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runtime: fix hard goal calculation

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The new GC pacer has a bug where the hard goal isn't set in relation to
the original heap goal, but rather to the one already extrapolated for
overshoot.

In practice, I have never once seen this case arise because the
extrapolated goal used for overshoot is conservative. No test because
writing a test for this case is impossible in the idealized model the
pacer tests create. It is possible to simulate but will take more work.
For now, just leave a TODO.

Change-Id: I24ff710016cd8100fad54f71b2c8cdea0f7dfa79
Reviewed-on: https://go-review.googlesource.com/c/go/+/361435
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
This commit is contained in:
Michael Anthony Knyszek 2021-11-04 21:09:34 +00:00 committed by Michael Knyszek
parent 2c32f29f2f
commit 156abe5122
2 changed files with 10 additions and 3 deletions
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7
src/runtime/mgcpacer.go
View File

@ -517,7 +517,7 @@ func (c *gcControllerState) revise() {
// growths. It's OK to use more memory this cycle to scan all the live heap,
// because the next GC cycle is inevitably going to use *at least* that much
// memory anyway.
heapGoal = int64(float64(heapGoal-int64(c.trigger))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.trigger)
extHeapGoal := int64(float64(heapGoal-int64(c.trigger))/float64(scanWorkExpected)*float64(maxScanWork)) + int64(c.trigger)
scanWorkExpected = maxScanWork
// hardGoal is a hard limit on the amount that we're willing to push back the
@ -528,9 +528,10 @@ func (c *gcControllerState) revise() {
// This maintains the invariant that we use no more memory than the next GC cycle
// will anyway.
hardGoal := int64((1.0 + float64(gcPercent)/100.0) * float64(heapGoal))
if heapGoal > hardGoal {
heapGoal = hardGoal
if extHeapGoal > hardGoal {
extHeapGoal = hardGoal
}
heapGoal = extHeapGoal
}
if int64(live) > heapGoal {
// We're already past our heap goal, even the extrapolated one.

6
src/runtime/mgcpacer_test.go
View File

@ -302,6 +302,12 @@ func TestGcPacer(t *testing.T) {
}
},
},
// TODO(mknyszek): Write a test that exercises the pacer's hard goal.
// This is difficult in the idealized model this testing framework places
// the pacer in, because the calculated overshoot is directly proportional
// to the runway for the case of the expected work.
// However, it is still possible to trigger this case if something exceptional
// happens between calls to revise; the framework just doesn't support this yet.
} {
e := e
t.Run(e.name, func(t *testing.T) {