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runtime: simplify fractional mark worker scheduler

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We haven't used non-zero gcForcePreemptNS for ages. Remove it and
declutter the code.

Change-Id: Id5cc62f526d21ca394d2b6ca17d34a72959535da
Reviewed-on: https://go-review.googlesource.com/68572
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
This commit is contained in:
Austin Clements 2017-10-05 12:22:09 -04:00
parent 315c28b788
commit b783930e63
1 changed files with 2 additions and 20 deletions
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22
src/runtime/mgc.go
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@ -711,22 +711,6 @@ func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
// This P has picked the token for the fractional worker.
// Is the GC currently under or at the utilization goal?
// If so, do more work.
//
// We used to check whether doing one time slice of work
// would remain under the utilization goal, but that has the
// effect of delaying work until the mutator has run for
// enough time slices to pay for the work. During those time
// slices, write barriers are enabled, so the mutator is running slower.
// Now instead we do the work whenever we're under or at the
// utilization work and pay for it by letting the mutator run later.
// This doesn't change the overall utilization averages, but it
// front loads the GC work so that the GC finishes earlier and
// write barriers can be turned off sooner, effectively giving
// the mutator a faster machine.
//
// The old, slower behavior can be restored by setting
// gcForcePreemptNS = forcePreemptNS.
const gcForcePreemptNS = 0
// TODO(austin): We could fast path this and basically
// eliminate contention on c.fractionalMarkWorkersNeeded by
@ -739,10 +723,8 @@ func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
// TODO(austin): Shorter preemption interval for mark
// worker to improve fairness and give this
// finer-grained control over schedule?
now := nanotime() - gcController.markStartTime
then := now + gcForcePreemptNS
timeUsed := c.fractionalMarkTime + gcForcePreemptNS
if then > 0 && float64(timeUsed)/float64(then) > c.fractionalUtilizationGoal {
delta := nanotime() - c.markStartTime
if delta > 0 && float64(c.fractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal {
// Nope, we'd overshoot the utilization goal
atomic.Xaddint64(&c.fractionalMarkWorkersNeeded, +1)
return nil