[gostls13.git] / src / 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"
        "internal/sysinfo"
        "io"
        "math"
        "os"
        "runtime"
        "sort"
        "strconv"
        "strings"
        "sync"
        "sync/atomic"
        "time"
        "unicode"
)

func initBenchmarkFlags() {
        matchBenchmarks = flag.String("test.bench", "", "run only benchmarks matching `regexp`")
        benchmarkMemory = flag.Bool("test.benchmem", false, "print memory allocations for benchmarks")
        flag.Var(&benchTime, "test.benchtime", "run each benchmark for duration `d` or N times if `d` is of the form Nx")
}

var (
        matchBenchmarks *string
        benchmarkMemory *bool

        benchTime = durationOrCountFlag{d: 1 * time.Second} // changed during test of testing package
)

type durationOrCountFlag struct {
        d         time.Duration
        n         int
        allowZero bool
}

func (f *durationOrCountFlag) String() string {
        if f.n > 0 {
                return fmt.Sprintf("%dx", f.n)
        }
        return f.d.String()
}

func (f *durationOrCountFlag) Set(s string) error {
        if strings.HasSuffix(s, "x") {
                n, err := strconv.ParseInt(s[:len(s)-1], 10, 0)
                if err != nil || n < 0 || (!f.allowZero && n == 0) {
                        return fmt.Errorf("invalid count")
                }
                *f = durationOrCountFlag{n: int(n)}
                return nil
        }
        d, err := time.ParseDuration(s)
        if err != nil || d < 0 || (!f.allowZero && d == 0) {
                return fmt.Errorf("invalid duration")
        }
        *f = durationOrCountFlag{d: d}
        return nil
}

// Global lock to ensure only one benchmark runs at a time.
var benchmarkLock sync.Mutex

// Used for every benchmark for measuring memory.
var memStats runtime.MemStats

// InternalBenchmark is an internal type but exported because it is cross-package;
// it is part of the implementation of the "go test" command.
type InternalBenchmark 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.
//
// A benchmark ends when its Benchmark function returns or calls any of the methods
// FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods must be called
// only from the goroutine running the Benchmark function.
// The other reporting methods, such as the variations of Log and Error,
// may be called simultaneously from multiple goroutines.
//
// Like in tests, benchmark logs are accumulated during execution
// and dumped to standard output when done. Unlike in tests, benchmark logs
// are always printed, so as not to hide output whose existence may be
// affecting benchmark results.
type B struct {
        common
        importPath       string // import path of the package containing the benchmark
        context          *benchContext
        N                int
        previousN        int           // number of iterations in the previous run
        previousDuration time.Duration // total duration of the previous run
        benchFunc        func(b *B)
        benchTime        durationOrCountFlag
        bytes            int64
        missingBytes     bool // one of the subbenchmarks does not have bytes set.
        timerOn          bool
        showAllocResult  bool
        result           BenchmarkResult
        parallelism      int // RunParallel creates parallelism*GOMAXPROCS goroutines
        // The initial states of memStats.Mallocs and memStats.TotalAlloc.
        startAllocs uint64
        startBytes  uint64
        // The net total of this test after being run.
        netAllocs uint64
        netBytes  uint64
        // Extra metrics collected by ReportMetric.
        extra map[string]float64
}

// StartTimer starts timing a test. This function is called automatically
// before a benchmark starts, but it can also be used to resume timing after
// a call to StopTimer.
func (b *B) StartTimer() {
        if !b.timerOn {
                runtime.ReadMemStats(&memStats)
                b.startAllocs = memStats.Mallocs
                b.startBytes = memStats.TotalAlloc
                b.start = time.Now()
                b.timerOn = true
        }
}

// 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.timerOn {
                b.duration += time.Since(b.start)
                runtime.ReadMemStats(&memStats)
                b.netAllocs += memStats.Mallocs - b.startAllocs
                b.netBytes += memStats.TotalAlloc - b.startBytes
                b.timerOn = false
        }
}

