diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go index 01f917bded95b1..2cc89e71218fa0 100644 --- a/src/runtime/export_test.go +++ b/src/runtime/export_test.go @@ -1230,3 +1230,81 @@ func GCTestPointerClass(p unsafe.Pointer) string { } const Raceenabled = raceenabled + +const ( + GCBackgroundUtilization = gcBackgroundUtilization + GCGoalUtilization = gcGoalUtilization +) + +type GCController struct { + gcControllerState +} + +func NewGCController(gcPercent int) *GCController { + // Force the controller to escape. We're going to + // do 64-bit atomics on it, and if it gets stack-allocated + // on a 32-bit architecture, it may get allocated unaligned + // space. + g := escape(new(GCController)).(*GCController) + g.init(int32(gcPercent)) + return g +} + +func (c *GCController) StartCycle(stackSize, globalsSize uint64, scannableFrac float64, gomaxprocs int) { + c.scannableStackSize = stackSize + c.globalsScan = globalsSize + c.heapLive = c.trigger + c.heapScan += uint64(float64(c.trigger-c.heapMarked) * scannableFrac) + c.startCycle(0, gomaxprocs) +} + +func (c *GCController) AssistWorkPerByte() float64 { + return c.assistWorkPerByte.Load() +} + +func (c *GCController) HeapGoal() uint64 { + return c.heapGoal +} + +func (c *GCController) HeapLive() uint64 { + return c.heapLive +} + +func (c *GCController) HeapMarked() uint64 { + return c.heapMarked +} + +func (c *GCController) Trigger() uint64 { + return c.trigger +} + +type GCControllerReviseDelta struct { + HeapLive int64 + HeapScan int64 + HeapScanWork int64 + StackScanWork int64 + GlobalsScanWork int64 +} + +func (c *GCController) Revise(d GCControllerReviseDelta) { + c.heapLive += uint64(d.HeapLive) + c.heapScan += uint64(d.HeapScan) + c.scanWork += d.HeapScanWork + d.StackScanWork + d.GlobalsScanWork + c.revise() +} + +func (c *GCController) EndCycle(bytesMarked uint64, assistTime, elapsed int64, gomaxprocs int) { + c.assistTime = assistTime + triggerRatio := c.endCycle(elapsed, gomaxprocs, false) + c.resetLive(bytesMarked) + c.commit(triggerRatio) +} + +var escapeSink interface{} + +//go:noinline +func escape(x interface{}) interface{} { + escapeSink = x + escapeSink = nil + return x +} diff --git a/src/runtime/mgcpacer_test.go b/src/runtime/mgcpacer_test.go new file mode 100644 index 00000000000000..5a9f46c6d17ae9 --- /dev/null +++ b/src/runtime/mgcpacer_test.go @@ -0,0 +1,590 @@ +// 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_test + +import ( + "fmt" + "math" + "math/rand" + . "runtime" + "testing" + "time" +) + +func TestGcPacer(t *testing.T) { + t.Parallel() + + const initialHeapBytes = 256 << 10 + for _, e := range []*gcExecTest{ + { + // The most basic test case: a steady-state heap. + // Growth to an O(MiB) heap, then constant heap size, alloc/scan rates. + name: "Steady", + gcPercent: 100, + globalsBytes: 32 << 10, + nCores: 8, + allocRate: constant(33.0), + scanRate: constant(1024.0), + growthRate: constant(2.0).sum(ramp(-1.0, 12)), + scannableFrac: constant(1.0), + stackBytes: constant(8192), + length: 50, + checker: func(t *testing.T, c []gcCycleResult) { + n := len(c) + if n >= 25 { + // Make sure the pacer settles into a non-degenerate state in at least 25 GC cycles. + assertInEpsilon(t, "GC utilization", c[n-1].gcUtilization, c[n-2].gcUtilization, 0.005) + assertInRange(t, "goal ratio", c[n-1].goalRatio(), 0.95, 1.05) + } + }, + }, + { + // This tests the GC pacer's response to a small change in allocation rate. + name: "StepAlloc", + gcPercent: 100, + globalsBytes: 32 << 10, + nCores: 8, + allocRate: constant(33.0).sum(ramp(66.0, 1).delay(50)), + scanRate: constant(1024.0), + growthRate: constant(2.0).sum(ramp(-1.0, 12)), + scannableFrac: constant(1.0), + stackBytes: constant(8192), + length: 100, + checker: func(t *testing.T, c []gcCycleResult) { + n := len(c) + if (n >= 25 && n < 50) || n >= 75 { + // Make sure the pacer settles into a non-degenerate state in at least 25 GC cycles + // and then is able to settle again after a significant jump in allocation rate. + assertInEpsilon(t, "GC utilization", c[n-1].gcUtilization, c[n-2].gcUtilization, 0.005) + assertInRange(t, "goal ratio", c[n-1].goalRatio(), 