diff options
Diffstat (limited to 'libgo/go/runtime/mgc.go')
| -rw-r--r-- | libgo/go/runtime/mgc.go | 935 |
1 files changed, 155 insertions, 780 deletions
diff --git a/libgo/go/runtime/mgc.go b/libgo/go/runtime/mgc.go index a9f2c1a..efb8012 100644 --- a/libgo/go/runtime/mgc.go +++ b/libgo/go/runtime/mgc.go @@ -113,8 +113,8 @@ // 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 +// (this mark is tracked in gcController.heapGoal 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 @@ -149,45 +149,16 @@ 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") } - // No sweep on the first cycle. - mheap_.sweepdone = 1 + mheap_.sweepDrained = 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()) + // Initialize GC pacer state. + // Use the environment variable GOGC for the initial gcPercent value. + gcController.init(readGOGC()) work.startSema = 1 work.markDoneSema = 1 @@ -196,16 +167,9 @@ 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 // gcenable is called after the bulk of the runtime initialization, // just before we're about to start letting user code run. @@ -213,41 +177,17 @@ func readgogc() int32 { // scavenger goroutine, and enables GC. func gcenable() { // Kick off sweeping and scavenging. - c := make(chan int, 2) + gcenable_setup = make(chan int, 2) expectSystemGoroutine() - go bgsweep(c) + go bgsweep() expectSystemGoroutine() - go bgscavenge(c) - <-c - <-c + go bgscavenge() + <-gcenable_setup + <-gcenable_setup + gcenable_setup = nil memstats.enablegc = true // now that runtime is initialized, GC is okay } -//go:linkname setGCPercent runtime_1debug.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 @@ -304,9 +244,11 @@ const ( // 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. + // an integer and using only dedicated workers would result in + // utilization too far from the target of gcBackgroundUtilization. + // 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 @@ -325,474 +267,6 @@ 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 - } - - // TODO: having atomic.Casint64 would be more pleasant. - if atomic.Cas64((*uint64)(unsafe.Pointer(ptr)), uint64(v), uint64(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. @@ -811,203 +285,6 @@ 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 @@ -1050,9 +327,11 @@ var work struct { nwait uint32 // Number of roots of various root types. Set by gcMarkRootPrepare. - nFlushCacheRoots int nDataRoots, nSpanRoots, nStackRoots int + // Base indexes of each root type. Set by gcMarkRootPrepare. + baseData, baseSpans, baseStacks, baseEnd uint32 + // 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 @@ -1086,7 +365,7 @@ var work struct { // program started if debug.gctrace > 0. totaltime int64 - // initialHeapLive is the value of memstats.heap_live at the + // initialHeapLive is the value of gcController.heapLive at the // beginning of this GC cycle. initialHeapLive uint64 @@ -1183,7 +462,7 @@ func GC() { // 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 { + for atomic.Load(&work.cycles) == n+1 && !isSweepDone() { Gosched() } @@ -1267,13 +546,13 @@ func (t gcTrigger) test() bool { } switch t.kind { case gcTriggerHeap: - // Non-atomic access to heap_live for performance. If + // Non-atomic access to gcController.heapLive for performance. If // we are going to trigger on this, this thread just - // atomically wrote heap_live anyway and we'll see our + // atomically wrote gcController.heapLive anyway and we'll see our // own write. - return memstats.heap_live >= memstats.gc_trigger + return gcController.heapLive >= gcController.trigger case gcTriggerTime: - if gcpercent < 0 { + if gcController.gcPercent < 0 { return false } lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime)) @@ -1367,7 +646,7 @@ func gcStart(trigger gcTrigger) { // so it can't be more than ncpu, even if GOMAXPROCS is. work.stwprocs = ncpu } - work.heap0 = atomic.Load64(&memstats.heap_live) + work.heap0 = atomic.Load64(&gcController.heapLive) work.pauseNS = 0 work.mode = mode @@ -1390,7 +669,7 @@ func gcStart(trigger gcTrigger) { work.cycles++ gcController.startCycle() - work.heapGoal = memstats.next_gc + work.heapGoal = gcController.heapGoal // In STW mode, disable scheduling of user Gs. This may also // disable scheduling of this goroutine, so it may block as @@ -1619,7 +898,7 @@ top: // endCycle depends on all gcWork cache stats being flushed. // The termination algorithm above ensured that up to // allocations since the ragged barrier. - nextTriggerRatio := gcController.endCycle() + nextTriggerRatio := gcController.endCycle(work.userForced) // Perform mark termination. This will restart the world. gcMarkTermination(nextTriggerRatio) @@ -1631,7 +910,7 @@ func gcMarkTermination(nextTriggerRatio float64) { // Start marktermination (write barrier remains enabled for now). setGCPhase(_GCmarktermination) - work.heap1 = memstats.heap_live + work.heap1 = gcController.heapLive startTime := nanotime() mp := acquirem() @@ -1693,12 +972,12 @@ func gcMarkTermination(nextTriggerRatio float64) { throw("gc done but gcphase != _GCoff") } - // Record next_gc and heap_inuse for scavenger. - memstats.last_next_gc = memstats.next_gc + // Record heapGoal and heap_inuse for scavenger. + gcController.lastHeapGoal = gcController.heapGoal memstats.last_heap_inuse = memstats.heap_inuse // Update GC trigger and pacing for the next cycle. - gcSetTriggerRatio(nextTriggerRatio) + gcController.commit(nextTriggerRatio) // Update timing memstats now := nanotime() @@ -1745,6 +1024,13 @@ func gcMarkTermination(nextTriggerRatio float64) { // so events don't leak into the wrong cycle. mProf_NextCycle() + // There may be stale spans in mcaches that need to be swept. + // Those aren't tracked in any sweep lists, so we need to + // count them against sweep completion until we ensure all + // those spans have been forced out. + sl := newSweepLocker() + sl.blockCompletion() + systemstack(func() { startTheWorldWithSema(true) }) // Flush the heap profile so we can start a new cycle next GC. @@ -1765,6 +1051,9 @@ func gcMarkTermination(nextTriggerRatio float64) { _p_.mcache.prepareForSweep() }) }) + // Now that we've swept stale spans in mcaches, they don't + // count against unswept spans. + sl.dispose() // Print gctrace before dropping worldsema. As soon as we drop // worldsema another cycle could start and smash the stats @@ -1986,15 +1275,11 @@ func gcBgMarkWorker() { // everything out of the run // queue so it can run // somewhere else. - lock(&sched.lock) - for { - gp, _ := runqget(pp) - if gp == nil { - break - } - globrunqput(gp) + if drainQ, n := runqdrain(pp); n > 0 { + lock(&sched.lock) + globrunqputbatch(&drainQ, int32(n)) + unlock(&sched.lock) } - unlock(&sched.lock) } // Go back to draining, this time // without preemption. @@ -2068,7 +1353,7 @@ func gcMarkWorkAvailable(p *p) bool { // gcMark runs the mark (or, for concurrent GC, mark termination) // All gcWork caches must be empty. // STW is in effect at this point. -func gcMark(start_time int64) { +func gcMark(startTime int64) { if debug.allocfreetrace > 0 { tracegc() } @@ -2076,7 +1361,7 @@ func gcMark(start_time int64) { if gcphase != _GCmarktermination { throw("in gcMark expecting to see gcphase as _GCmarktermination") } - work.tstart = start_time + work.tstart = startTime // Check that there's no marking work remaining. if work.full != 0 || work.markrootNext < work.markrootJobs { @@ -2138,25 +1423,25 @@ func gcMark(start_time int64) { } // Update the marked heap stat. - memstats.heap_marked = work.bytesMarked + gcController.heapMarked = work.bytesMarked // Flush scanAlloc from each mcache since we're about to modify - // heap_scan directly. If we were to flush this later, then scanAlloc + // heapScan directly. If we were to flush this later, then scanAlloc // might have incorrect information. for _, p := range allp { c := p.mcache if