// ResetTimer zeroes the elapsed benchmark time and memory allocation counters
// and deletes user-reported metrics.
// It does not affect whether the timer is running.
func (b *B) ResetTimer() {
        if b.extra == nil {
                // Allocate the extra map before reading memory stats.
                // Pre-size it to make more allocation unlikely.
                b.extra = make(map[string]float64, 16)
        } else {
                clear(b.extra)
        }
        if b.timerOn {
                runtime.ReadMemStats(&memStats)
                b.startAllocs = memStats.Mallocs
                b.startBytes = memStats.TotalAlloc
                b.start = time.Now()
        }
        b.duration = 0
        b.netAllocs = 0
        b.netBytes = 0
}

// SetBytes records the number of bytes processed in a single operation.
// If this is called, the benchmark will report ns/op and MB/s.
func (b *B) SetBytes(n int64) { b.bytes = n }

// ReportAllocs enables malloc statistics for this benchmark.
// It is equivalent to setting -test.benchmem, but it only affects the
// benchmark function that calls ReportAllocs.
func (b *B) ReportAllocs() {
        b.showAllocResult = true
}

// runN runs a single benchmark for the specified number of iterations.
func (b *B) runN(n int) {
        benchmarkLock.Lock()
        defer benchmarkLock.Unlock()
        defer func() {
                b.runCleanup(normalPanic)
                b.checkRaces()
        }()
        // Try to get a comparable environment for each run
        // by clearing garbage from previous runs.
        runtime.GC()
        b.resetRaces()
        b.N = n
        b.parallelism = 1
        b.ResetTimer()
        b.StartTimer()
        b.benchFunc(b)
        b.StopTimer()
        b.previousN = n
        b.previousDuration = b.duration
}

// run1 runs the first iteration of benchFunc. It reports whether more
// iterations of this benchmarks should be run.
func (b *B) run1() bool {
        if ctx := b.context; ctx != nil {
                // Extend maxLen, if needed.
                if n := len(b.name) + ctx.extLen + 1; n > ctx.maxLen {
                        ctx.maxLen = n + 8 // Add additional slack to avoid too many jumps in size.
                }
        }
        go func() {
                // Signal that we're done whether we return normally
                // or by FailNow's runtime.Goexit.
                defer func() {
                        b.signal <- true
                }()

                b.runN(1)
        }()
        <-b.signal
        if b.failed {
                fmt.Fprintf(b.w, "%s--- FAIL: %s\n%s", b.chatty.prefix(), b.name, b.output)
                return false
        }
        // Only print the output if we know we are not going to proceed.
        // Otherwise it is printed in processBench.
        b.mu.RLock()
        finished := b.finished
        b.mu.RUnlock()
        if b.hasSub.Load() || finished {
                tag := "BENCH"
                if b.skipped {
                        tag = "SKIP"
                }
                if b.chatty != nil && (len(b.output) > 0 || finished) {
                        b.trimOutput()
                        fmt.Fprintf(b.w, "%s--- %s: %s\n%s", b.chatty.prefix(), tag, b.name, b.output)
                }
                return false
        }
        return true
}

var labelsOnce sync.Once

// run executes the benchmark in a separate goroutine, including all of its
// subbenchmarks. b must not have subbenchmarks.
func (b *B) run() {
        labelsOnce.Do(func() {
                fmt.Fprintf(b.w, "goos: %s\n", runtime.GOOS)
                fmt.Fprintf(b.w, "goarch: %s\n", runtime.GOARCH)
                if b.importPath != "" {
                        fmt.Fprintf(b.w, "pkg: %s\n", b.importPath)
                }
                if cpu := sysinfo.CPUName(); cpu != "" {
                        fmt.Fprintf(b.w, "cpu: %s\n", cpu)
                }
        })
        if b.context != nil {
                // Running go test --test.bench
                b.context.processBench(b) // Must call doBench.
        } else {
                // Running func Benchmark.
                b.doBench()
        }
}

func (b *B) doBench() BenchmarkResult {
        go b.launch()
        <-b.signal
        return b.result
}