0.95, 1.05) + } + }, + }, + { + // This tests the GC pacer's response to a large change in allocation rate. + name: "HeavyStepAlloc", + gcPercent: 100, + globalsBytes: 32 << 10, + nCores: 8, + allocRate: constant(33).sum(ramp(330, 1).delay(50)), + scanRate: constant(1024.0), + growthRate: constant(2.0).sum(ramp(-1.0, 12)), + scannableFrac: constant(1.0), + stackBytes: constant(8192), + length: 100, + checker: func(t *testing.T, c []gcCycleResult) { + n := len(c) + if (n >= 25 && n < 50) || n >= 75 { + // Make sure the pacer settles into a non-degenerate state in at least 25 GC cycles + // and then is able to settle again after a significant jump in allocation rate. + assertInEpsilon(t, "GC utilization", c[n-1].gcUtilization, c[n-2].gcUtilization, 0.005) + assertInRange(t, "goal ratio", c[n-1].goalRatio(), 0.95, 1.05) + } + }, + }, + { + // This tests the GC pacer's response to a change in the fraction of the scannable heap. + name: "StepScannableFrac", + gcPercent: 100, + globalsBytes: 32 << 10, + nCores: 8, + allocRate: constant(128.0), + scanRate: constant(1024.0), + growthRate: constant(2.0).sum(ramp(-1.0, 12)), + scannableFrac: constant(0.2).sum(unit(0.5).delay(50)), + stackBytes: constant(8192), + length: 100, + checker: func(t *testing.T, c []gcCycleResult) { + n := len(c) + if (n >= 25 && n < 50) || n >= 75 { + // Make sure the pacer settles into a non-degenerate state in at least 25 GC cycles + // and then is able to settle again after a significant jump in allocation rate. + assertInEpsilon(t, "GC utilization", c[n-1].gcUtilization, c[n-2].gcUtilization, 0.005) + assertInRange(t, "goal ratio", c[n-1].goalRatio(), 0.95, 1.05) + } + }, + }, + { + // This test makes sure that in the face of a varying (in this case, oscillating) allocation + // rate, the pacer does a reasonably good job of staying abreast of the changes. + name: "OscAlloc", + gcPercent: 100, + globalsBytes: 32 << 10, + nCores: 8, + allocRate: oscillate(13, 0, 8).offset(67), + scanRate: constant(1024.0), + growthRate: constant(2.0).sum(ramp(-1.0, 12)), + scannableFrac: constant(1.0), + stackBytes: constant(8192), + length: 50, + checker: func(t *testing.T, c []gcCycleResult) { + n := len(c) + if n > 12 { + // After the 12th GC, the heap will stop growing. Now, just make sure that: + // 1. Utilization isn't varying _too_ much, and + // 2. The pacer is mostly keeping up with the goal. + assertInRange(t, "goal ratio", c[n-1].goalRatio(), 0.95, 1.05) + assertInRange(t, "GC utilization", c[n-1].gcUtilization, 0.25, 0.4) + } + }, + }, + { + // This test is the same as OscAlloc, but instead of oscillating, the allocation rate is jittery. + name: "JitterAlloc", + gcPercent: 100, + globalsBytes: 32 << 10, + nCores: 8, + allocRate: random(13, 0xf).offset(132), + scanRate: constant(1024.0), + growthRate: constant(2.0).sum(ramp(-1.0, 12), random(0.01, 0xe)), + scannableFrac: constant(1.0), + stackBytes: constant(8192), + length: 50, + checker: func(t *testing.T, c []gcCycleResult) { + n := len(c) + if n > 12 { + // After the 12th GC, the heap will stop growing. Now, just make sure that: + // 1. Utilization isn't varying _too_ much, and + // 2. The pacer is mostly keeping up with the goal. + assertInRange(t, "goal ratio", c[n-1].goalRatio(), 0.95, 1.05) + assertInRange(t, "GC utilization", c[n-1].gcUtilization, 0.25, 0.4) + } + }, + }, + { + // This test is the same as JitterAlloc, but with a much higher allocation rate. + // The jitter is proportionally the same. + name: "HeavyJitterAlloc", + gcPercent: 100, + globalsBytes: 32 << 10, + nCores: 8, + allocRate: random(33.0, 0x0).offset(330), + scanRate: constant(1024.0), + growthRate: constant(2.0).sum(ramp(-1.0, 12), random(0.01, 0x152)), + scannableFrac: constant(1.0), + stackBytes: constant(8192), + length: 50, + checker: func(t *testing.T, c []gcCycleResult) { + n := len(c) + if n > 13 { + // After the 12th GC, the heap will stop growing. Now, just make sure that: + // 1. Utilization isn't varying _too_ much, and + // 2. The pacer is mostly keeping up with the goal. + // We start at the 13th here because we want to use the 12th as a reference. + assertInRange(t, "goal