mgclimit_test.go

Documentation: runtime

     1  // Copyright 2022 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package runtime_test
     6  
     7  import (
     8  	. "runtime"
     9  	"testing"
    10  	"time"
    11  )
    12  
    13  func TestGCCPULimiter(t *testing.T) {
    14  	const procs = 14
    15  
    16  	// Create mock time.
    17  	ticks := int64(0)
    18  	advance := func(d time.Duration) int64 {
    19  		t.Helper()
    20  		ticks += int64(d)
    21  		return ticks
    22  	}
    23  
    24  	// assistTime computes the CPU time for assists using frac of GOMAXPROCS
    25  	// over the wall-clock duration d.
    26  	assistTime := func(d time.Duration, frac float64) int64 {
    27  		t.Helper()
    28  		return int64(frac * float64(d) * procs)
    29  	}
    30  
    31  	l := NewGCCPULimiter(ticks, procs)
    32  
    33  	// Do the whole test twice to make sure state doesn't leak across.
    34  	var baseOverflow uint64 // Track total overflow across iterations.
    35  	for i := 0; i < 2; i++ {
    36  		t.Logf("Iteration %d", i+1)
    37  
    38  		if l.Capacity() != procs*CapacityPerProc {
    39  			t.Fatalf("unexpected capacity: %d", l.Capacity())
    40  		}
    41  		if l.Fill() != 0 {
    42  			t.Fatalf("expected empty bucket to start")
    43  		}
    44  
    45  		// Test filling the bucket with just mutator time.
    46  
    47  		l.Update(advance(10 * time.Millisecond))
    48  		l.Update(advance(1 * time.Second))
    49  		l.Update(advance(1 * time.Hour))
    50  		if l.Fill() != 0 {
    51  			t.Fatalf("expected empty bucket from only accumulating mutator time, got fill of %d cpu-ns", l.Fill())
    52  		}
    53  
    54  		// Test needUpdate.
    55  
    56  		if l.NeedUpdate(advance(GCCPULimiterUpdatePeriod / 2)) {
    57  			t.Fatal("need update even though updated half a period ago")
    58  		}
    59  		if !l.NeedUpdate(advance(GCCPULimiterUpdatePeriod)) {
    60  			t.Fatal("doesn't need update even though updated 1.5 periods ago")
    61  		}
    62  		l.Update(advance(0))
    63  		if l.NeedUpdate(advance(0)) {
    64  			t.Fatal("need update even though just updated")
    65  		}
    66  
    67  		// Test transitioning the bucket to enable the GC.
    68  
    69  		l.StartGCTransition(true, advance(109*time.Millisecond))
    70  		l.FinishGCTransition(advance(2*time.Millisecond + 1*time.Microsecond))
    71  
    72  		if expect := uint64((2*time.Millisecond + 1*time.Microsecond) * procs); l.Fill() != expect {
    73  			t.Fatalf("expected fill of %d, got %d cpu-ns", expect, l.Fill())
    74  		}
    75  
    76  		// Test passing time without assists during a GC. Specifically, just enough to drain the bucket to
    77  		// exactly procs nanoseconds (easier to get to because of rounding).
    78  		//
    79  		// The window we need to drain the bucket is 1/(1-2*gcBackgroundUtilization) times the current fill:
    80  		//
    81  		//   fill + (window * procs * gcBackgroundUtilization - window * procs * (1-gcBackgroundUtilization)) = n
    82  		//   fill = n - (window * procs * gcBackgroundUtilization - window * procs * (1-gcBackgroundUtilization))
    83  		//   fill = n + window * procs * ((1-gcBackgroundUtilization) - gcBackgroundUtilization)
    84  		//   fill = n + window * procs * (1-2*gcBackgroundUtilization)
    85  		//   window = (fill - n) / (procs * (1-2*gcBackgroundUtilization)))
    86  		//
    87  		// And here we want n=procs:
    88  		factor := (1 / (1 - 2*GCBackgroundUtilization))
    89  		fill := (2*time.Millisecond + 1*time.Microsecond) * procs
    90  		l.Update(advance(time.Duration(factor * float64(fill-procs) / procs)))
    91  		if l.Fill() != procs {
    92  			t.Fatalf("expected fill %d cpu-ns from draining after a GC started, got fill of %d cpu-ns", procs, l.Fill())
    93  		}
    94  
    95  		// Drain to zero for the rest of the test.
