1 files changed, 255 insertions, 0 deletions
diff --git a/src/runtime/mgclimit_test.go b/src/runtime/mgclimit_test.go
new file mode 100644
index 0000000..124da03
--- /dev/null
+++ b/src/runtime/mgclimit_test.go
@@ -0,0 +1,255 @@
+// Copyright 2022 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 (
+	. "runtime"
+	"testing"
+	"time"
+)
+
+func TestGCCPULimiter(t *testing.T) {
+	const procs = 14
+
+	// Create mock time.
+	ticks := int64(0)
+	advance := func(d time.Duration) int64 {
+		t.Helper()
+		ticks += int64(d)
+		return ticks
+	}
+
+	// assistTime computes the CPU time for assists using frac of GOMAXPROCS
+	// over the wall-clock duration d.
+	assistTime := func(d time.Duration, frac float64) int64 {
+		t.Helper()
+		return int64(frac * float64(d) * procs)
+	}
+
+	l := NewGCCPULimiter(ticks, procs)
+
+	// Do the whole test twice to make sure state doesn't leak across.
+	var baseOverflow uint64 // Track total overflow across iterations.
+	for i := 0; i < 2; i++ {
+		t.Logf("Iteration %d", i+1)
+
+		if l.Capacity() != procs*CapacityPerProc {
+			t.Fatalf("unexpected capacity: %d", l.Capacity())
+		}
+		if l.Fill() != 0 {
+			t.Fatalf("expected empty bucket to start")
+		}
+
+		// Test filling the bucket with just mutator time.
+
+		l.Update(advance(10 * time.Millisecond))
+		l.Update(advance(1 * time.Second))
+		l.Update(advance(1 * time.Hour))
+		if l.Fill() != 0 {
+			t.Fatalf("expected empty bucket from only accumulating mutator time, got fill of %d cpu-ns", l.Fill())
+		}
+
+		// Test needUpdate.
+
+		if l.NeedUpdate(advance(GCCPULimiterUpdatePeriod / 2)) {
+			t.Fatal("need update even though updated half a period ago")
+		}
+		if !l.NeedUpdate(advance(GCCPULimiterUpdatePeriod)) {
+			t.Fatal("doesn't need update even though updated 1.5 periods ago")
+		}
+		l.Update(advance(0))
+		if l.NeedUpdate(advance(0)) {
+			t.Fatal("need update even though just updated")
+		}
+
+		// Test transitioning the bucket to enable the GC.
+
+		l.StartGCTransition(true, advance(109*time.Millisecond))
+		l.FinishGCTransition(advance(2*time.Millisecond + 1*time.Microsecond))
+
+		if expect := uint64((2*time.Millisecond + 1*time.Microsecond) * procs); l.Fill() != expect {
+			t.Fatalf("expected fill of %d, got %d cpu-ns", expect, l.Fill())
+		}
+
+		// Test passing time without assists during a GC. Specifically, just enough to drain the bucket to
+		// exactly procs nanoseconds (easier to get to because of rounding).
+		//
+		// The window we need to drain the bucket is 1/(1-2*gcBackgroundUtilization) times the current fill:
+		//
+		//   fill + (window * procs * gcBackgroundUtilization - window * procs * (1-gcBackgroundUtilization)) = n
+		//   fill = n - (window * procs * gcBackgroundUtilization - window * procs * (1-gcBackgroundUtilization))
+		//   fill = n + window * procs * ((1-gcBackgroundUtilization) - gcBackgroundUtilization)
+		//   fill = n + window * procs * (1-2*gcBackgroundUtilization)
+		//   window = (fill - n) / (procs * (1-2*gcBackgroundUtilization)))
+		//
+		// And here we want n=procs:
+		factor := (1 / (1 - 2*GCBackgroundUtilization))
+		fill := (2*time.Millisecond + 1*time.Microsecond) * procs
+		l.Update(advance(time.Duration(factor * float64(fill-procs) / procs)))
+		if l.Fill() != procs {
+			t.Fatalf("expected fill %d cpu-ns from draining after a GC started, got fill of %d cpu-ns", procs, l.Fill())
+		}
+
+		// Drain to zero for the rest of the test.
+		l.Update(advance(2 * procs * CapacityPerProc))
+		if l.Fill() != 0 {
+			t.Fatalf("expected empty bucket from draining, got fill of %d cpu-ns", l.Fill())
+		}
+
+		// Test filling up the bucket with 50% total GC work (so, not moving the bucket at all).
+		l.AddAssistTime(assistTime(10*time.Millisecond, 0.5-GCBackgroundUtilization))
+		l.Update(advance(10 * time.Millisecond))
+		if l.Fill() != 0 {
+			t.Fatalf("expected empty bucket from 50%% GC work, got fill of %d cpu-ns", l.Fill())
+		}
+
+		// Test adding to the bucket overall with 100% GC work.
+		l.AddAssistTime(assistTime(time.Millisecond, 1.0-GCBackgroundUtilization))
+		l.Update(advance(time.Millisecond))
+		if expect := uint64(procs * time.Millisecond); l.Fill() != expect {
+			t.Errorf("expected %d fill from 100%% GC CPU, got fill of %d cpu-ns", expect, l.Fill())
+		}
+		if l.Limiting() {
+			t.Errorf("limiter is enabled after filling bucket but shouldn't be")
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Test filling the bucket exactly full.