ratio", c[n-1].goalRatio(), 0.95, 1.05) + // Unlike the other tests, GC utilization here will vary more and tend higher. + // Just make sure it's not going too crazy. + assertInEpsilon(t, "GC utilization", c[n-1].gcUtilization, c[n-2].gcUtilization, 0.05) + assertInEpsilon(t, "GC utilization", c[n-1].gcUtilization, c[11].gcUtilization, 0.07) + } + }, + }, + } { + e := e + t.Run(e.name, func(t *testing.T) { + t.Parallel() + + c := NewGCController(e.gcPercent) + var bytesAllocatedBlackLast int64 + results := make([]gcCycleResult, 0, e.length) + for i := 0; i < e.length; i++ { + cycle := e.next() + c.StartCycle(cycle.stackBytes, e.globalsBytes, cycle.scannableFrac, e.nCores) + + // Update pacer incrementally as we complete scan work. + const ( + revisePeriod = 500 * time.Microsecond + rateConv = 1024 * float64(revisePeriod) / float64(time.Millisecond) + ) + var nextHeapMarked int64 + if i == 0 { + nextHeapMarked = initialHeapBytes + } else { + nextHeapMarked = int64(float64(int64(c.HeapMarked())-bytesAllocatedBlackLast) * cycle.growthRate) + } + globalsScanWorkLeft := int64(e.globalsBytes) + stackScanWorkLeft := int64(cycle.stackBytes) + heapScanWorkLeft := int64(float64(nextHeapMarked) * cycle.scannableFrac) + doWork := func(work int64) (int64, int64, int64) { + var deltas [3]int64 + + // Do globals work first, then stacks, then heap. + for i, workLeft := range []*int64{&globalsScanWorkLeft, &stackScanWorkLeft, &heapScanWorkLeft} { + if *workLeft == 0 { + continue + } + if *workLeft > work { + deltas[i] += work + *workLeft -= work + work = 0 + break + } else { + deltas[i] += *workLeft + work -= *workLeft + *workLeft = 0 + } + } + return deltas[0], deltas[1], deltas[2] + } + var ( + gcDuration int64 + assistTime int64 + bytesAllocatedBlack int64 + ) + for heapScanWorkLeft+stackScanWorkLeft+globalsScanWorkLeft > 0 { + // Simulate GC assist pacing. + // + // Note that this is an idealized view of the GC assist pacing + // mechanism. + + // From the assist ratio and the alloc and scan rates, we can idealize what + // the GC CPU utilization looks like. + // + // We start with assistRatio = (bytes of scan work) / (bytes of runway) (by definition). + // + // Over revisePeriod, we can also calculate how many bytes are scanned and + // allocated, given some GC CPU utilization u: + // + // bytesScanned = scanRate * rateConv * nCores * u + // bytesAllocated = allocRate * rateConv * nCores * (1 - u) + // + // During revisePeriod, assistRatio is kept constant, and GC assists kick in to + // maintain it. Specifically, they act to prevent too many bytes being allocated + // compared to how many bytes are scanned. It directly defines the ratio of + // bytesScanned to bytesAllocated over this period, hence: + // + // assistRatio = bytesScanned / bytesAllocated + // + // From this, we can solve for utilization, because everything else has already + // been determined: + // + // assistRatio = (scanRate * rateConv * nCores * u) / (allocRate * rateConv * nCores * (1 - u)) + // assistRatio = (scanRate * u) / (allocRate * (1 - u)) + // assistRatio * allocRate * (1-u) = scanRate * u + // assistRatio * allocRate - assistRatio * allocRate * u = scanRate * u + // assistRatio * allocRate = assistRatio * allocRate * u + scanRate * u + // assistRatio * allocRate = (assistRatio * allocRate + scanRate) * u + // u = (assistRatio * allocRate) / (assistRatio * allocRate + scanRate) + // + // Note that this may give a utilization that is _less_ than GCBackgroundUtilization, + // which isn't possible in practice because of dedicated workers. Thus, this case + // must be interpreted as GC assists not kicking in at all, and just round up. All + // downstream values will then have this accounted for. + assistRatio := c.AssistWorkPerByte() + utilization := assistRatio * cycle.allocRate / (assistRatio*cycle.allocRate + cycle.scanRate) + if utilization < GCBackgroundUtilization { + utilization = GCBackgroundUtilization + } + + // Knowing the utilization, calculate bytesScanned and bytesAllocated. + bytesScanned := int64(cycle.scanRate * rateConv * float64(e.nCores) * utilization) + bytesAllocated := int64(cycle.allocRate * rateConv * float64(e.nCores) * (1 - utilization)) + + // Subtract work from our model. + globalsScanned, stackScanned, heapScanned := doWork(bytesScanned) + + // doWork may not use all of bytesScanned. + // In this case, the GC actually ends sometime in this period. + // Let's figure out when, exactly, and adjust bytesAllocated too. + actualElapsed := revisePeriod + actualAllocated := bytesAllocated + if actualScanned := globalsScanned + stackScanned + heapScanned; actualScanned < bytesScanned { + // actualScanned = scanRate * rateConv * (t / revisePeriod) * nCores * u + // => t = actualScanned * revisePeriod / (scanRate * rateConv * nCores * u) + actualElapsed = time.Duration(float64(actualScanned) * float64(revisePeriod) / (cycle.scanRate * rateConv * float64(e.nCores) * utilization)) + actualAllocated = int64(cycle.allocRate * rateConv * float64(actualElapsed) / float64(revisePeriod) * float64(e.nCores) * (1 - utilization)) + } + + // Ask the pacer to revise. + c.Revise(GCControllerReviseDelta{ + HeapLive: actualAllocated, + HeapScan: int64(float64(actualAllocated) * cycle.scannableFrac), + HeapScanWork: heapScanned, + StackScanWork: stackScanned, + GlobalsScanWork: globalsScanned, + }) + + // Accumulate variables. + assistTime += int64(float64(actualElapsed) * float64(e.nCores) * (utilization - GCBackgroundUtilization)) + gcDuration += int64(actualElapsed) + bytesAllocatedBlack += actualAllocated + } + + // Put together the results, log them, and concatenate them. + result := gcCycleResult{ + cycle: i + 1, + heapLive: c.HeapMarked(), + heapScannable: int64(float64(int64(c.HeapMarked())-bytesAllocatedBlackLast) * cycle.scannableFrac), + heapTrigger: c.Trigger(), + heapPeak: c.HeapLive(), + heapGoal: c.HeapGoal(), + gcUtilization: float64(assistTime)/(float64(gcDuration)*float64(e.nCores)) + GCBackgroundUtilization, + } + t.Log("GC", result.String()) + results = append(results, result) + + // Run the checker for this test. + e.check(t, results) + + c.EndCycle(uint64(nextHeapMarked+bytesAllocatedBlack), assistTime, gcDuration, e.nCores) + + bytesAllocatedBlackLast = bytesAllocatedBlack + } + }) + } +} + +type gcExecTest struct { + name string + + gcPercent int + globalsBytes uint64 + nCores int + + allocRate float64Stream // > 0, KiB / cpu-ms + scanRate float64Stream // > 0, KiB / cpu-ms + growthRate float64Stream // > 0 + scannableFrac float64Stream // Clamped to [0, 1] + stackBytes float64Stream // Multiple of 2048. + length int + + checker func(*testing.T, []gcCycleResult) +} + +// minRate is an arbitrary minimum for allocRate, scanRate, and growthRate. +// These values just cannot be zero. +const minRate = 0.0001 + +func (e *gcExecTest) next() gcCycle { + return gcCycle{ + allocRate: e.allocRate.min(minRate)(), + scanRate: e.scanRate.min(minRate)(), + growthRate: e.growthRate.min(minRate)(), + scannableFrac: e.scannableFrac.limit(0, 1)(), + stackBytes: uint64(e.stackBytes.quantize(2048).min(0)()), + } +} + +func (e *gcExecTest) check(t *testing.T, results []gcCycleResult) { + t.Helper() + + // Do some basic general checks first. + n := len(results) + switch n { + case 0: + t.Fatal("no results passed to check") + return + case 1: + if results[0].cycle != 1 { + t.Error("first cycle has incorrect number") + } + default: + if results[n-1].cycle != results[n-2].cycle+1 { + t.Error("cycle numbers out of order") + } + } + if u := results[n-1].gcUtilization; u < 0 || u > 1 { + t.Fatal("GC utilization not within acceptable bounds") + } + if s := results[n-1].heapScannable; s < 0 { + t.Fatal("heapScannable is negative") + } + if e.checker == nil { + t.Fatal("test-specific checker is missing") + } + + // Run the test-specific checker. + e.checker(t, results) +} + +type gcCycle struct { + allocRate float64 + scanRate float64 + growthRate float64 + scannableFrac float64 + stackBytes uint64 +} + +type gcCycleResult struct { + cycle int + + // These come directly from the pacer, so uint64. + heapLive uint64 + heapTrigger uint64 + heapGoal uint64 + heapPeak uint64 + + // These are produced by the simulation, so int64 and + // float64 are more appropriate, so that we can check for + // bad states in