From 89b7a08aea13d750f0bb48d88fea7dc867c9295c Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Tue, 14 Nov 2017 15:08:32 -0800 Subject: [PATCH] runtime: fix gctrace STW CPU time and CPU fraction The CPU time reported in the gctrace for STW phases is simply work.stwprocs times the wall-clock duration of these phases. However, work.stwprocs is set to gcprocs(), which is wrong for multiple reasons: 1. gcprocs is intended to limit the number of Ms used for mark termination based on how well the garbage collector actually scales, but the gctrace wants to report how much CPU time is being stolen from the application. During STW, that's *all* of the CPU, regardless of how many the garbage collector can actually use. 2. gcprocs assumes it's being called during STW, so it limits its result to sched.nmidle+1. However, we're not calling it during STW, so sched.nmidle is typically quite small, even if GOMAXPROCS is quite large. Fix this by setting work.stwprocs to min(ncpu, GOMAXPROCS). This also fixes the overall GC CPU fraction, which is based on the computed CPU times. Fixes #22725. Change-Id: I64b5ce87e28dbec6870aa068ce7aecdd28c058d1 Reviewed-on: https://go-review.googlesource.com/77710 Run-TryBot: Austin Clements TryBot-Result: Gobot Gobot Reviewed-by: Rick Hudson --- src/runtime/mgc.go | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go index f4523ee7a2..a28ecdec2d 100644 --- a/src/runtime/mgc.go +++ b/src/runtime/mgc.go @@ -1282,7 +1282,12 @@ func gcStart(mode gcMode, trigger gcTrigger) { gcResetMarkState() - work.stwprocs, work.maxprocs = gcprocs(), gomaxprocs + work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs + if work.stwprocs > ncpu { + // This is used to compute CPU time of the STW phases, + // so it can't be more than ncpu, even if GOMAXPROCS is. + work.stwprocs = ncpu + } work.heap0 = atomic.Load64(&memstats.heap_live) work.pauseNS = 0 work.mode = mode -- 2.50.0