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// run
//go:build darwin || linux
// Copyright 2014 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.
// Test that dequeuing from a pending channel doesn't
// take linear time.
package main
import (
"fmt"
"runtime"
"time"
)
// checkLinear asserts that the running time of f(n) is in O(n).
// tries is the initial number of iterations.
func checkLinear(typ string, tries int, f func(n int)) {
// Depending on the machine and OS, this test might be too fast
// to measure with accurate enough granularity. On failure,
// make it run longer, hoping that the timing granularity
// is eventually sufficient.
timeF := func(n int) time.Duration {
t1 := time.Now()
f(n)
return time.Since(t1)
}
t0 := time.Now()
n := tries
fails := 0
for {
runtime.GC()
t1 := timeF(n)
runtime.GC()
t2 := timeF(2 * n)
// should be 2x (linear); allow up to 3x
if t2 < 3*t1 {
if false {
fmt.Println(typ, "\t", time.Since(t0))
}
return
}
// If n ops run in under a second and the ratio
// doesn't work out, make n bigger, trying to reduce
// the effect that a constant amount of overhead has
// on the computed ratio.
if t1 < 1*time.Second {
n *= 2
continue
}
// Once the test runs long enough for n ops,
// try to get the right ratio at least once.
// If five in a row all fail, give up.
if fails++; fails >= 5 {
panic(fmt.Sprintf("%s: too slow: %d channels: %v; %d channels: %v\n",
typ, n, t1, 2*n, t2))
}
}
}
func main() {
checkLinear("chanSelect", 1000, func(n int) {
const messages = 10
c := make(chan bool) // global channel
var a []chan bool // local channels for each goroutine
for i := 0; i < n; i++ {
d := make(chan bool)
a = append(a, d)
go func() {
for j := 0; j < messages; j++ {
// queue ourselves on the global channel
select {
case <-c:
case <-d:
}
}
}()
}
for i := 0; i < messages; i++ {
// wake each goroutine up, forcing it to dequeue and then enqueue
// on the global channel.
for _, d := range a {
d <- true
}
}
})
}