Source file src/testing/benchmark.go

Documentation: testing

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package testing
     6  
     7  import (
     8  	"flag"
     9  	"fmt"
    10  	"internal/race"
    11  	"io"
    12  	"math"
    13  	"os"
    14  	"runtime"
    15  	"sort"
    16  	"strconv"
    17  	"strings"
    18  	"sync"
    19  	"sync/atomic"
    20  	"time"
    21  	"unicode"
    22  )
    23  
    24  func initBenchmarkFlags() {
    25  	matchBenchmarks = flag.String("test.bench", "", "run only benchmarks matching `regexp`")
    26  	benchmarkMemory = flag.Bool("test.benchmem", false, "print memory allocations for benchmarks")
    27  	flag.Var(&benchTime, "test.benchtime", "run each benchmark for duration `d`")
    28  }
    29  
    30  var (
    31  	matchBenchmarks *string
    32  	benchmarkMemory *bool
    33  
    34  	benchTime = benchTimeFlag{d: 1 * time.Second} // changed during test of testing package
    35  )
    36  
    37  type benchTimeFlag struct {
    38  	d time.Duration
    39  	n int
    40  }
    41  
    42  func (f *benchTimeFlag) String() string {
    43  	if f.n > 0 {
    44  		return fmt.Sprintf("%dx", f.n)
    45  	}
    46  	return time.Duration(f.d).String()
    47  }
    48  
    49  func (f *benchTimeFlag) Set(s string) error {
    50  	if strings.HasSuffix(s, "x") {
    51  		n, err := strconv.ParseInt(s[:len(s)-1], 10, 0)
    52  		if err != nil || n <= 0 {
    53  			return fmt.Errorf("invalid count")
    54  		}
    55  		*f = benchTimeFlag{n: int(n)}
    56  		return nil
    57  	}
    58  	d, err := time.ParseDuration(s)
    59  	if err != nil || d <= 0 {
    60  		return fmt.Errorf("invalid duration")
    61  	}
    62  	*f = benchTimeFlag{d: d}
    63  	return nil
    64  }
    65  
    66  // Global lock to ensure only one benchmark runs at a time.
    67  var benchmarkLock sync.Mutex
    68  
    69  // Used for every benchmark for measuring memory.
    70  var memStats runtime.MemStats
    71  
    72  // An internal type but exported because it is cross-package; part of the implementation
    73  // of the "go test" command.
    74  type InternalBenchmark struct {
    75  	Name string
    76  	F    func(b *B)
    77  }
    78  
    79  // B is a type passed to Benchmark functions to manage benchmark
    80  // timing and to specify the number of iterations to run.
    81  //
    82  // A benchmark ends when its Benchmark function returns or calls any of the methods
    83  // FailNow, Fatal, Fatalf, SkipNow, Skip, or Skipf. Those methods must be called
    84  // only from the goroutine running the Benchmark function.
    85  // The other reporting methods, such as the variations of Log and Error,
    86  // may be called simultaneously from multiple goroutines.
    87  //
    88  // Like in tests, benchmark logs are accumulated during execution
    89  // and dumped to standard error when done. Unlike in tests, benchmark logs
    90  // are always printed, so as not to hide output whose existence may be
    91  // affecting benchmark results.
    92  type B struct {
    93  	common
    94  	importPath       string // import path of the package containing the benchmark
    95  	context          *benchContext
    96  	N                int
    97  	previousN        int           // number of iterations in the previous run
    98  	previousDuration time.Duration // total duration of the previous run
    99  	benchFunc        func(b *B)
   100  	benchTime        benchTimeFlag
   101  	bytes            int64
   102  	missingBytes     bool // one of the subbenchmarks does not have bytes set.
   103  	timerOn          bool
   104  	showAllocResult  bool
   105  	result           BenchmarkResult
   106  	parallelism      int // RunParallel creates parallelism*GOMAXPROCS goroutines
   107  	// The initial states of memStats.Mallocs and memStats.TotalAlloc.
