runtime: don't always adjust timers

Some programs have a lot of timers that they adjust both forward and
backward in time. This can cause a large number of timerModifiedEarlier
timers. In practice these timers are used for I/O deadlines and are
rarely reached. The effect is that the runtime spends a lot of time
in adjusttimers making sure that there are no timerModifiedEarlier
timers, but the effort is wasted because none of the adjusted timers
are near the top of the timer heap anyhow.

Avoid much of this extra work by keeping track of the earliest known
timerModifiedEarlier timer. This lets us skip adjusttimers if we know
that none of the timers will be ready to run anyhow. We will still
eventually run it, when we reach the deadline of the earliest known
timerModifiedEarlier, although in practice that timer has likely
been removed. When we do run adjusttimers, we will reset all of the
timerModifiedEarlier timers, and clear our notion of when we need
to run adjusttimers again.

This effect should be to significantly reduce the number of times we
walk through the timer list in adjusttimers.

Fixes #41699

Change-Id: I38eb2be611fb34e3017bb33d0a9ed40d75fb414f
Reviewed-on: https://go-review.googlesource.com/c/go/+/258303
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 26 insertions, 26 deletions

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 6feecef985..87d4b6e568 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -3017,40 +3017,40 @@ func dropg() {
 // We pass now in and out to avoid extra calls of nanotime.
 //go:yeswritebarrierrec
 func checkTimers(pp *p, now int64) (rnow, pollUntil int64, ran bool) {
-	// If there are no timers to adjust, and the first timer on
-	// the heap is not yet ready to run, then there is nothing to do.
-	if atomic.Load(&pp.adjustTimers) == 0 {
-		next := int64(atomic.Load64(&pp.timer0When))
-		if next == 0 {
-			return now, 0, false
-		}
-		if now == 0 {
-			now = nanotime()
-		}
-		if now < next {
-			// Next timer is not ready to run.
-			// But keep going if we would clear deleted timers.
-			// This corresponds to the condition below where
-			// we decide whether to call clearDeletedTimers.
-			if pp != getg().m.p.ptr() || int(atomic.Load(&pp.deletedTimers)) <= int(atomic.Load(&pp.numTimers)/4) {
-				return now, next, false
-			}
+	// If it's not yet time for the first timer, or the first adjusted
+	// timer, then there is nothing to do.
+	next := int64(atomic.Load64(&pp.timer0When))
+	nextAdj := int64(atomic.Load64(&pp.timerModifiedEarliest))
+	if next == 0 || (nextAdj != 0 && nextAdj < next) {
+		next = nextAdj
+	}
+
+	if next == 0 {
+		// No timers to run or adjust.
+		return now, 0, false
+	}
+
+	if now == 0 {
+		now = nanotime()
+	}
+	if now < next {
+		// Next timer is not ready to run, but keep going
+		// if we would clear deleted timers.
+		// This corresponds to the condition below where
+		// we decide whether to call clearDeletedTimers.
+		if pp != getg().m.p.ptr() || int(atomic.Load(&pp.deletedTimers)) <= int(atomic.Load(&pp.numTimers)/4) {
+			return now, next, false
 		}
 	}
 
 	lock(&pp.timersLock)
 
-	adjusttimers(pp)
-
-	rnow = now
 	if len(pp.timers) > 0 {
-		if rnow == 0 {
-			rnow = nanotime()
-		}
+		adjusttimers(pp, now)
 		for len(pp.timers) > 0 {
 			// Note that runtimer may temporarily unlock
 			// pp.timersLock.
-			if tw := runtimer(pp, rnow); tw != 0 {
+			if tw := runtimer(pp, now); tw != 0 {
 				if tw > 0 {
 					pollUntil = tw
 				}
@@ -3069,7 +3069,7 @@ func checkTimers(pp *p, now int64) (rnow, pollUntil int64, ran bool) {
 
 	unlock(&pp.timersLock)
 
-	return rnow, pollUntil, ran
+	return now, pollUntil, ran
 }
 
 func parkunlock_c(gp *g, lock unsafe.Pointer) bool {