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runtime: run background mark helpers only if work is available

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Prior to this CL whenever the GC marking was enabled and
a P was looking for work we supplied a G to help
the GC do its marking tasks. Once this G finished all
the marking available it would release the P to find another
available G. In the case where there was no work the P would drop
into findrunnable which would execute the mark helper G which would
immediately return and the P would drop into findrunnable again repeating
the process. Since the P was always given a G to run it never blocks.
This CL first checks if the GC mark helper G has available work and if
not the P immediately falls through to its blocking logic.

Fixes #10901

Change-Id: I94ac9646866ba64b7892af358888bc9950de23b5
Reviewed-on: https://go-review.googlesource.com/10189
Reviewed-by: Austin Clements <austin@google.com>
This commit is contained in:
Rick Hudson 2015-05-18 16:02:37 -04:00
parent f4d51eb2f5
commit 913db7685e
3 changed files with 20 additions and 1 deletions
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2
src/runtime/proc1.go
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@ -1479,7 +1479,7 @@ stop:
// We have nothing to do. If we're in the GC mark phase and can
// safely scan and blacken objects, run idle-time marking
// rather than give up the P.
if _p_ := _g_.m.p.ptr(); gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != nil {
if _p_ := _g_.m.p.ptr(); gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != nil && gcMarkWorkAvailable(_p_) {
_p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
gp := _p_.gcBgMarkWorker
casgstatus(gp, _Gwaiting, _Grunnable)