   108  	startAllocs uint64
   109  	startBytes  uint64
   110  	// The net total of this test after being run.
   111  	netAllocs uint64
   112  	netBytes  uint64
   113  	// Extra metrics collected by ReportMetric.
   114  	extra map[string]float64
   115  }
   116  
   117  // StartTimer starts timing a test. This function is called automatically
   118  // before a benchmark starts, but it can also be used to resume timing after
   119  // a call to StopTimer.
   120  func (b *B) StartTimer() {
   121  	if !b.timerOn {
   122  		runtime.ReadMemStats(&memStats)
   123  		b.startAllocs = memStats.Mallocs
   124  		b.startBytes = memStats.TotalAlloc
   125  		b.start = time.Now()
   126  		b.timerOn = true
   127  	}
   128  }
   129  
   130  // StopTimer stops timing a test. This can be used to pause the timer
   131  // while performing complex initialization that you don't
   132  // want to measure.
   133  func (b *B) StopTimer() {
   134  	if b.timerOn {
   135  		b.duration += time.Since(b.start)
   136  		runtime.ReadMemStats(&memStats)
   137  		b.netAllocs += memStats.Mallocs - b.startAllocs
   138  		b.netBytes += memStats.TotalAlloc - b.startBytes
   139  		b.timerOn = false
   140  	}
   141  }
   142  
   143  // ResetTimer zeroes the elapsed benchmark time and memory allocation counters
   144  // and deletes user-reported metrics.
   145  // It does not affect whether the timer is running.
   146  func (b *B) ResetTimer() {
   147  	if b.extra == nil {
   148  		// Allocate the extra map before reading memory stats.
   149  		// Pre-size it to make more allocation unlikely.
   150  		b.extra = make(map[string]float64, 16)
   151  	} else {
   152  		for k := range b.extra {
   153  			delete(b.extra, k)
   154  		}
   155  	}
   156  	if b.timerOn {
   157  		runtime.ReadMemStats(&memStats)
   158  		b.startAllocs = memStats.Mallocs
   159  		b.startBytes = memStats.TotalAlloc
   160  		b.start = time.Now()
   161  	}
   162  	b.duration = 0
   163  	b.netAllocs = 0
   164  	b.netBytes = 0
   165  }
   166  
   167  // SetBytes records the number of bytes processed in a single operation.
   168  // If this is called, the benchmark will report ns/op and MB/s.
   169  func (b *B) SetBytes(n int64) { b.bytes = n }
   170  
   171  // ReportAllocs enables malloc statistics for this benchmark.
   172  // It is equivalent to setting -test.benchmem, but it only affects the
   173  // benchmark function that calls ReportAllocs.
   174  func (b *B) ReportAllocs() {
   175  	b.showAllocResult = true
   176  }
   177  
   178  // runN runs a single benchmark for the specified number of iterations.
   179  func (b *B) runN(n int) {
   180  	benchmarkLock.Lock()
   181  	defer benchmarkLock.Unlock()
   182  	// Try to get a comparable environment for each run
   183  	// by clearing garbage from previous runs.
   184  	runtime.GC()
   185  	b.raceErrors = -race.Errors()
   186  	b.N = n
   187  	b.parallelism = 1
   188  	b.ResetTimer()
   189  	b.StartTimer()
   190  	b.benchFunc(b)
   191  	b.StopTimer()
   192  	b.previousN = n
   193  	b.previousDuration = b.duration
   194  	b.raceErrors += race.Errors()
   195  	if b.raceErrors > 0 {
   196  		b.Errorf("race detected during execution of benchmark")
   197  	}
   198  }
   199  
   200  func min(x, y int64) int64 {
   201  	if x > y {
   202  		return y
   203  	}
   204  	return x
   205  }
   206  
   207  func max(x, y int64) int64 {
   208  	if x < y {
   209  		return y
   210  	}
   211  	return x
   212  }
   213  
   214  // run1 runs the first iteration of benchFunc. It reports whether more
   215  // iterations of this benchmarks should be run.
