mirror of
https://github.com/golang/go
synced 2024-09-28 20:24:29 -06:00
[release-branch.go1.9] runtime: use simple, more robust fastrandn
CL 36932 (speed up fastrandn) made it faster but introduced bad interference with some properties of fastrand itself, making fastrandn not very random in certain ways. In particular, certain selects are demonstrably unfair. For Go 1.10 the new faster fastrandn has induced a new fastrand, which in turn has caused other follow-on bugs that are still being discovered and fixed. For Go 1.9.2, just go back to the barely slower % implementation that we used in Go 1.8 and earlier. This should restore fairness in select and any other problems caused by the clever fastrandn. The test in this CL is copied from CL 62530. Fixes #22253. Change-Id: Ibcf948a7bce981452e05c90dbdac122043f6f813 Reviewed-on: https://go-review.googlesource.com/70991 Run-TryBot: Russ Cox <rsc@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
parent
78952c06c5
commit
d93cb46280
@ -5,6 +5,7 @@
|
||||
package runtime_test
|
||||
|
||||
import (
|
||||
"math"
|
||||
"runtime"
|
||||
"sync"
|
||||
"sync/atomic"
|
||||
@ -430,6 +431,62 @@ func TestSelectStress(t *testing.T) {
|
||||
wg.Wait()
|
||||
}
|
||||
|
||||
func TestSelectFairness(t *testing.T) {
|
||||
const trials = 10000
|
||||
c1 := make(chan byte, trials+1)
|
||||
c2 := make(chan byte, trials+1)
|
||||
for i := 0; i < trials+1; i++ {
|
||||
c1 <- 1
|
||||
c2 <- 2
|
||||
}
|
||||
c3 := make(chan byte)
|
||||
c4 := make(chan byte)
|
||||
out := make(chan byte)
|
||||
done := make(chan byte)
|
||||
var wg sync.WaitGroup
|
||||
wg.Add(1)
|
||||
go func() {
|
||||
defer wg.Done()
|
||||
for {
|
||||
var b byte
|
||||
select {
|
||||
case b = <-c3:
|
||||
case b = <-c4:
|
||||
case b = <-c1:
|
||||
case b = <-c2:
|
||||
}
|
||||
select {
|
||||
case out <- b:
|
||||
case <-done:
|
||||
return
|
||||
}
|
||||
}
|
||||
}()
|
||||
cnt1, cnt2 := 0, 0
|
||||
for i := 0; i < trials; i++ {
|
||||
switch b := <-out; b {
|
||||
case 1:
|
||||
cnt1++
|
||||
case 2:
|
||||
cnt2++
|
||||
default:
|
||||
t.Fatalf("unexpected value %d on channel", b)
|
||||
}
|
||||
}
|
||||
// If the select in the goroutine is fair,
|
||||
// cnt1 and cnt2 should be about the same value.
|
||||
// With 10,000 trials, the expected margin of error at
|
||||
// a confidence level of five nines is 4.4172 / (2 * Sqrt(10000)).
|
||||
r := float64(cnt1) / trials
|
||||
e := math.Abs(r - 0.5)
|
||||
t.Log(cnt1, cnt2, r, e)
|
||||
if e > 4.4172/(2*math.Sqrt(trials)) {
|
||||
t.Errorf("unfair select: in %d trials, results were %d, %d", trials, cnt1, cnt2)
|
||||
}
|
||||
close(done)
|
||||
wg.Wait()
|
||||
}
|
||||
|
||||
func TestChanSendInterface(t *testing.T) {
|
||||
type mt struct{}
|
||||
m := &mt{}
|
||||
|
@ -105,9 +105,11 @@ func fastrand() uint32 {
|
||||
|
||||
//go:nosplit
|
||||
func fastrandn(n uint32) uint32 {
|
||||
// This is similar to fastrand() % n, but faster.
|
||||
// See http://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
|
||||
return uint32(uint64(fastrand()) * uint64(n) >> 32)
|
||||
// Don't be clever.
|
||||
// fastrand is not good enough for cleverness.
|
||||
// Just use mod.
|
||||
// See golang.org/issue/21806.
|
||||
return fastrand() % n
|
||||
}
|
||||
|
||||
//go:linkname sync_fastrand sync.fastrand
|
||||
|
Loading…
Reference in New Issue
Block a user