// launch launches the benchmark function. It gradually increases the number
// of benchmark iterations until the benchmark runs for the requested benchtime.
// launch is run by the doBench function as a separate goroutine.
// run1 must have been called on b.
func (b *B) launch() {
        // Signal that we're done whether we return normally
        // or by FailNow's runtime.Goexit.
        defer func() {
                b.signal <- true
        }()

        // Run the benchmark for at least the specified amount of time.
        if b.benchTime.n > 0 {
                // We already ran a single iteration in run1.
                // If -benchtime=1x was requested, use that result.
                // See https://golang.org/issue/32051.
                if b.benchTime.n > 1 {
                        b.runN(b.benchTime.n)
                }
        } else {
                d := b.benchTime.d
                for n := int64(1); !b.failed && b.duration < d && n < 1e9; {
                        last := n
                        // Predict required iterations.
                        goalns := d.Nanoseconds()
                        prevIters := int64(b.N)
                        prevns := b.duration.Nanoseconds()
                        if prevns <= 0 {
                                // Round up, to avoid div by zero.
                                prevns = 1
                        }
                        // Order of operations matters.
                        // For very fast benchmarks, prevIters ~= prevns.
                        // If you divide first, you get 0 or 1,
                        // which can hide an order of magnitude in execution time.
                        // So multiply first, then divide.
                        n = goalns * prevIters / prevns
                        // Run more iterations than we think we'll need (1.2x).
                        n += n / 5
                        // Don't grow too fast in case we had timing errors previously.
                        n = min(n, 100*last)
                        // Be sure to run at least one more than last time.
                        n = max(n, last+1)
                        // Don't run more than 1e9 times. (This also keeps n in int range on 32 bit platforms.)
                        n = min(n, 1e9)
                        b.runN(int(n))
                }
        }
        b.result = BenchmarkResult{b.N, b.duration, b.bytes, b.netAllocs, b.netBytes, b.extra}
}

// Elapsed returns the measured elapsed time of the benchmark.
// The duration reported by Elapsed matches the one measured by
// StartTimer, StopTimer, and ResetTimer.
func (b *B) Elapsed() time.Duration {
        d := b.duration
        if b.timerOn {
                d += time.Since(b.start)
        }
        return d
}

// ReportMetric adds "n unit" to the reported benchmark results.
// If the metric is per-iteration, the caller should divide by b.N,
// and by convention units should end in "/op".
// ReportMetric overrides any previously reported value for the same unit.
// ReportMetric panics if unit is the empty string or if unit contains
// any whitespace.
// If unit is a unit normally reported by the benchmark framework itself
// (such as "allocs/op"), ReportMetric will override that metric.
// Setting "ns/op" to 0 will suppress that built-in metric.
func (b *B) ReportMetric(n float64, unit string) {
        if unit == "" {
                panic("metric unit must not be empty")
        }
        if strings.IndexFunc(unit, unicode.IsSpace) >= 0 {
                panic("metric unit must not contain whitespace")
        }
        b.extra[unit] = n
}

// BenchmarkResult contains the results of a benchmark run.
type BenchmarkResult struct {
        N         int           // The number of iterations.
        T         time.Duration // The total time taken.
        Bytes     int64         // Bytes processed in one iteration.
        MemAllocs uint64        // The total number of memory allocations.
        MemBytes  uint64        // The total number of bytes allocated.