   216  func (b *B) run1() bool {
   217  	if ctx := b.context; ctx != nil {
   218  		// Extend maxLen, if needed.
   219  		if n := len(b.name) + ctx.extLen + 1; n > ctx.maxLen {
   220  			ctx.maxLen = n + 8 // Add additional slack to avoid too many jumps in size.
   221  		}
   222  	}
   223  	go func() {
   224  		// Signal that we're done whether we return normally
   225  		// or by FailNow's runtime.Goexit.
   226  		defer func() {
   227  			b.signal <- true
   228  		}()
   229  
   230  		b.runN(1)
   231  	}()
   232  	<-b.signal
   233  	if b.failed {
   234  		fmt.Fprintf(b.w, "--- FAIL: %s\n%s", b.name, b.output)
   235  		return false
   236  	}
   237  	// Only print the output if we know we are not going to proceed.
   238  	// Otherwise it is printed in processBench.
   239  	if atomic.LoadInt32(&b.hasSub) != 0 || b.finished {
   240  		tag := "BENCH"
   241  		if b.skipped {
   242  			tag = "SKIP"
   243  		}
   244  		if b.chatty && (len(b.output) > 0 || b.finished) {
   245  			b.trimOutput()
   246  			fmt.Fprintf(b.w, "--- %s: %s\n%s", tag, b.name, b.output)
   247  		}
   248  		return false
   249  	}
   250  	return true
   251  }
   252  
   253  var labelsOnce sync.Once
   254  
   255  // run executes the benchmark in a separate goroutine, including all of its
   256  // subbenchmarks. b must not have subbenchmarks.
   257  func (b *B) run() {
   258  	labelsOnce.Do(func() {
   259  		fmt.Fprintf(b.w, "goos: %s\n", runtime.GOOS)
   260  		fmt.Fprintf(b.w, "goarch: %s\n", runtime.GOARCH)
   261  		if b.importPath != "" {
   262  			fmt.Fprintf(b.w, "pkg: %s\n", b.importPath)
   263  		}
   264  	})
   265  	if b.context != nil {
   266  		// Running go test --test.bench
   267  		b.context.processBench(b) // Must call doBench.
   268  	} else {
   269  		// Running func Benchmark.
   270  		b.doBench()
   271  	}
   272  }
   273  
   274  func (b *B) doBench() BenchmarkResult {
   275  	go b.launch()
   276  	<-b.signal
   277  	return b.result
   278  }
   279  
   280  // launch launches the benchmark function. It gradually increases the number
   281  // of benchmark iterations until the benchmark runs for the requested benchtime.
   282  // launch is run by the doBench function as a separate goroutine.
   283  // run1 must have been called on b.
   284  func (b *B) launch() {
   285  	// Signal that we're done whether we return normally
   286  	// or by FailNow's runtime.Goexit.
   287  	defer func() {
   288  		b.signal <- true
   289  	}()
   290  
   291  	// Run the benchmark for at least the specified amount of time.
   292  	if b.benchTime.n > 0 {
   293  		b.runN(b.benchTime.n)
   294  	} else {
   295  		d := b.benchTime.d
   296  		for n := int64(1); !b.failed && b.duration < d && n < 1e9; {
   297  			last := n
   298  			// Predict required iterations.
   299  			goalns := d.Nanoseconds()
   300  			prevIters := int64(b.N)
   301  			prevns := b.duration.Nanoseconds()
   302  			if prevns <= 0 {
   303  				// Round up, to avoid div by zero.
   304  				prevns = 1
   305  			}
   306  			// Order of operations matters.
   307  			// For very fast benchmarks, prevIters ~= prevns.
   308  			// If you divide first, you get 0 or 1,
   309  			// which can hide an order of magnitude in execution time.