the simulation. + heapScannable int64 + gcUtilization float64 +} + +func (r *gcCycleResult) goalRatio() float64 { + return float64(r.heapPeak) / float64(r.heapGoal) +} + +func (r *gcCycleResult) String() string { + return fmt.Sprintf("%d %2.1f%% %d->%d->%d (goal: %d)", r.cycle, r.gcUtilization*100, r.heapLive, r.heapTrigger, r.heapPeak, r.heapGoal) +} + +func assertInEpsilon(t *testing.T, name string, a, b, epsilon float64) { + t.Helper() + assertInRange(t, name, a, b-epsilon, b+epsilon) +} + +func assertInRange(t *testing.T, name string, a, min, max float64) { + t.Helper() + if a < min || a > max { + t.Errorf("%s not in range (%f, %f): %f", name, min, max, a) + } +} + +// float64Stream is a function that generates an infinite stream of +// float64 values when called repeatedly. +type float64Stream func() float64 + +// constant returns a stream that generates the value c. +func constant(c float64) float64Stream { + return func() float64 { + return c + } +} + +// unit returns a stream that generates a single peak with +// amplitude amp, followed by zeroes. +// +// In another manner of speaking, this is the Kronecker delta. +func unit(amp float64) float64Stream { + dropped := false + return func() float64 { + if dropped { + return 0 + } + dropped = true + return amp + } +} + +// oscillate returns a stream that oscillates sinusoidally +// with the given amplitude, phase, and period. +func oscillate(amp, phase float64, period int) float64Stream { + var cycle int + return func() float64 { + p := float64(cycle)/float64(period)*2*math.Pi + phase + cycle++ + if cycle == period { + cycle = 0 + } + return math.Sin(p) * amp + } +} + +// ramp returns a stream that moves from zero to height +// over the course of length steps. +func ramp(height float64, length int) float64Stream { + var cycle int + return func() float64 { + h := height * float64(cycle) / float64(length) + if cycle < length { + cycle++ + } + return h + } +} + +// random returns a stream that generates random numbers +// between -amp and amp. +func random(amp float64, seed int64) float64Stream { + r := rand.New(rand.NewSource(seed)) + return func() float64 { + return ((r.Float64() - 0.5) * 2) * amp + } +} + +// delay returns a new stream which is a buffered version +// of f: it returns zero for cycles steps, followed by f. +func (f float64Stream) delay(cycles int) float64Stream { + zeroes := 0 + return func() float64 { + if zeroes < cycles { + zeroes++ + return 0 + } + return f() + } +} + +// scale returns a new stream that is f, but attenuated by a +// constant factor. +func (f float64Stream) scale(amt float64) float64Stream { + return func() float64 { + return f() * amt + } +} + +// offset returns a new stream that is f but offset by amt +// at each step. +func (f float64Stream) offset(amt float64) float64Stream { + return func() float64 { + old := f() + return old + amt + } +} + +// sum returns a new stream that is the sum of all input streams +// at each step. +func (f float64Stream) sum(fs ...float64Stream) float64Stream { + return func() float64 { + sum := f() + for _, s := range fs { + sum += s() + } + return sum + } +} + +// quantize returns a new stream that rounds f to a multiple +// of mult at each step. +func (f float64Stream) quantize(mult float64) float64Stream { + return func() float64 { + r := f() / mult + if r < 0 { + return math.Ceil(r) * mult + } + return math.Floor(r) * mult + } +} + +// min returns a new stream that replaces all values produced +// by f lower than min with min. +func (f float64Stream) min(min float64) float64Stream { + return func() float64 { + return math.Max(min, f()) + } +} + +// max returns a new stream that replaces all values produced +// by f higher than max with max. +func (f float64Stream) max(max float64) float64Stream { + return func() float64 { + return math.Min(max, f()) + } +} + +// limit returns a new stream that replaces all values produced +// by f lower than min with min and higher than max with max. +func (f float64Stream) limit(min, max float64) float64Stream { + return func() float64 { + v := f() + if v < min { + v = min + } else if v > max { + v = max + } + return v + } +}