Function Start

fgprof.go:33–75

Start begins profiling the goroutines of the program and returns a function that needs to be invoked by the caller to stop the profiling and write the results to w using the given format.

(w io.Writer, format Format)

// that needs to be invoked by the caller to stop the profiling and write the
// results to w using the given format.
func Start(w io.Writer, format Format) func() error {
	startTime := time.Now()

	// Go's CPU profiler uses 100hz, but 99hz might be less likely to result in
	// accidental synchronization with the program we're profiling.
	const hz = 99
	ticker := time.NewTicker(time.Second / hz)
	stopCh := make(chan struct{})
	prof := &profiler{}
	profile := newWallclockProfile()

	var sampleCount int64

	go func() {
		defer ticker.Stop()

		for {
			select {
			case <-ticker.C:
				sampleCount++

				stacks := prof.GoroutineProfile()
				profile.Add(stacks)
			case <-stopCh:
				return
			}
		}
	}()

	return func() error {
		stopCh <- struct{}{}
		endTime := time.Now()
		profile.Ignore(prof.SelfFrames()...)

		// Compute actual sample rate in case, due to performance issues, we
		// were not actually able to sample at the given hz. Converting
		// everything to float avoids integers being rounded in the wrong
		// direction and improves the correctness of times in profiles.
		duration := endTime.Sub(startTime)
		actualHz := float64(sampleCount) / (float64(duration) / 1e9)
		return profile.Export(w, format, int(math.Round(actualHz)), startTime, endTime)
	}
}

// profiler provides a convenient and performant way to access
// runtime.GoroutineProfile().