   310  			// So multiply first, then divide.
   311  			n = goalns * prevIters / prevns
   312  			// Run more iterations than we think we'll need (1.2x).
   313  			n += n / 5
   314  			// Don't grow too fast in case we had timing errors previously.
   315  			n = min(n, 100*last)
   316  			// Be sure to run at least one more than last time.
   317  			n = max(n, last+1)
   318  			// Don't run more than 1e9 times. (This also keeps n in int range on 32 bit platforms.)
   319  			n = min(n, 1e9)
   320  			b.runN(int(n))
   321  		}
   322  	}
   323  	b.result = BenchmarkResult{b.N, b.duration, b.bytes, b.netAllocs, b.netBytes, b.extra}
   324  }
   325  
   326  // ReportMetric adds "n unit" to the reported benchmark results.
   327  // If the metric is per-iteration, the caller should divide by b.N,
   328  // and by convention units should end in "/op".
   329  // ReportMetric overrides any previously reported value for the same unit.
   330  // ReportMetric panics if unit is the empty string or if unit contains
   331  // any whitespace.
   332  // If unit is a unit normally reported by the benchmark framework itself
   333  // (such as "allocs/op"), ReportMetric will override that metric.
   334  // Setting "ns/op" to 0 will suppress that built-in metric.
   335  func (b *B) ReportMetric(n float64, unit string) {
   336  	if unit == "" {
   337  		panic("metric unit must not be empty")
   338  	}
   339  	if strings.IndexFunc(unit, unicode.IsSpace) >= 0 {
   340  		panic("metric unit must not contain whitespace")
   341  	}
   342  	b.extra[unit] = n
   343  }
   344  
   345  // The results of a benchmark run.
   346  type BenchmarkResult struct {
   347  	N         int           // The number of iterations.
   348  	T         time.Duration // The total time taken.
   349  	Bytes     int64         // Bytes processed in one iteration.
   350  	MemAllocs uint64        // The total number of memory allocations.
   351  	MemBytes  uint64        // The total number of bytes allocated.
   352  
   353  	// Extra records additional metrics reported by ReportMetric.
   354  	Extra map[string]float64
   355  }
   356  
   357  // NsPerOp returns the "ns/op" metric.
   358  func (r BenchmarkResult) NsPerOp() int64 {
   359  	if v, ok := r.Extra["ns/op"]; ok {
   360  		return int64(v)
   361  	}
   362  	if r.N <= 0 {
   363  		return 0
   364  	}
   365  	return r.T.Nanoseconds() / int64(r.N)
   366  }
   367  
   368  // mbPerSec returns the "MB/s" metric.
   369  func (r BenchmarkResult) mbPerSec() float64 {
   370  	if v, ok := r.Extra["MB/s"]; ok {
   371  		return v
   372  	}
   373  	if r.Bytes <= 0 || r.T <= 0 || r.N <= 0 {
   374  		return 0
   375  	}
   376  	return (float64(r.Bytes) * float64(r.N) / 1e6) / r.T.Seconds()
   377  }
   378  
   379  // AllocsPerOp returns the "allocs/op" metric,
   380  // which is calculated as r.MemAllocs / r.N.
   381  func (r BenchmarkResult) AllocsPerOp() int64 {
   382  	if v, ok := r.Extra["allocs/op"]; ok {
   383  		return int64(v)
   384  	}
   385  	if r.N <= 0 {
   386  		return 0
   387  	}
   388  	return int64(r.MemAllocs) / int64(r.N)
   389  }
   390  
   391  // AllocedBytesPerOp returns the "B/op" metric,
   392  // which is calculated as r.MemBytes / r.N.
   393  func (r BenchmarkResult) AllocedBytesPerOp() int64 {
   394  	if v, ok := r.Extra["B/op"]; ok {
   395  		return int64(v)
   396  	}
   397  	if r.N <= 0 {
   398  		return 0
   399  	}
   400  	return int64(r.MemBytes) / int64(r.N)
   401  }
   402  
   403  // String returns a summary of the benchmark results.