        // Extra records additional metrics reported by ReportMetric.
        Extra map[string]float64
}

// NsPerOp returns the "ns/op" metric.
func (r BenchmarkResult) NsPerOp() int64 {
        if v, ok := r.Extra["ns/op"]; ok {
                return int64(v)
        }
        if r.N <= 0 {
                return 0
        }
        return r.T.Nanoseconds() / int64(r.N)
}

// mbPerSec returns the "MB/s" metric.
func (r BenchmarkResult) mbPerSec() float64 {
        if v, ok := r.Extra["MB/s"]; ok {
                return v
        }
        if r.Bytes <= 0 || r.T <= 0 || r.N <= 0 {
                return 0
        }
        return (float64(r.Bytes) * float64(r.N) / 1e6) / r.T.Seconds()
}

// AllocsPerOp returns the "allocs/op" metric,
// which is calculated as r.MemAllocs / r.N.
func (r BenchmarkResult) AllocsPerOp() int64 {
        if v, ok := r.Extra["allocs/op"]; ok {
                return int64(v)
        }
        if r.N <= 0 {
                return 0
        }
        return int64(r.MemAllocs) / int64(r.N)
}

// AllocedBytesPerOp returns the "B/op" metric,
// which is calculated as r.MemBytes / r.N.
func (r BenchmarkResult) AllocedBytesPerOp() int64 {
        if v, ok := r.Extra["B/op"]; ok {
                return int64(v)
        }
        if r.N <= 0 {
                return 0
        }
        return int64(r.MemBytes) / int64(r.N)
}

// String returns a summary of the benchmark results.
// It follows the benchmark result line format from
// https://golang.org/design/14313-benchmark-format, not including the
// benchmark name.
// Extra metrics override built-in metrics of the same name.
// String does not include allocs/op or B/op, since those are reported
// by MemString.
func (r BenchmarkResult) String() string {
        buf := new(strings.Builder)
        fmt.Fprintf(buf, "%8d", r.N)

        // Get ns/op as a float.
        ns, ok := r.Extra["ns/op"]
        if !ok {
                ns = float64(r.T.Nanoseconds()) / float64(r.N)
        }
        if ns != 0 {
                buf.WriteByte('\t')
                prettyPrint(buf, ns, "ns/op")
        }

        if mbs := r.mbPerSec(); mbs != 0 {
                fmt.Fprintf(buf, "\t%7.2f MB/s", mbs)
        }

        // Print extra metrics that aren't represented in the standard
        // metrics.
        var extraKeys []string
        for k := range r.Extra {
                switch k {
                case "ns/op", "MB/s", "B/op", "allocs/op":
                        // Built-in metrics reported elsewhere.
                        continue
                }
                extraKeys = append(extraKeys, k)
        }
        sort.Strings(extraKeys)
        for _, k := range extraKeys {
                buf.WriteByte('\t')
                prettyPrint(buf, r.Extra[k], k)
        }
        return buf.String()
}

func prettyPrint(w io.Writer, x float64, unit string) {
        // Print all numbers with 10 places before the decimal point
        // and small numbers with four sig figs. Field widths are
        // chosen to fit the whole part in 10 places while aligning
        // the decimal point of all fractional formats.
        var format string
        switch y := math.Abs(x); {
        case y == 0 || y >= 999.95:
                format = "%10.0f %s"
        case y >= 99.995:
                format = "%12.1f %s"
        case y >= 9.9995:
                format = "%13.2f %s"
        case y >= 0.99995:
                format = "%14.3f %s"
        case y >= 0.099995:
                format = "%15.4f %s"
        case y >= 0.0099995:
                format = "%16.5f %s"
        case y >= 0.00099995:
                format = "%17.6f %s"
        default:
                format = "%18.7f %s"
        }
        fmt.Fprintf(w, format, x, unit)
}

// MemString returns r.AllocedBytesPerOp and r.AllocsPerOp in the same format as 'go test'.
func (r BenchmarkResult) MemString() string {
        return fmt.Sprintf("%8d B/op\t%8d allocs/op",
                r.AllocedBytesPerOp(), r.AllocsPerOp())
}

