func run()

in benchmarks/read_full_file/main.go [46:162]
86 lines of code
13 McCabe index (conditional complexity)

func run() (err error) {
	if *fDir == "" {
		err = errors.New("you must set --dir")
		return
	}

	// Create a temporary file.
	log.Printf("Creating a temporary file in %s.", *fDir)

	f, err := os.CreateTemp(*fDir, "sequential_read")
	if err != nil {
		err = fmt.Errorf("TempFile: %w", err)
		return
	}

	path := f.Name()

	// Make sure we clean it up later.
	defer func() {
		log.Printf("Deleting %s.", path)
		os.Remove(path)
	}()

	// Fill it with random content.
	log.Printf("Writing %d random bytes.", *fFileSize)
	_, err = io.Copy(f, io.LimitReader(rand.Reader, *fFileSize))
	if err != nil {
		err = fmt.Errorf("copying random bytes: %w", err)
		return
	}

	// Finish off the file.
	err = f.Close()
	if err != nil {
		err = fmt.Errorf("closing file: %w", err)
		return
	}

	// Run several iterations.
	log.Printf("Measuring for %v...", *fDuration)

	var fullFileRead percentile.DurationSlice
	var singleReadCall percentile.DurationSlice
	buf := make([]byte, *fReadSize)

	overallStartTime := time.Now()
	for len(fullFileRead) == 0 || time.Since(overallStartTime) < *fDuration {
		// Open the file for reading.
		f, err = os.Open(path)
		if err != nil {
			err = fmt.Errorf("opening file: %w", err)
			return
		}

		// Read the whole thing.
		fileStartTime := time.Now()
		for err == nil {
			readStartTime := time.Now()
			_, err = f.Read(buf)
			singleReadCall = append(singleReadCall, time.Since(readStartTime))
		}

		fullFileRead = append(fullFileRead, time.Since(fileStartTime))

		switch {
		case err == io.EOF:
			err = nil

		case err != nil:
			err = fmt.Errorf("reading: %w", err)
			return
		}

		// Close the file.
		err = f.Close()
		if err != nil {
			err = fmt.Errorf("closing file after reading: %w", err)
			return
		}
	}

	sort.Sort(fullFileRead)
	sort.Sort(singleReadCall)

	log.Printf(
		"Read the file %d times, using %d calls to read(2).",
		len(fullFileRead),
		len(singleReadCall))

	// Report.
	ptiles := []int{50, 90, 98}

	reportSlice := func(
		name string,
		bytesPerObservation int64,
		observations percentile.DurationSlice) {
		fmt.Printf("\n%s:\n", name)
		for _, ptile := range ptiles {
			d := percentile.Duration(observations, ptile)
			seconds := float64(d) / float64(time.Second)
			bandwidthBytesPerSec := float64(bytesPerObservation) / seconds

			fmt.Printf(
				"  %02dth ptile: %10v (%s/s)\n",
				ptile,
				d,
				format.Bytes(bandwidthBytesPerSec))
		}
	}

	reportSlice("Full-file read times", *fFileSize, fullFileRead)
	reportSlice("read(2) latencies", *fReadSize, singleReadCall)

	fmt.Println()

	return
}