   404  // It follows the benchmark result line format from
   405  // https://golang.org/design/14313-benchmark-format, not including the
   406  // benchmark name.
   407  // Extra metrics override built-in metrics of the same name.
   408  // String does not include allocs/op or B/op, since those are reported
   409  // by MemString.
   410  func (r BenchmarkResult) String() string {
   411  	buf := new(strings.Builder)
   412  	fmt.Fprintf(buf, "%8d", r.N)
   413  
   414  	// Get ns/op as a float.
   415  	ns, ok := r.Extra["ns/op"]
   416  	if !ok {
   417  		ns = float64(r.T.Nanoseconds()) / float64(r.N)
   418  	}
   419  	if ns != 0 {
   420  		buf.WriteByte('\t')
   421  		prettyPrint(buf, ns, "ns/op")
   422  	}
   423  
   424  	if mbs := r.mbPerSec(); mbs != 0 {
   425  		fmt.Fprintf(buf, "\t%7.2f MB/s", mbs)
   426  	}
   427  
   428  	// Print extra metrics that aren't represented in the standard
   429  	// metrics.
   430  	var extraKeys []string
   431  	for k := range r.Extra {
   432  		switch k {
   433  		case "ns/op", "MB/s", "B/op", "allocs/op":
   434  			// Built-in metrics reported elsewhere.
   435  			continue
   436  		}
   437  		extraKeys = append(extraKeys, k)
   438  	}
   439  	sort.Strings(extraKeys)
   440  	for _, k := range extraKeys {
   441  		buf.WriteByte('\t')
   442  		prettyPrint(buf, r.Extra[k], k)
   443  	}
   444  	return buf.String()
   445  }
   446  
   447  func prettyPrint(w io.Writer, x float64, unit string) {
   448  	// Print all numbers with 10 places before the decimal point
   449  	// and small numbers with three sig figs.
   450  	var format string
   451  	switch y := math.Abs(x); {
   452  	case y == 0 || y >= 99.95:
   453  		format = "%10.0f %s"
   454  	case y >= 9.995:
   455  		format = "%12.1f %s"
   456  	case y >= 0.9995:
   457  		format = "%13.2f %s"
   458  	case y >= 0.09995:
   459  		format = "%14.3f %s"
   460  	case y >= 0.009995:
   461  		format = "%15.4f %s"
   462  	case y >= 0.0009995:
   463  		format = "%16.5f %s"
   464  	default:
   465  		format = "%17.6f %s"
   466  	}
   467  	fmt.Fprintf(w, format, x, unit)
   468  }
   469  
   470  // MemString returns r.AllocedBytesPerOp and r.AllocsPerOp in the same format as 'go test'.
   471  func (r BenchmarkResult) MemString() string {
   472  	return fmt.Sprintf("%8d B/op\t%8d allocs/op",
   473  		r.AllocedBytesPerOp(), r.AllocsPerOp())
   474  }
   475  
   476  // benchmarkName returns full name of benchmark including procs suffix.
   477  func benchmarkName(name string, n int) string {
   478  	if n != 1 {
   479  		return fmt.Sprintf("%s-%d", name, n)
   480  	}
   481  	return name
   482  }
   483  
   484  type benchContext struct {
   485  	match *matcher
   486  
   487  	maxLen int // The largest recorded benchmark name.
   488  	extLen int // Maximum extension length.
   489  }
   490  
   491  // An internal function but exported because it is cross-package; part of the implementation
   492  // of the "go test" command.
   493  func RunBenchmarks(matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) {
   494  	runBenchmarks("", matchString, benchmarks)
   495  }
   496  
   497  func runBenchmarks(importPath string, matchString func(pat, str string) (bool, error), benchmarks []InternalBenchmark) bool {
   498  	// If no flag was specified, don't run benchmarks.
