a worker enters gcMarkDone it may park without rejoining the + // pool, thus freeing a P with no corresponding worker. + // gcMarkDone never depends on another worker doing work, so it + // is safe to simply do nothing here. + // + // If gcMarkDone bails out without completing the mark phase, + // it will always do so with queued global work. Thus, that P + // will be immediately eligible to re-run the worker G it was + // just using, ensuring work can complete. + return nil + } + + decIfPositive := func(ptr *int64) bool { + for { + v := atomic.Loadint64(ptr) + if v <= 0 { + return false + } + + if atomic.Casint64(ptr, v, v-1) { + return true + } + } + } + + 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. + gcBgMarkWorkerPool.push(&node.node) + 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. + gcBgMarkWorkerPool.push(&node.node) + return nil + } + // Run a fractional worker. + _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode + } + + // Run the background mark worker. + gp := node.gp.ptr() + casgstatus(gp, _Gwaiting, _Grunnable) + if trace.enabled { + traceGoUnpark(gp, 0) + } + return gp +} + +// 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) { + assertWorldStoppedOrLockHeld(&mheap_.lock) + + // 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 + } + + // Set the trigger ratio, capped to reasonable bounds. + if gcpercent >= 0 { + scalingFactor := float64(gcpercent) / 100 + // Ensure there's always a little margin so that the + // mutator assist ratio isn't infinity. + maxTriggerRatio := 0.95 * scalingFactor + if triggerRatio > maxTriggerRatio { + triggerRatio = maxTriggerRatio + } + + // If we let triggerRatio go too low, then if the application + // is allocating very rapidly we might end up in a situation + // where we're allocating black during a nearly always-on GC. + // The result of this is a growing heap and ultimately an + // increase in RSS. By capping us at a point >0, we're essentially + // saying that we're OK using more CPU during the GC to prevent + // this growth in RSS. + // + // The current constant was chosen empirically: given a sufficiently + // fast/scalable allocator with 48 Ps that could drive the trigger ratio + // to <0.05, this constant causes applications to retain the same peak + // RSS compared to not having this allocator. + minTriggerRatio := 0.6 * scalingFactor + if triggerRatio < minTriggerRatio { + triggerRatio = minTriggerRatio + } + } else if triggerRatio < 0 { + // gcpercent < 0, so just make sure we're not getting a negative + // triggerRatio. This case isn't expected to happen in practice, + // and doesn't really matter because if gcpercent < 0 then we won't + // ever consume triggerRatio further on in this function, but let's + // just be defensive here; the triggerRatio being negative is almost + // certainly undesirable. + triggerRatio = 0 + } + 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 !isSweepDone() { + // 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 + 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") + } + if trigger > goal { + // 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 + } + } + + // Commit to the trigger and goal. + memstats.gc_trigger = trigger + atomic.Store64(&memstats.next_gc, goal) + if trace.enabled { + traceNextGC() + } + + // Update mark pacing. + if gcphase != _GCoff { + gcController.revise() + } + + // Update sweep pacing. + if isSweepDone() { + 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) + pagesInUse := atomic.Load64(&mheap_.pagesInUse) + sweepDistancePages := int64(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) + } + } + + gcPaceScavenger() +} + +// gcEffectiveGrowthRatio returns the current effective heap growth +// ratio (GOGC/100) based on heap_marked from the previous GC and +// next_gc for the current GC. +// +// This may differ from gcpercent/100 because of various upper and +// lower bounds on gcpercent. For example, if the heap is smaller than +// heapminimum, this can be higher than gcpercent/100. +// +// mheap_.lock must be held or the world must be stopped. +func gcEffectiveGrowthRatio() float64 { + assertWorldStoppedOrLockHeld(&mheap_.lock) + + egogc := float64(atomic.Load64(&memstats.next_gc)-memstats.heap_marked) / float64(memstats.heap_marked) + if egogc < 0 { + // Shouldn't happen, but just in case. + egogc = 0 + } + return egogc +} + +//go:linkname setGCPercent runtime/debug.setGCPercent +func setGCPercent(in int32) (out int32) { + // Run on the system stack since we grab the heap lock. + systemstack(func() { + 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 mark to + // finish so we always return with no GC running. + if in < 0 { + gcWaitOnMark(atomic.Load(&work.cycles)) + } + + return out +} + +func readgogc() int32 { + p := gogetenv("GOGC") + if p == "off" { + return -1 + } + if n, ok := atoi32(p); ok { + return n + } + return 100 +}