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runtime: tune P retake logic 

When GOMAXPROCS>1 the last P in syscall is never retaken
(because there are already idle P's -- npidle>0).
This prevents sysmon thread from sleeping.
On a darwin machine the program from issue 6673 constantly
consumes ~0.2% CPU. With this change it stably consumes 0.0% CPU.
Fixes #6673.

R=golang-codereviews, r
CC=bradfitz, golang-codereviews, iant, khr
https://golang.org/cl/56990045
This commit is contained in:
Dmitriy Vyukov 2014-01-27 23:17:46 +04:00
parent ea86752206
commit 179d41fecc
1 changed files with 6 additions and 3 deletions
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9
src/pkg/runtime/proc.c
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@ -2534,16 +2534,19 @@ retake(int64 now)
pd = &pdesc[i];
s = p->status;
if(s == Psyscall) {
// Retake P from syscall if it's there for more than 1 sysmon tick (20us).
// But only if there is other work to do.
// Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
t = p->syscalltick;
if(pd->syscalltick != t) {
pd->syscalltick = t;
pd->syscallwhen = now;
continue;
}
// On the one hand we don't want to retake Ps if there is no other work to do,
// but on the other hand we want to retake them eventually
// because they can prevent the sysmon thread from deep sleep.
if(p->runqhead == p->runqtail &&
runtime·atomicload(&runtime·sched.nmspinning) + runtime·atomicload(&runtime·sched.npidle) > 0)
runtime·atomicload(&runtime·sched.nmspinning) + runtime·atomicload(&runtime·sched.npidle) > 0 &&
pd->syscallwhen + 10*1000*1000 > now)
continue;
// Need to decrement number of idle locked M's
// (pretending that one more is running) before the CAS.