   499  	if len(*matchBenchmarks) == 0 {
   500  		return true
   501  	}
   502  	// Collect matching benchmarks and determine longest name.
   503  	maxprocs := 1
   504  	for _, procs := range cpuList {
   505  		if procs > maxprocs {
   506  			maxprocs = procs
   507  		}
   508  	}
   509  	ctx := &benchContext{
   510  		match:  newMatcher(matchString, *matchBenchmarks, "-test.bench"),
   511  		extLen: len(benchmarkName("", maxprocs)),
   512  	}
   513  	var bs []InternalBenchmark
   514  	for _, Benchmark := range benchmarks {
   515  		if _, matched, _ := ctx.match.fullName(nil, Benchmark.Name); matched {
   516  			bs = append(bs, Benchmark)
   517  			benchName := benchmarkName(Benchmark.Name, maxprocs)
   518  			if l := len(benchName) + ctx.extLen + 1; l > ctx.maxLen {
   519  				ctx.maxLen = l
   520  			}
   521  		}
   522  	}
   523  	main := &B{
   524  		common: common{
   525  			name:   "Main",
   526  			w:      os.Stdout,
   527  			chatty: *chatty,
   528  		},
   529  		importPath: importPath,
   530  		benchFunc: func(b *B) {
   531  			for _, Benchmark := range bs {
   532  				b.Run(Benchmark.Name, Benchmark.F)
   533  			}
   534  		},
   535  		benchTime: benchTime,
   536  		context:   ctx,
   537  	}
   538  	main.runN(1)
   539  	return !main.failed
   540  }
   541  
   542  // processBench runs bench b for the configured CPU counts and prints the results.
   543  func (ctx *benchContext) processBench(b *B) {
   544  	for i, procs := range cpuList {
   545  		for j := uint(0); j < *count; j++ {
   546  			runtime.GOMAXPROCS(procs)
   547  			benchName := benchmarkName(b.name, procs)
   548  			fmt.Fprintf(b.w, "%-*s\t", ctx.maxLen, benchName)
   549  			// Recompute the running time for all but the first iteration.
   550  			if i > 0 || j > 0 {
   551  				b = &B{
   552  					common: common{
   553  						signal: make(chan bool),
   554  						name:   b.name,
   555  						w:      b.w,
   556  						chatty: b.chatty,
   557  					},
   558  					benchFunc: b.benchFunc,
   559  					benchTime: b.benchTime,
   560  				}
   561  				b.run1()
   562  			}
   563  			r := b.doBench()
   564  			if b.failed {
   565  				// The output could be very long here, but probably isn't.
   566  				// We print it all, regardless, because we don't want to trim the reason
   567  				// the benchmark failed.
   568  				fmt.Fprintf(b.w, "--- FAIL: %s\n%s", benchName, b.output)
   569  				continue
   570  			}
   571  			results := r.String()
   572  			if *benchmarkMemory || b.showAllocResult {
   573  				results += "\t" + r.MemString()
   574  			}
   575  			fmt.Fprintln(b.w, results)
   576  			// Unlike with tests, we ignore the -chatty flag and always print output for
   577  			// benchmarks since the output generation time will skew the results.
   578  			if len(b.output) > 0 {
   579  				b.trimOutput()
   580  				fmt.Fprintf(b.w, "--- BENCH: %s\n%s", benchName, b.output)
   581  			}
   582  			if p := runtime.GOMAXPROCS(-1); p != procs {
   583  				fmt.Fprintf(os.Stderr, "testing: %s left GOMAXPROCS set to %d\n", benchName, p)
   584  			}
   585  		}
   586  	}
   587  }
   588  
   589  // Run benchmarks f as a subbenchmark with the given name. It reports
   590  // whether there were any failures.
   591  //
   592  // A subbenchmark is like any other benchmark. A benchmark that calls Run at
   593  // least once will not be measured itself and will be called once with N=1.
