runtime: test for linear enqueue/dequeue behavior

Make sure dequeueing from a channel queue does not exhibit quadratic time behavior.

Change-Id: Ifb7c709b026f74c7e783610d4914dd92909a441b
Reviewed-on: https://go-review.googlesource.com/1212Reviewed-by: Russ Cox <rsc@golang.org>

test/chanlinear.go

+// +build darwin linux
+// run
+
+// 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 dequeueing 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
+			}
+		}
+	})
+}