c == nil { continue } - memstats.heap_scan += uint64(c.scanAlloc) + gcController.heapScan += uint64(c.scanAlloc) c.scanAlloc = 0 } // 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) + // flushallmcaches (which modifies gcController.heapLive). + gcController.heapLive = work.bytesMarked + gcController.heapScan = uint64(gcController.scanWork) if trace.enabled { traceHeapAlloc() @@ -2178,7 +1463,7 @@ func gcSweep(mode gcMode) { lock(&mheap_.lock) mheap_.sweepgen += 2 - mheap_.sweepdone = 0 + mheap_.sweepDrained = 0 mheap_.pagesSwept = 0 mheap_.sweepArenas = mheap_.allArenas mheap_.reclaimIndex = 0 @@ -2229,14 +1514,12 @@ func gcSweep(mode gcMode) { // //go:systemstack func gcResetMarkState() { - // This may be called during a concurrent phase, so make sure + // This may be called during a concurrent phase, so lock to make sure // allgs doesn't change. - lock(&allglock) - for _, gp := range allgs { + forEachG(func(gp *g) { gp.gcscandone = false // set to true in gcphasework gp.gcAssistBytes = 0 - } - unlock(&allglock) + }) // Clear page marks. This is just 1MB per 64GB of heap, so the // time here is pretty trivial. @@ -2251,7 +1534,7 @@ func gcResetMarkState() { } work.bytesMarked = 0 - work.initialHeapLive = atomic.Load64(&memstats.heap_live) + work.initialHeapLive = atomic.Load64(&gcController.heapLive) } // Hooks for other packages @@ -2334,3 +1617,95 @@ func fmtNSAsMS(buf []byte, ns uint64) []byte { } return itoaDiv(buf, x, dec) } + +// Helpers for testing GC. + +// gcTestIsReachable performs a GC and returns a bit set where bit i +// is set if ptrs[i] is reachable. +func gcTestIsReachable(ptrs ...unsafe.Pointer) (mask uint64) { + // This takes the pointers as unsafe.Pointers in order to keep + // them live long enough for us to attach specials. After + // that, we drop our references to them. + + if len(ptrs) > 64 { + panic("too many pointers for uint64 mask") + } + + // Block GC while we attach specials and drop our references + // to ptrs. Otherwise, if a GC is in progress, it could mark + // them reachable via this function before we have a chance to + // drop them. + semacquire(&gcsema) + + // Create reachability specials for ptrs. + specials := make([]*specialReachable, len(ptrs)) + for i, p := range ptrs { + lock(&mheap_.speciallock) + s := (*specialReachable)(mheap_.specialReachableAlloc.alloc()) + unlock(&mheap_.speciallock) + s.special.kind = _KindSpecialReachable + if !addspecial(p, &s.special) { + throw("already have a reachable special (duplicate pointer?)") + } + specials[i] = s + // Make sure we don't retain ptrs. + ptrs[i] = nil + } + + semrelease(&gcsema) + + // Force a full GC and sweep. + GC() + + // Process specials. + for i, s := range specials { + if !s.done { + printlock() + println("runtime: object", i, "was not swept") + throw("IsReachable failed") + } + if s.reachable { + mask |= 1 << i + } + lock(&mheap_.speciallock) + mheap_.specialReachableAlloc.free(unsafe.Pointer(s)) + unlock(&mheap_.speciallock) + } + + return mask +} + +// onCurrentStack reports whether the argument is on the current stack. +// It is implemented in C. +func onCurrentStack(uintptr) bool + +// getBSS returns the start of the BSS section. +// It is implemented in C. +func getBSS() uintptr + +// gcTestPointerClass returns the category of what p points to, one of: +// "heap", "stack", "data", "bss", "other". This is useful for checking +// that a test is doing what it's intended to do. +// +// This is nosplit simply to avoid extra pointer shuffling that may +// complicate a test. +// +//go:nosplit +func gcTestPointerClass(p unsafe.Pointer) string { + p2 := uintptr(noescape(p)) + if onCurrentStack(p2) { + return "stack" + } + if base, _, _ := findObject(p2, 0, 0, false); base != 0 { + return "heap" + } + bss := getBSS() + if p2 >= getText() && p2 < bss { + return "data" + } + if p2 >= bss && p2 < getEnd() { + return "bss" + } + KeepAlive(p) + return "other" +} |