   594  func (b *B) Run(name string, f func(b *B)) bool {
   595  	// Since b has subbenchmarks, we will no longer run it as a benchmark itself.
   596  	// Release the lock and acquire it on exit to ensure locks stay paired.
   597  	atomic.StoreInt32(&b.hasSub, 1)
   598  	benchmarkLock.Unlock()
   599  	defer benchmarkLock.Lock()
   600  
   601  	benchName, ok, partial := b.name, true, false
   602  	if b.context != nil {
   603  		benchName, ok, partial = b.context.match.fullName(&b.common, name)
   604  	}
   605  	if !ok {
   606  		return true
   607  	}
   608  	var pc [maxStackLen]uintptr
   609  	n := runtime.Callers(2, pc[:])
   610  	sub := &B{
   611  		common: common{
   612  			signal:  make(chan bool),
   613  			name:    benchName,
   614  			parent:  &b.common,
   615  			level:   b.level + 1,
   616  			creator: pc[:n],
   617  			w:       b.w,
   618  			chatty:  b.chatty,
   619  		},
   620  		importPath: b.importPath,
   621  		benchFunc:  f,
   622  		benchTime:  b.benchTime,
   623  		context:    b.context,
   624  	}
   625  	if partial {
   626  		// Partial name match, like -bench=X/Y matching BenchmarkX.
   627  		// Only process sub-benchmarks, if any.
   628  		atomic.StoreInt32(&sub.hasSub, 1)
   629  	}
   630  	if sub.run1() {
   631  		sub.run()
   632  	}
   633  	b.add(sub.result)
   634  	return !sub.failed
   635  }
   636  
   637  // add simulates running benchmarks in sequence in a single iteration. It is
   638  // used to give some meaningful results in case func Benchmark is used in
   639  // combination with Run.
   640  func (b *B) add(other BenchmarkResult) {
   641  	r := &b.result
   642  	// The aggregated BenchmarkResults resemble running all subbenchmarks as
   643  	// in sequence in a single benchmark.
   644  	r.N = 1
   645  	r.T += time.Duration(other.NsPerOp())
   646  	if other.Bytes == 0 {
   647  		// Summing Bytes is meaningless in aggregate if not all subbenchmarks
   648  		// set it.
   649  		b.missingBytes = true
   650  		r.Bytes = 0
   651  	}
   652  	if !b.missingBytes {
   653  		r.Bytes += other.Bytes
   654  	}
   655  	r.MemAllocs += uint64(other.AllocsPerOp())
   656  	r.MemBytes += uint64(other.AllocedBytesPerOp())
   657  }
   658  
   659  // trimOutput shortens the output from a benchmark, which can be very long.
   660  func (b *B) trimOutput() {
   661  	// The output is likely to appear multiple times because the benchmark
   662  	// is run multiple times, but at least it will be seen. This is not a big deal
   663  	// because benchmarks rarely print, but just in case, we trim it if it's too long.
   664  	const maxNewlines = 10
   665  	for nlCount, j := 0, 0; j < len(b.output); j++ {
   666  		if b.output[j] == '\n' {
   667  			nlCount++
   668  			if nlCount >= maxNewlines {
   669  				b.output = append(b.output[:j], "\n\t... [output truncated]\n"...)
   670  				break
   671  			}
   672  		}
   673  	}
   674  }
   675  
   676  // A PB is used by RunParallel for running parallel benchmarks.
   677  type PB struct {
   678  	globalN *uint64 // shared between all worker goroutines iteration counter
   679  	grain   uint64  // acquire that many iterations from globalN at once
   680  	cache   uint64  // local cache of acquired iterations
   681  	bN      uint64  // total number of iterations to execute (b.N)
   682  }
   683  
   684  // Next reports whether there are more iterations to execute.