+		l.AddAssistTime(assistTime(CapacityPerProc-time.Millisecond, 1.0-GCBackgroundUtilization))
+		l.Update(advance(CapacityPerProc - time.Millisecond))
+		if l.Fill() != l.Capacity() {
+			t.Errorf("expected bucket filled to capacity %d, got %d", l.Capacity(), l.Fill())
+		}
+		if !l.Limiting() {
+			t.Errorf("limiter is not enabled after filling bucket but should be")
+		}
+		if l.Overflow() != 0+baseOverflow {
+			t.Errorf("bucket filled exactly should not have overflow, found %d", l.Overflow())
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Test adding with a delta of exactly zero. That is, GC work is exactly 50% of all resources.
+		// Specifically, the limiter should still be on, and no overflow should accumulate.
+		l.AddAssistTime(assistTime(1*time.Second, 0.5-GCBackgroundUtilization))
+		l.Update(advance(1 * time.Second))
+		if l.Fill() != l.Capacity() {
+			t.Errorf("expected bucket filled to capacity %d, got %d", l.Capacity(), l.Fill())
+		}
+		if !l.Limiting() {
+			t.Errorf("limiter is not enabled after filling bucket but should be")
+		}
+		if l.Overflow() != 0+baseOverflow {
+			t.Errorf("bucket filled exactly should not have overflow, found %d", l.Overflow())
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Drain the bucket by half.
+		l.AddAssistTime(assistTime(CapacityPerProc, 0))
+		l.Update(advance(CapacityPerProc))
+		if expect := l.Capacity() / 2; l.Fill() != expect {
+			t.Errorf("failed to drain to %d, got fill %d", expect, l.Fill())
+		}
+		if l.Limiting() {
+			t.Errorf("limiter is enabled after draining bucket but shouldn't be")
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Test overfilling the bucket.
+		l.AddAssistTime(assistTime(CapacityPerProc, 1.0-GCBackgroundUtilization))
+		l.Update(advance(CapacityPerProc))
+		if l.Fill() != l.Capacity() {
+			t.Errorf("failed to fill to capacity %d, got fill %d", l.Capacity(), l.Fill())
+		}
+		if !l.Limiting() {
+			t.Errorf("limiter is not enabled after overfill but should be")
+		}
+		if expect := uint64(CapacityPerProc * procs / 2); l.Overflow() != expect+baseOverflow {
+			t.Errorf("bucket overfilled should have overflow %d, found %d", expect, l.Overflow())
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Test ending the cycle with some assists left over.
+		l.AddAssistTime(assistTime(1*time.Millisecond, 1.0-GCBackgroundUtilization))
+		l.StartGCTransition(false, advance(1*time.Millisecond))
+		if l.Fill() != l.Capacity() {
+			t.Errorf("failed to maintain fill to capacity %d, got fill %d", l.Capacity(), l.Fill())
+		}
+		if !l.Limiting() {
+			t.Errorf("limiter is not enabled after overfill but should be")
+		}
+		if expect := uint64((CapacityPerProc/2 + time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
+			t.Errorf("bucket overfilled should have overflow %d, found %d", expect, l.Overflow())
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Make sure the STW adds to the bucket.
+		l.FinishGCTransition(advance(5 * time.Millisecond))
+		if l.Fill() != l.Capacity() {
+			t.Errorf("failed to maintain fill to capacity %d, got fill %d", l.Capacity(), l.Fill())
+		}
+		if !l.Limiting() {
+			t.Errorf("limiter is not enabled after overfill but should be")
+		}
+		if expect := uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
+			t.Errorf("bucket overfilled should have overflow %d, found %d", expect, l.Overflow())
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Resize procs up and make sure limiting stops.
+		expectFill := l.Capacity()
+		l.ResetCapacity(advance(0), procs+10)
+		if l.Fill() != expectFill {
+			t.Errorf("failed to maintain fill at old capacity %d, got fill %d", expectFill, l.Fill())
+		}
+		if l.Limiting() {
+			t.Errorf("limiter is enabled after resetting capacity higher")
+		}
+		if expect := uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
+			t.Errorf("bucket overflow %d should have remained constant, found %d", expect, l.Overflow())
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Resize procs down and make sure limiting begins again.
+		// Also make sure resizing doesn't affect overflow. This isn't
+		// a case where we want to report overflow, because we're not
+		// actively doing work to achieve it. It's that we have fewer
+		// CPU resources now.
+		l.ResetCapacity(advance(0), procs-10)
+		if l.Fill() != l.Capacity() {
+			t.Errorf("failed lower fill to new capacity %d, got fill %d", l.Capacity(), l.Fill())
+		}
+		if !l.Limiting() {
+			t.Errorf("limiter is disabled after resetting capacity lower")
+		}
+		if expect := uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
+			t.Errorf("bucket overflow %d should have remained constant, found %d", expect, l.Overflow())
+		}
+		if t.Failed() {
+			t.FailNow()
+		}
+
+		// Get back to a zero state. The top of the loop will double check.
+		l.ResetCapacity(advance(CapacityPerProc*procs), procs)
+
+		// Track total overflow for future iterations.
+		baseOverflow += uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs)
+	}
+}