// benchmarkName returns full name of benchmark including procs suffix.
func benchmarkName(name string, n int) string {
        if n != 1 {
                return fmt.Sprintf("%s-%d", name, n)
        }
        return name
}

type benchContext struct {
        match *matcher

        maxLen int // The largest recorded benchmark name.
        extLen int // Maximum extension length.
}

// RunBenchmarks is an internal function but exported because it is cross-package;
// it is part of the implementation of the "go test" command.
func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) {
        runBenchmarks("", matchString, benchmarks)
}

func runBenchmarks(importPath string, matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) bool {
        // If no flag was specified, don't run benchmarks.
        if len(*matchBenchmarks) == 0 {
                return true
        }
        // Collect matching benchmarks and determine longest name.
        maxprocs := 1
        for _, procs := range cpuList {
                if procs > maxprocs {
                        maxprocs = procs
                }
        }
        ctx := &benchContext{
                match:  newMatcher(matchString, *matchBenchmarks, "-test.bench", *skip),
                extLen: len(benchmarkName("", maxprocs)),
        }
        var bs []InternalBenchmark
        for _, Benchmark := range benchmarks {
                if _, matched, _ := ctx.match.fullName(nil, Benchmark.Name); matched {
                        bs = append(bs, Benchmark)
                        benchName := benchmarkName(Benchmark.Name, maxprocs)
                        if l := len(benchName) + ctx.extLen + 1; l > ctx.maxLen {
                                ctx.maxLen = l
                        }
                }
        }
        main := &B{
                common: common{
                        name:  "Main",
                        w:     os.Stdout,
                        bench: true,
                },
                importPath: importPath,
                benchFunc: func(b *B) {
                        for _, Benchmark := range bs {
                                b.Run(Benchmark.Name, Benchmark.F)
                        }
                },
                benchTime: benchTime,
                context:   ctx,
        }
        if Verbose() {
                main.chatty = newChattyPrinter(main.w)
        }
        main.runN(1)
        return !main.failed
}

// processBench runs bench b for the configured CPU counts and prints the results.
func (ctx *benchContext) processBench(b *B) {
        for i, procs := range cpuList {
                for j := uint(0); j < *count; j++ {
                        runtime.GOMAXPROCS(procs)
                        benchName := benchmarkName(b.name, procs)

                        // If it's chatty, we've already printed this information.
                        if b.chatty == nil {
                                fmt.Fprintf(b.w, "%-*s\t", ctx.maxLen, benchName)
                        }
                        // Recompute the running time for all but the first iteration.
                        if i > 0 || j > 0 {
                                b = &B{
                                        common: common{
                                                signal: make(chan bool),
                                                name:   b.name,
                                                w:      b.w,
                                                chatty: b.chatty,
                                                bench:  true,
                                        },
                                        benchFunc: b.benchFunc,
                                        benchTime: b.benchTime,
                                }
                                b.run1()
                        }
                        r := b.doBench()
                        if b.failed {
                                // The output could be very long here, but probably isn't.
                                // We print it all, regardless, because we don't want to trim the reason
                                // the benchmark failed.
                                fmt.Fprintf(b.w, "%s--- FAIL: %s\n%s", b.chatty.prefix(), benchName, b.output)
                                continue
                        }
                        results := r.String()
                        if b.chatty != nil {
                                fmt.Fprintf(b.w, "%-*s\t", ctx.maxLen, benchName)
                        }
                        if *benchmarkMemory || b.showAllocResult {
                                results += "\t" + r.MemString()
                        }
                        fmt.Fprintln(b.w, results)
                        // Unlike with tests, we ignore the -chatty flag and always print output for
                        // benchmarks since the output generation time will skew the results.
                        if len(b.output) > 0 {
                                b.trimOutput()
                                fmt.Fprintf(b.w, "%s--- BENCH: %s\n%s", b.chatty.prefix(), benchName, b.output)
                        }
                        if p := runtime.GOMAXPROCS(-1); p != procs {
                                fmt.Fprintf(os.Stderr, "testing: %s left GOMAXPROCS set to %d\n", benchName, p)
                        }
                        if b.chatty != nil && b.chatty.json {
                                b.chatty.Updatef("", "=== NAME  %s\n", "")
                        }
                }
        }
}

