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runtime: remove overlap in the GC CPU pause time metrics

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Currently the GC CPU pause time metrics start measuring before the STW
is complete. This results in a slightly less accurate measurement and
creates some overlap with other timings (for example, the idle time of
idle Ps) that will cause double-counting.

This CL adds a field to worldStop to track the point at which the world
actually stopped and uses that as the basis for the GC CPU pause time
metrics, basically eliminating this overlap.

Note that this will cause Ps in _Pgcstop before the world is fully
stopped to be counted as user time. A follow-up CL will fix this
discrepancy.

Change-Id: I287731f08415ffd97d327f582ddf7e5d2248a6f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/570258
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Nicolas Hillegeer <aktau@google.com>
This commit is contained in:
Michael Anthony Knyszek 2024-03-08 23:17:23 +00:00 committed by Gopher Robot
parent 017478a963
commit d4395ebc0c
2 changed files with 12 additions and 10 deletions
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10
src/runtime/mgc.go
View File

@ -749,12 +749,12 @@ func gcStart(trigger gcTrigger) {
// Use maxprocs instead of stwprocs here because the total time
// computed in the CPU stats is based on maxprocs, and we want them
// to be comparable.
work.cpuStats.accumulateGCPauseTime(nanotime()-work.tSweepTerm, work.maxprocs)
work.cpuStats.accumulateGCPauseTime(nanotime()-stw.finishedStopping, work.maxprocs)
// Concurrent mark.
systemstack(func() {
now = startTheWorldWithSema(0, stw)
work.pauseNS += now - stw.start
work.pauseNS += now - stw.startedStopping
work.tMark = now
// Release the CPU limiter.
@ -895,7 +895,7 @@ top:
getg().m.preemptoff = ""
systemstack(func() {
now := startTheWorldWithSema(0, stw)
work.pauseNS += now - stw.start
work.pauseNS += now - stw.startedStopping
})
semrelease(&worldsema)
goto top
@ -1015,7 +1015,7 @@ func gcMarkTermination(stw worldStop) {
now := nanotime()
sec, nsec, _ := time_now()
unixNow := sec*1e9 + int64(nsec)
work.pauseNS += now - stw.start
work.pauseNS += now - stw.startedStopping
work.tEnd = now
atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user
atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us
@ -1031,7 +1031,7 @@ func gcMarkTermination(stw worldStop) {
//
// Pass gcMarkPhase=true to accumulate so we can get all the latest GC CPU stats
// in there too.
work.cpuStats.accumulateGCPauseTime(work.tEnd-work.tMarkTerm, work.maxprocs)
work.cpuStats.accumulateGCPauseTime(now-stw.finishedStopping, work.maxprocs)
work.cpuStats.accumulate(now, true)
// Compute overall GC CPU utilization.

12
src/runtime/proc.go
View File

@ -1322,8 +1322,9 @@ var stwReasonStrings = [...]string{
// worldStop provides context from the stop-the-world required by the
// start-the-world.
type worldStop struct {
reason stwReason
start int64
reason stwReason
startedStopping int64
finishedStopping int64
}
// Temporary variable for stopTheWorld, when it can't write to the stack.
@ -1522,7 +1523,8 @@ func stopTheWorldWithSema(reason stwReason) worldStop {
}
}
startTime := nanotime() - start
finish := nanotime()
startTime := finish - start
if reason.isGC() {
sched.stwStoppingTimeGC.record(startTime)
} else {
@ -1554,7 +1556,7 @@ func stopTheWorldWithSema(reason stwReason) worldStop {
worldStopped()
return worldStop{reason: reason, start: start}
return worldStop{reason: reason, startedStopping: start, finishedStopping: finish}
}
// reason is the same STW reason passed to stopTheWorld. start is the start
@ -1610,7 +1612,7 @@ func startTheWorldWithSema(now int64, w worldStop) int64 {
if now == 0 {
now = nanotime()
}
totalTime := now - w.start
totalTime := now - w.startedStopping
if w.reason.isGC() {
sched.stwTotalTimeGC.record(totalTime)
} else {