runtime: simplify fractional mark worker scheduler

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>

diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index 83eb14f772939cd5bac96579e244d25678988440..2726d293d1d68233bb55a42cb42478deeecb304b 100644 (file)
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -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