// If hideStdoutForTesting is true, Run does not print the benchName.
// This avoids a spurious print during 'go test' on package testing itself,
// which invokes b.Run in its own tests (see sub_test.go).
var hideStdoutForTesting = false

// Run benchmarks f as a subbenchmark with the given name. It reports
// whether there were any failures.
//
// A subbenchmark is like any other benchmark. A benchmark that calls Run at
// least once will not be measured itself and will be called once with N=1.
func (b *B) Run(name string, f func(b *B)) bool {
        // Since b has subbenchmarks, we will no longer run it as a benchmark itself.
        // Release the lock and acquire it on exit to ensure locks stay paired.
        b.hasSub.Store(true)
        benchmarkLock.Unlock()
        defer benchmarkLock.Lock()

        benchName, ok, partial := b.name, true, false
        if b.context != nil {
                benchName, ok, partial = b.context.match.fullName(&b.common, name)
        }
        if !ok {
                return true
        }
        var pc [maxStackLen]uintptr
        n := runtime.Callers(2, pc[:])
        sub := &B{
                common: common{
                        signal:  make(chan bool),
                        name:    benchName,
                        parent:  &b.common,
                        level:   b.level + 1,
                        creator: pc[:n],
                        w:       b.w,
                        chatty:  b.chatty,
                        bench:   true,
                },
                importPath: b.importPath,
                benchFunc:  f,
                benchTime:  b.benchTime,
                context:    b.context,
        }
        if partial {
                // Partial name match, like -bench=X/Y matching BenchmarkX.
                // Only process sub-benchmarks, if any.
                sub.hasSub.Store(true)
        }

        if b.chatty != nil {
                labelsOnce.Do(func() {
                        fmt.Printf("goos: %s\n", runtime.GOOS)
                        fmt.Printf("goarch: %s\n", runtime.GOARCH)
                        if b.importPath != "" {
                                fmt.Printf("pkg: %s\n", b.importPath)
                        }
                        if cpu := sysinfo.CPUName(); cpu != "" {
                                fmt.Printf("cpu: %s\n", cpu)
                        }
                })

                if !hideStdoutForTesting {
                        if b.chatty.json {
                                b.chatty.Updatef(benchName, "=== RUN   %s\n", benchName)
                        }
                        fmt.Println(benchName)
                }
        }

        if sub.run1() {
                sub.run()
        }
        b.add(sub.result)
        return !sub.failed
}

// add simulates running benchmarks in sequence in a single iteration. It is
// used to give some meaningful results in case func Benchmark is used in
// combination with Run.
func (b *B) add(other BenchmarkResult) {
        r := &b.result
        // The aggregated BenchmarkResults resemble running all subbenchmarks as
        // in sequence in a single benchmark.
        r.N = 1
        r.T += time.Duration(other.NsPerOp())
        if other.Bytes == 0 {
                // Summing Bytes is meaningless in aggregate if not all subbenchmarks
                // set it.
                b.missingBytes = true
                r.Bytes = 0
        }
        if !b.missingBytes {
                r.Bytes += other.Bytes
        }
        r.MemAllocs += uint64(other.AllocsPerOp())
        r.MemBytes += uint64(other.AllocedBytesPerOp())
}

// trimOutput shortens the output from a benchmark, which can be very long.
func (b *B) trimOutput() {
        // The output is likely to appear multiple times because the benchmark
        // is run multiple times, but at least it will be seen. This is not a big deal
        // because benchmarks rarely print, but just in case, we trim it if it's too long.
        const maxNewlines = 10
        for nlCount, j := 0, 0; j < len(b.output); j++ {
                if b.output[j] == '\n' {
                        nlCount++
                        if nlCount >= maxNewlines {
                                b.output = append(b.output[:j], "\n\t... [output truncated]\n"...)
                                break
                        }
                }
        }
}

