code review 4650054: sync: add benchmark for Once.

src/pkg/sync/once_test.go

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package sync_test
 
 import (
         . "sync"
         "sync/atomic"
         "runtime"
@@ skipping 22 matching lines @@
         for i := 0; i < N; i++ {
                 <-c
         }
         if *o != 1 {
                 t.Errorf("once failed: %d is not 1", *o)
         }
 }
 
 func BenchmarkOnce(b *testing.B) {
         const CallsPerSched = 1000
-        b.SetBytes(1)

[rsc, 2011/06/27 15:59:24]

Since this is not processing data, it seems
better just to omit this call, which will make
the benchmark output drop the MB/s part.

         procs := runtime.GOMAXPROCS(-1)
         N := int32(b.N / CallsPerSched)

[rsc, 2011/06/27 15:59:24]

Should this be b.N / CallsPerSched / procs?

-        once := new(Once)

[rsc, 2011/06/27 15:59:24]

var once Once

+        var once Once
         f := func() {}
         c := make(chan bool, procs)
         for p := 0; p < procs; p++ {
                 go func() {
                         for atomic.AddInt32(&N, -1) >= 0 {
                                 runtime.Gosched()
                                 for g := 0; g < CallsPerSched; g++ {
                                         once.Do(f)
                                 }
                         }
                         c <- true
                 }()
         }
         for p := 0; p < procs; p++ {
                 <-c
         }
 }