code review 4673054: runtime: eliminate false sharing on runtime.goidgen

runtime: eliminate false sharing on runtime.goidgen
runtime.goidgen can be quite frequently modified and
shares cache line with the following variables,
it leads to false sharing.
50c6b0 b nfname
50c6b4 b nfunc
50c6b8 b nfunc$17
50c6bc b nhist$17
50c6c0 B runtime.checking
50c6c4 B runtime.gcwaiting
50c6c8 B runtime.goidgen
50c6cc B runtime.gomaxprocs
50c6d0 B runtime.panicking
50c6d4 B strconv.IntSize
50c6d8 B src/pkg/runtime/_xtest_.ss
50c6e0 B src/pkg/runtime/_xtest_.stop
50c6e8 b addrfree
50c6f0 b addrmem
50c6f8 b argv

[dvyukov, 2011-07-11 13:39:23]

Hello golang-dev@googlegroups.com (cc: golang-dev@googlegroups.com),

I'd like you to review this change to
https://go.googlecode.com/hg/

[dvyukov, 2011-07-11 13:42:43]

I don't think that the following benchmark worth committing

func spawn(compl *bool) {
	if *compl == false {
		go spawn(compl)
	}
}

func BenchmarkRuntimeContention(b *testing.B) {
	const CallsPerSched = 1000
	procs := runtime.GOMAXPROCS(-1)
	if procs == 1 {
		procs = 2
		defer runtime.GOMAXPROCS(1)
	}
	procs--
	N := int32(b.N / CallsPerSched)
	c := make(chan bool, procs)
	var compl bool
	spawn(&compl)
	for p := 0; p < procs; p++ {
		go func() {
			m := make(map[int]int)
			for atomic.AddInt32(&N, -1) >= 0 {
				for g := 0; g < CallsPerSched; g++ {
					_, _ = m[0]
				}
			}
			c <- true
		}()
	}
	for p := 0; p < procs; p++ {
		<-c
	}
}

but I get the following improvement on it:

benchmark                                        old ns/op    new ns/op    delta
BenchmarkRuntimeContention                           40.50        39.80   -1.73%
BenchmarkRuntimeContention-2                         61.80        40.10  -35.11%
BenchmarkRuntimeContention-4                         29.60        13.40  -54.73%
BenchmarkRuntimeContention-8                         16.70         6.87  -58.86%
BenchmarkRuntimeContention-16                        10.40         4.72  -54.62%

The false sharing seems to be between runtime.goidgen and runtime.gcwaiting,
which is accessed in all chan and map operations.

[rsc, 2011-07-12 05:25:01]

LGTM

Thanks for finding this!

[rsc, 2011-07-12 05:25:18]

*** Submitted as http://code.google.com/p/go/source/detail?r=538318748eec ***

runtime: eliminate false sharing on runtime.goidgen
runtime.goidgen can be quite frequently modified and
shares cache line with the following variables,
it leads to false sharing.
50c6b0 b nfname
50c6b4 b nfunc
50c6b8 b nfunc$17
50c6bc b nhist$17
50c6c0 B runtime.checking
50c6c4 B runtime.gcwaiting
50c6c8 B runtime.goidgen
50c6cc B runtime.gomaxprocs
50c6d0 B runtime.panicking
50c6d4 B strconv.IntSize
50c6d8 B src/pkg/runtime/_xtest_.ss
50c6e0 B src/pkg/runtime/_xtest_.stop
50c6e8 b addrfree
50c6f0 b addrmem
50c6f8 b argv

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/4673054

Committer: Russ Cox <rsc@golang.org>