go/src/pkg/testing/benchmark.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 testing

import (
	"flag";
	"fmt";
	"os";
	"time";
)

var matchBenchmarks = flag.String("benchmarks", "", "regular expression to select benchmarks to run")

// An internal type but exported because it is cross-package; part of the implementation
// of gotest.
type Benchmark struct {
	Name	string;
	F	func(b *B);
}

// B is a type passed to Benchmark functions to manage benchmark
// timing and to specify the number of iterations to run.
type B struct {
	N		int;
	benchmark	Benchmark;
	ns		int64;
	start		int64;
}

// StartTimer starts timing a test.  This function is called automatically
// before a benchmark starts, but it can also used to resume timing after
// a call to StopTimer.
func (b *B) StartTimer()	{ b.start = time.Nanoseconds() }

// StopTimer stops timing a test.  This can be used to pause the timer
// while performing complex initialization that you don't
// want to measure.
func (b *B) StopTimer() {
	if b.start > 0 {
		b.ns += time.Nanoseconds() - b.start
	}
	b.start = 0;
}

// ResetTimer stops the timer and sets the elapsed benchmark time to zero.
func (b *B) ResetTimer() {
	b.start = 0;
	b.ns = 0;
}

func (b *B) nsPerOp() int64 {
	if b.N <= 0 {
		return 0
	}
	return b.ns / int64(b.N);
}

// runN runs a single benchmark for the specified number of iterations.
func (b *B) runN(n int) {
	b.N = n;
	b.ResetTimer();
	b.StartTimer();
	b.benchmark.F(b);
	b.StopTimer();
}

func min(x, y int) int {
	if x > y {
		return y
	}
	return x;
}

// roundDown10 rounds a number down to the nearest power of 10.
func roundDown10(n int) int {
	var tens = 0;
	// tens = floor(log_10(n))
	for n > 10 {
		n = n / 10;
		tens++;
	}
	// result = 10^tens
	result := 1;
	for i := 0; i < tens; i++ {
		result *= 10
	}
	return result;
}

// roundUp rounds x up to a number of the form [1eX, 2eX, 5eX].
func roundUp(n int) int {
	base := roundDown10(n);
	if n < (2 * base) {
		return 2 * base
	}
	if n < (5 * base) {
		return 5 * base
	}
	return 10 * base;
}

// run times the benchmark function.  It gradually increases the number
// of benchmark iterations until the benchmark runs for a second in order
// to get a reasonable measurement.  It prints timing information in this form
//		testing.BenchmarkHello	100000		19 ns/op
func (b *B) run() {
	// Run the benchmark for a single iteration in case it's expensive.
	n := 1;
	b.runN(n);
	// Run the benchmark for at least a second.
	for b.ns < 1e9 && n < 1e9 {
		last := n;
		// Predict iterations/sec.
		if b.nsPerOp() == 0 {
			n = 1e9
		} else {
			n = 1e9 / int(b.nsPerOp())
		}
		// Run more iterations than we think we'll need for a second (1.5x).
		// Don't grow too fast in case we had timing errors previously.
		n = min(int(1.5*float(n)), 100*last);
		// Round up to something easy to read.
		n = roundUp(n);
		b.runN(n);
	}
	fmt.Printf("%s\t%d\t%10d ns/op\n", b.benchmark.Name, b.N, b.nsPerOp());
}

// An internal function but exported because it is cross-package; part of the implementation
// of gotest.
func RunBenchmarks(benchmarks []Benchmark) {
	// If no flag was specified, don't run benchmarks.
	if len(*matchBenchmarks) == 0 {
		return
	}
	re, err := CompileRegexp(*matchBenchmarks);
	if err != "" {
		println("invalid regexp for -benchmarks:", err);
		os.Exit(1);
	}
	for _, Benchmark := range benchmarks {
		if !re.MatchString(Benchmark.Name) {
			continue
		}
		b := &B{benchmark: Benchmark};
		b.run();
	}
}