   685  func (pb *PB) Next() bool {
   686  	if pb.cache == 0 {
   687  		n := atomic.AddUint64(pb.globalN, pb.grain)
   688  		if n <= pb.bN {
   689  			pb.cache = pb.grain
   690  		} else if n < pb.bN+pb.grain {
   691  			pb.cache = pb.bN + pb.grain - n
   692  		} else {
   693  			return false
   694  		}
   695  	}
   696  	pb.cache--
   697  	return true
   698  }
   699  
   700  // RunParallel runs a benchmark in parallel.
   701  // It creates multiple goroutines and distributes b.N iterations among them.
   702  // The number of goroutines defaults to GOMAXPROCS. To increase parallelism for
   703  // non-CPU-bound benchmarks, call SetParallelism before RunParallel.
   704  // RunParallel is usually used with the go test -cpu flag.
   705  //
   706  // The body function will be run in each goroutine. It should set up any
   707  // goroutine-local state and then iterate until pb.Next returns false.
   708  // It should not use the StartTimer, StopTimer, or ResetTimer functions,
   709  // because they have global effect. It should also not call Run.
   710  func (b *B) RunParallel(body func(*PB)) {
   711  	if b.N == 0 {
   712  		return // Nothing to do when probing.
   713  	}
   714  	// Calculate grain size as number of iterations that take ~100µs.
   715  	// 100µs is enough to amortize the overhead and provide sufficient
   716  	// dynamic load balancing.
   717  	grain := uint64(0)
   718  	if b.previousN > 0 && b.previousDuration > 0 {
   719  		grain = 1e5 * uint64(b.previousN) / uint64(b.previousDuration)
   720  	}
   721  	if grain < 1 {
   722  		grain = 1
   723  	}
   724  	// We expect the inner loop and function call to take at least 10ns,
   725  	// so do not do more than 100µs/10ns=1e4 iterations.
   726  	if grain > 1e4 {
   727  		grain = 1e4
   728  	}
   729  
   730  	n := uint64(0)
   731  	numProcs := b.parallelism * runtime.GOMAXPROCS(0)
   732  	var wg sync.WaitGroup
   733  	wg.Add(numProcs)
   734  	for p := 0; p < numProcs; p++ {
   735  		go func() {
   736  			defer wg.Done()
   737  			pb := &PB{
   738  				globalN: &n,
   739  				grain:   grain,
   740  				bN:      uint64(b.N),
   741  			}
   742  			body(pb)
   743  		}()
   744  	}
   745  	wg.Wait()
   746  	if n <= uint64(b.N) && !b.Failed() {
   747  		b.Fatal("RunParallel: body exited without pb.Next() == false")
   748  	}
   749  }
   750  
   751  // SetParallelism sets the number of goroutines used by RunParallel to p*GOMAXPROCS.
   752  // There is usually no need to call SetParallelism for CPU-bound benchmarks.
   753  // If p is less than 1, this call will have no effect.
   754  func (b *B) SetParallelism(p int) {
   755  	if p >= 1 {
   756  		b.parallelism = p
   757  	}
   758  }
   759  
   760  // Benchmark benchmarks a single function. It is useful for creating
   761  // custom benchmarks that do not use the "go test" command.
   762  //
   763  // If f depends on testing flags, then Init must be used to register
   764  // those flags before calling Benchmark and before calling flag.Parse.
   765  //
   766  // If f calls Run, the result will be an estimate of running all its
   767  // subbenchmarks that don't call Run in sequence in a single benchmark.
   768  func Benchmark(f func(b *B)) BenchmarkResult {
   769  	b := &B{
   770  		common: common{
   771  			signal: make(chan bool),
   772  			w:      discard{},
   773  		},
   774  		benchFunc: f,
   775  		benchTime: benchTime,
   776  	}
   777  	if b.run1() {
   778  		b.run()
   779  	}
   780  	return b.result
   781  }
   782  
   783  type discard struct{}
   784  
   785  func (discard) Write(b []byte) (n int, err error) { return len(b), nil }
   786  

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