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runtime: simplify timers.siftDown

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No effect on benchmarks, but the code is clearer.

goos: linux
goarch: amd64
pkg: time
cpu: AMD Ryzen 9 7950X 16-Core Processor
                                   │  s7base.txt  │               s7.txt                │
                                   │    sec/op    │    sec/op     vs base               │
AdjustTimers10000-32                 195.9µ ± 13%   198.1µ ±  2%       ~ (p=0.436 n=10)
AdjustTimers10000SingleThread-32     1.573m ± 13%   1.566m ± 10%       ~ (p=0.739 n=10)
AdjustTimers10000NoReset-32          170.6µ ±  1%   170.4µ ±  1%       ~ (p=0.912 n=10)
AdjustTimers10000NoSleep-32          183.9µ ±  2%   181.4µ ±  2%  -1.39% (p=0.045 n=10)
AdjustTimers10000NoResetNoSleep-32   151.3µ ±  1%   150.0µ ±  1%  -0.90% (p=0.007 n=10)

goos: darwin
goarch: arm64
pkg: time
cpu: Apple M3 Pro
                                   │ m3base.txt  │               m3.txt               │
                                   │   sec/op    │   sec/op     vs base               │
AdjustTimers10000-12                 234.2µ ± 1%   234.5µ ± 1%       ~ (p=0.796 n=10)
AdjustTimers10000SingleThread-12     1.191m ± 1%   1.272m ± 1%  +6.81% (p=0.000 n=10)
AdjustTimers10000NoReset-12          239.6µ ± 2%   236.3µ ± 9%       ~ (p=0.971 n=10)
AdjustTimers10000NoSleep-12          223.3µ ± 2%   221.4µ ± 3%       ~ (p=0.579 n=10)
AdjustTimers10000NoResetNoSleep-12   209.2µ ± 2%   209.0µ ± 4%       ~ (p=0.796 n=10)


Change-Id: Id48aa893235d652814b7fa4605037f09b0b4d73b
Reviewed-on: https://go-review.googlesource.com/c/go/+/574897
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
This commit is contained in:
Russ Cox 2024-03-27 22:14:27 -04:00
parent bb7a29991c
commit d0051be847
1 changed files with 19 additions and 23 deletions
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42
src/runtime/time.go
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@ -1077,8 +1077,8 @@ func (ts *timers) verify() {
continue
}
// The heap is 4-ary. See siftupTimer and siftdownTimer.
p := (i - 1) / 4
// The heap is timerHeapN-ary. See siftupTimer and siftdownTimer.
p := int(uint(i-1) / timerHeapN)
if tw.when < ts.heap[p].when {
print("bad timer heap at ", i, ": ", p, ": ", ts.heap[p].when, ", ", i, ": ", tw.when, "\n")
throw("bad timer heap")
@ -1139,6 +1139,8 @@ func timeSleepUntil() int64 {
return next
}
const timerHeapN = 4
// Heap maintenance algorithms.
// These algorithms check for slice index errors manually.
// Slice index error can happen if the program is using racy
@ -1160,7 +1162,7 @@ func (ts *timers) siftUp(i int) {
badTimer()
}
for i > 0 {
p := (i - 1) / 4 // parent
p := int(uint(i-1) / timerHeapN) // parent
if when >= heap[p].when {
break
}
@ -1180,34 +1182,28 @@ func (ts *timers) siftDown(i int) {
if i >= n {
badTimer()
}
if i*timerHeapN+1 >= n {
return
}
tw := heap[i]
when := tw.when
if when <= 0 {
badTimer()
}
for {
c := i*4 + 1 // left child
c3 := c + 2 // mid child
if c >= n {
leftChild := i*timerHeapN + 1
if leftChild >= n {
break
}
w := heap[c].when
if c+1 < n && heap[c+1].when < w {
w = heap[c+1].when
c++
}
if c3 < n {
w3 := heap[c3].when
if c3+1 < n && heap[c3+1].when < w3 {
w3 = heap[c3+1].when
c3++
}
if w3 < w {
w = w3
c = c3
w := when
c := -1
for j, tw := range heap[leftChild:min(leftChild+timerHeapN, n)] {
if tw.when < w {
w = tw.when
c = leftChild + j
}
}
if w >= when {
if c < 0 {
break
}
heap[i] = heap[c]
@ -1222,11 +1218,11 @@ func (ts *timers) siftDown(i int) {
// It takes O(n) time for n=len(ts.heap), not the O(n log n) of n repeated add operations.
func (ts *timers) initHeap() {
// Last possible element that needs sifting down is parent of last element;
// last element is len(t)-1; parent of last element is (len(t)-1-1)/4.
// last element is len(t)-1; parent of last element is (len(t)-1-1)/timerHeapN.
if len(ts.heap) <= 1 {
return
}
for i := (len(ts.heap) - 1 - 1) / 4; i >= 0; i-- {
for i := int(uint(len(ts.heap)-1-1) / timerHeapN); i >= 0; i-- {
ts.siftDown(i)
}
}