    96  		l.Update(advance(2 * procs * CapacityPerProc))
    97  		if l.Fill() != 0 {
    98  			t.Fatalf("expected empty bucket from draining, got fill of %d cpu-ns", l.Fill())
    99  		}
   100  
   101  		// Test filling up the bucket with 50% total GC work (so, not moving the bucket at all).
   102  		l.AddAssistTime(assistTime(10*time.Millisecond, 0.5-GCBackgroundUtilization))
   103  		l.Update(advance(10 * time.Millisecond))
   104  		if l.Fill() != 0 {
   105  			t.Fatalf("expected empty bucket from 50%% GC work, got fill of %d cpu-ns", l.Fill())
   106  		}
   107  
   108  		// Test adding to the bucket overall with 100% GC work.
   109  		l.AddAssistTime(assistTime(time.Millisecond, 1.0-GCBackgroundUtilization))
   110  		l.Update(advance(time.Millisecond))
   111  		if expect := uint64(procs * time.Millisecond); l.Fill() != expect {
   112  			t.Errorf("expected %d fill from 100%% GC CPU, got fill of %d cpu-ns", expect, l.Fill())
   113  		}
   114  		if l.Limiting() {
   115  			t.Errorf("limiter is enabled after filling bucket but shouldn't be")
   116  		}
   117  		if t.Failed() {
   118  			t.FailNow()
   119  		}
   120  
   121  		// Test filling the bucket exactly full.
   122  		l.AddAssistTime(assistTime(CapacityPerProc-time.Millisecond, 1.0-GCBackgroundUtilization))
   123  		l.Update(advance(CapacityPerProc - time.Millisecond))
   124  		if l.Fill() != l.Capacity() {
   125  			t.Errorf("expected bucket filled to capacity %d, got %d", l.Capacity(), l.Fill())
   126  		}
   127  		if !l.Limiting() {
   128  			t.Errorf("limiter is not enabled after filling bucket but should be")
   129  		}
   130  		if l.Overflow() != 0+baseOverflow {
   131  			t.Errorf("bucket filled exactly should not have overflow, found %d", l.Overflow())
   132  		}
   133  		if t.Failed() {
   134  			t.FailNow()
   135  		}
   136  
   137  		// Test adding with a delta of exactly zero. That is, GC work is exactly 50% of all resources.
   138  		// Specifically, the limiter should still be on, and no overflow should accumulate.
   139  		l.AddAssistTime(assistTime(1*time.Second, 0.5-GCBackgroundUtilization))
   140  		l.Update(advance(1 * time.Second))
   141  		if l.Fill() != l.Capacity() {
   142  			t.Errorf("expected bucket filled to capacity %d, got %d", l.Capacity(), l.Fill())
   143  		}
   144  		if !l.Limiting() {
   145  			t.Errorf("limiter is not enabled after filling bucket but should be")
   146  		}
   147  		if l.Overflow() != 0+baseOverflow {
   148  			t.Errorf("bucket filled exactly should not have overflow, found %d", l.Overflow())
   149  		}
   150  		if t.Failed() {
   151  			t.FailNow()
   152  		}
   153  
   154  		// Drain the bucket by half.
   155  		l.AddAssistTime(assistTime(CapacityPerProc, 0))
   156  		l.Update(advance(CapacityPerProc))
   157  		if expect := l.Capacity() / 2; l.Fill() != expect {
   158  			t.Errorf("failed to drain to %d, got fill %d", expect, l.Fill())
   159  		}
   160  		if l.Limiting() {
   161  			t.Errorf("limiter is enabled after draining bucket but shouldn't be")
   162  		}
   163  		if t.Failed() {
   164  			t.FailNow()
   165  		}
   166  
   167  		// Test overfilling the bucket.
