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[release-branch.go1.16] runtime: remove adjustTimers counter
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In CL 336432 we changed adjusttimers so that it no longer cleared
timerModifiedEarliest if there were no timersModifiedEarlier timers.
This caused some Google internal tests to time out, presumably due
to the increased contention on timersLock.  We can avoid that by
simply not skipping the loop in adjusttimers, which lets us safely
clear timerModifiedEarliest.  And if we don't skip the loop, then there
isn't much reason to keep the count of timerModifiedEarlier timers at all.
So remove it.

The effect will be that for programs that create some timerModifiedEarlier
timers and then remove them all, the program will do an occasional
additional loop over all the timers.  And, programs that have some
timerModifiedEarlier timers will always loop over all the timers,
without the quicker exit when they have all been seen.  But the loops
should not occur all that often, due to timerModifiedEarliest.

For #47329
For #47332

Change-Id: I7b244c1244d97b169a3c7fbc8f8a8b115731ddee
Reviewed-on: https://go-review.googlesource.com/c/go/+/337309
Trust: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
(cherry picked from commit bfbb288)
Reviewed-on: https://go-review.googlesource.com/c/go/+/338649
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ianlancetaylor committed Jul 30, 2021
1 parent ed8cbbc commit ae7943e
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Showing 3 changed files with 8 additions and 52 deletions.
1 change: 0 additions & 1 deletion src/runtime/proc.go
Original file line number Diff line number Diff line change
Expand Up @@ -4751,7 +4751,6 @@ func (pp *p) destroy() {
moveTimers(plocal, pp.timers)
pp.timers = nil
pp.numTimers = 0
pp.adjustTimers = 0
pp.deletedTimers = 0
atomic.Store64(&pp.timer0When, 0)
unlock(&pp.timersLock)
Expand Down
6 changes: 0 additions & 6 deletions src/runtime/runtime2.go
Original file line number Diff line number Diff line change
Expand Up @@ -699,12 +699,6 @@ type p struct {
// Modified using atomic instructions.
numTimers uint32

// Number of timerModifiedEarlier timers on P's heap.
// This should only be modified while holding timersLock,
// or while the timer status is in a transient state
// such as timerModifying.
adjustTimers uint32

// Number of timerDeleted timers in P's heap.
// Modified using atomic instructions.
deletedTimers uint32
Expand Down
53 changes: 8 additions & 45 deletions src/runtime/time.go
Original file line number Diff line number Diff line change
Expand Up @@ -333,7 +333,6 @@ func deltimer(t *timer) bool {
// Must fetch t.pp before setting status
// to timerDeleted.
tpp := t.pp.ptr()
atomic.Xadd(&tpp.adjustTimers, -1)
if !atomic.Cas(&t.status, timerModifying, timerDeleted) {
badTimer()
}
Expand Down Expand Up @@ -510,20 +509,9 @@ loop:

tpp := t.pp.ptr()

// Update the adjustTimers field. Subtract one if we
// are removing a timerModifiedEarlier, add one if we
// are adding a timerModifiedEarlier.
adjust := int32(0)
if status == timerModifiedEarlier {
adjust--
}
if newStatus == timerModifiedEarlier {
adjust++
updateTimerModifiedEarliest(tpp, when)
}
if adjust != 0 {
atomic.Xadd(&tpp.adjustTimers, adjust)
}

// Set the new status of the timer.
if !atomic.Cas(&t.status, timerModifying, newStatus) {
Expand Down Expand Up @@ -591,9 +579,6 @@ func cleantimers(pp *p) {
// Move t to the right position.
dodeltimer0(pp)
doaddtimer(pp, t)
if s == timerModifiedEarlier {
atomic.Xadd(&pp.adjustTimers, -1)
}
if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
badTimer()
}
Expand Down Expand Up @@ -664,32 +649,23 @@ func moveTimers(pp *p, timers []*timer) {
// it also moves timers that have been modified to run later,
// and removes deleted timers. The caller must have locked the timers for pp.
func adjusttimers(pp *p, now int64) {
if atomic.Load(&pp.adjustTimers) == 0 {
if verifyTimers {
verifyTimerHeap(pp)
}
return
}

// If we haven't yet reached the time of the first timerModifiedEarlier
// timer, don't do anything. This speeds up programs that adjust
// a lot of timers back and forth if the timers rarely expire.
// We'll postpone looking through all the adjusted timers until
// one would actually expire.
if first := atomic.Load64(&pp.timerModifiedEarliest); first != 0 {
if int64(first) > now {
if verifyTimers {
verifyTimerHeap(pp)
}
return
first := atomic.Load64(&pp.timerModifiedEarliest)
if first == 0 || int64(first) > now {
if verifyTimers {
verifyTimerHeap(pp)
}

// We are going to clear all timerModifiedEarlier timers.
atomic.Store64(&pp.timerModifiedEarliest, 0)
return
}

// We are going to clear all timerModifiedEarlier timers.
atomic.Store64(&pp.timerModifiedEarliest, 0)

var moved []*timer
loop:
for i := 0; i < len(pp.timers); i++ {
t := pp.timers[i]
if t.pp.ptr() != pp {
Expand All @@ -716,11 +692,6 @@ loop:
// loop to skip some other timer.
dodeltimer(pp, i)
moved = append(moved, t)
if s == timerModifiedEarlier {
if n := atomic.Xadd(&pp.adjustTimers, -1); int32(n) <= 0 {
break loop
}
}
// Look at this heap position again.
i--
}
Expand Down Expand Up @@ -819,9 +790,6 @@ func runtimer(pp *p, now int64) int64 {
t.when = t.nextwhen
dodeltimer0(pp)
doaddtimer(pp, t)
if s == timerModifiedEarlier {
atomic.Xadd(&pp.adjustTimers, -1)
}
if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
badTimer()
}
Expand Down Expand Up @@ -916,7 +884,6 @@ func clearDeletedTimers(pp *p) {
atomic.Store64(&pp.timerModifiedEarliest, 0)

cdel := int32(0)
cearlier := int32(0)
to := 0
changedHeap := false
timers := pp.timers
Expand All @@ -941,9 +908,6 @@ nextTimer:
if !atomic.Cas(&t.status, timerMoving, timerWaiting) {
badTimer()
}
if s == timerModifiedEarlier {
cearlier++
}
continue nextTimer
}
case timerDeleted:
Expand Down Expand Up @@ -980,7 +944,6 @@ nextTimer:

atomic.Xadd(&pp.deletedTimers, -cdel)
atomic.Xadd(&pp.numTimers, -cdel)
atomic.Xadd(&pp.adjustTimers, -cearlier)

timers = timers[:to]
pp.timers = timers
Expand Down

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