// A PB is used by RunParallel for running parallel benchmarks.
type PB struct {
        globalN *uint64 // shared between all worker goroutines iteration counter
        grain   uint64  // acquire that many iterations from globalN at once
        cache   uint64  // local cache of acquired iterations
        bN      uint64  // total number of iterations to execute (b.N)
}

// Next reports whether there are more iterations to execute.
func (pb *PB) Next() bool {
        if pb.cache == 0 {
                n := atomic.AddUint64(pb.globalN, pb.grain)
                if n <= pb.bN {
                        pb.cache = pb.grain
                } else if n < pb.bN+pb.grain {
                        pb.cache = pb.bN + pb.grain - n
                } else {
                        return false
                }
        }
        pb.cache--
        return true
}

// RunParallel runs a benchmark in parallel.
// It creates multiple goroutines and distributes b.N iterations among them.
// The number of goroutines defaults to GOMAXPROCS. To increase parallelism for
// non-CPU-bound benchmarks, call SetParallelism before RunParallel.
// RunParallel is usually used with the go test -cpu flag.
//
// The body function will be run in each goroutine. It should set up any
// goroutine-local state and then iterate until pb.Next returns false.
// It should not use the StartTimer, StopTimer, or ResetTimer functions,
// because they have global effect. It should also not call Run.
//
// RunParallel reports ns/op values as wall time for the benchmark as a whole,
// not the sum of wall time or CPU time over each parallel goroutine.
func (b *B) RunParallel(body func(*PB)) {
        if b.N == 0 {
                return // Nothing to do when probing.
        }
        // Calculate grain size as number of iterations that take ~100µs.
        // 100µs is enough to amortize the overhead and provide sufficient
        // dynamic load balancing.
        grain := uint64(0)
        if b.previousN > 0 && b.previousDuration > 0 {
                grain = 1e5 * uint64(b.previousN) / uint64(b.previousDuration)
        }
        if grain < 1 {
                grain = 1
        }
        // We expect the inner loop and function call to take at least 10ns,
        // so do not do more than 100µs/10ns=1e4 iterations.
        if grain > 1e4 {
                grain = 1e4
        }

        n := uint64(0)
        numProcs := b.parallelism * runtime.GOMAXPROCS(0)
        var wg sync.WaitGroup
        wg.Add(numProcs)
        for p := 0; p < numProcs; p++ {
                go func() {
                        defer wg.Done()
                        pb := &PB{
                                globalN: &n,
                                grain:   grain,
                                bN:      uint64(b.N),
                        }
                        body(pb)
                }()
        }
        wg.Wait()
        if n <= uint64(b.N) && !b.Failed() {
                b.Fatal("RunParallel: body exited without pb.Next() == false")
        }
}

// SetParallelism sets the number of goroutines used by RunParallel to p*GOMAXPROCS.
// There is usually no need to call SetParallelism for CPU-bound benchmarks.
// If p is less than 1, this call will have no effect.
func (b *B) SetParallelism(p int) {
        if p >= 1 {
                b.parallelism = p
        }
}

// Benchmark benchmarks a single function. It is useful for creating
// custom benchmarks that do not use the "go test" command.
//
// If f depends on testing flags, then Init must be used to register
// those flags before calling Benchmark and before calling flag.Parse.
//
// If f calls Run, the result will be an estimate of running all its
// subbenchmarks that don't call Run in sequence in a single benchmark.
func Benchmark(f func(b *B)) BenchmarkResult {
        b := &B{
                common: common{
                        signal: make(chan bool),
                        w:      discard{},
                },
                benchFunc: f,
                benchTime: benchTime,
        }
        if b.run1() {
                b.run()
        }
        return b.result
}

type discard struct{}

func (discard) Write(b []byte) (n int, err error) { return len(b), nil }