   168  		l.AddAssistTime(assistTime(CapacityPerProc, 1.0-GCBackgroundUtilization))
   169  		l.Update(advance(CapacityPerProc))
   170  		if l.Fill() != l.Capacity() {
   171  			t.Errorf("failed to fill to capacity %d, got fill %d", l.Capacity(), l.Fill())
   172  		}
   173  		if !l.Limiting() {
   174  			t.Errorf("limiter is not enabled after overfill but should be")
   175  		}
   176  		if expect := uint64(CapacityPerProc * procs / 2); l.Overflow() != expect+baseOverflow {
   177  			t.Errorf("bucket overfilled should have overflow %d, found %d", expect, l.Overflow())
   178  		}
   179  		if t.Failed() {
   180  			t.FailNow()
   181  		}
   182  
   183  		// Test ending the cycle with some assists left over.
   184  		l.AddAssistTime(assistTime(1*time.Millisecond, 1.0-GCBackgroundUtilization))
   185  		l.StartGCTransition(false, advance(1*time.Millisecond))
   186  		if l.Fill() != l.Capacity() {
   187  			t.Errorf("failed to maintain fill to capacity %d, got fill %d", l.Capacity(), l.Fill())
   188  		}
   189  		if !l.Limiting() {
   190  			t.Errorf("limiter is not enabled after overfill but should be")
   191  		}
   192  		if expect := uint64((CapacityPerProc/2 + time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
   193  			t.Errorf("bucket overfilled should have overflow %d, found %d", expect, l.Overflow())
   194  		}
   195  		if t.Failed() {
   196  			t.FailNow()
   197  		}
   198  
   199  		// Make sure the STW adds to the bucket.
   200  		l.FinishGCTransition(advance(5 * time.Millisecond))
   201  		if l.Fill() != l.Capacity() {
   202  			t.Errorf("failed to maintain fill to capacity %d, got fill %d", l.Capacity(), l.Fill())
   203  		}
   204  		if !l.Limiting() {
   205  			t.Errorf("limiter is not enabled after overfill but should be")
   206  		}
   207  		if expect := uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
   208  			t.Errorf("bucket overfilled should have overflow %d, found %d", expect, l.Overflow())
   209  		}
   210  		if t.Failed() {
   211  			t.FailNow()
   212  		}
   213  
   214  		// Resize procs up and make sure limiting stops.
   215  		expectFill := l.Capacity()
   216  		l.ResetCapacity(advance(0), procs+10)
   217  		if l.Fill() != expectFill {
   218  			t.Errorf("failed to maintain fill at old capacity %d, got fill %d", expectFill, l.Fill())
   219  		}
   220  		if l.Limiting() {
   221  			t.Errorf("limiter is enabled after resetting capacity higher")
   222  		}
   223  		if expect := uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
   224  			t.Errorf("bucket overflow %d should have remained constant, found %d", expect, l.Overflow())
   225  		}
   226  		if t.Failed() {
   227  			t.FailNow()
   228  		}
   229  
   230  		// Resize procs down and make sure limiting begins again.
   231  		// Also make sure resizing doesn't affect overflow. This isn't
   232  		// a case where we want to report overflow, because we're not
   233  		// actively doing work to achieve it. It's that we have fewer
   234  		// CPU resources now.
   235  		l.ResetCapacity(advance(0), procs-10)
   236  		if l.Fill() != l.Capacity() {
   237  			t.Errorf("failed lower fill to new capacity %d, got fill %d", l.Capacity(), l.Fill())
   238  		}
   239  		if !l.Limiting() {
   240  			t.Errorf("limiter is disabled after resetting capacity lower")
   241  		}
   242  		if expect := uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs); l.Overflow() != expect+baseOverflow {
   243  			t.Errorf("bucket overflow %d should have remained constant, found %d", expect, l.Overflow())
   244  		}
   245  		if t.Failed() {
   246  			t.FailNow()
   247  		}
   248  
   249  		// Get back to a zero state. The top of the loop will double check.
   250  		l.ResetCapacity(advance(CapacityPerProc*procs), procs)
   251  
   252  		// Track total overflow for future iterations.
   253  		baseOverflow += uint64((CapacityPerProc/2 + 6*time.Millisecond) * procs)
   254  	}
   255  }
   256
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