perf: Files

Command benchstat

Benchstat computes and compares statistics about benchmarks.


benchstat [-delta-test name] [-geomean] [-html] old.txt [new.txt] [more.txt ...]

Each input file should contain the concatenated output of a number of runs of “go test -bench.” For each different benchmark listed in an input file, benchstat computes the mean, minimum, and maximum run time, after removing outliers using the interquartile range rule.

If invoked on a single input file, benchstat prints the per-benchmark statistics for that file.

If invoked on a pair of input files, benchstat adds to the output a column showing the statistics from the second file and a column showing the percent change in mean from the first to the second file. Next to the percent change, benchstat shows the p-value and sample sizes from a test of the two distributions of benchmark times. Small p-values indicate that the two distributions are significantly different. If the test indicates that there was no significant change between the two benchmarks (defined as p > 0.05), benchstat displays a single ~ instead of the percent change.

The -delta-test option controls which significance test is applied: utest (Mann-Whitney U-test), ttest (two-sample Welch t-test), or none. The default is the U-test, sometimes also referred to as the Wilcoxon rank sum test.

If invoked on more than two input files, benchstat prints the per-benchmark statistics for all the files, showing one column of statistics for each file, with no column for percent change or statistical significance.

The -html option causes benchstat to print the results as an HTML table.


Suppose we collect benchmark results from running “go test -bench=Encode” five times before and after a particular change.

The file old.txt contains:

BenchmarkGobEncode   	100	  13552735 ns/op	  56.63 MB/s
BenchmarkJSONEncode  	 50	  32395067 ns/op	  59.90 MB/s
BenchmarkGobEncode   	100	  13553943 ns/op	  56.63 MB/s
BenchmarkJSONEncode  	 50	  32334214 ns/op	  60.01 MB/s
BenchmarkGobEncode   	100	  13606356 ns/op	  56.41 MB/s
BenchmarkJSONEncode  	 50	  31992891 ns/op	  60.65 MB/s
BenchmarkGobEncode   	100	  13683198 ns/op	  56.09 MB/s
BenchmarkJSONEncode  	 50	  31735022 ns/op	  61.15 MB/s

The file new.txt contains:

BenchmarkGobEncode   	 100	  11773189 ns/op	  65.19 MB/s
BenchmarkJSONEncode  	  50	  32036529 ns/op	  60.57 MB/s
BenchmarkGobEncode   	 100	  11942588 ns/op	  64.27 MB/s
BenchmarkJSONEncode  	  50	  32156552 ns/op	  60.34 MB/s
BenchmarkGobEncode   	 100	  11786159 ns/op	  65.12 MB/s
BenchmarkJSONEncode  	  50	  31288355 ns/op	  62.02 MB/s
BenchmarkGobEncode   	 100	  11628583 ns/op	  66.00 MB/s
BenchmarkJSONEncode  	  50	  31559706 ns/op	  61.49 MB/s
BenchmarkGobEncode   	 100	  11815924 ns/op	  64.96 MB/s
BenchmarkJSONEncode  	  50	  31765634 ns/op	  61.09 MB/s

The order of the lines in the file does not matter, except that the output lists benchmarks in order of appearance.

If run with just one input file, benchstat summarizes that file:

$ benchstat old.txt
name        time/op
GobEncode   13.6ms ± 1%
JSONEncode  32.1ms ± 1%

If run with two input files, benchstat summarizes and compares:

$ benchstat old.txt new.txt
name        old time/op  new time/op  delta
GobEncode   13.6ms ± 1%  11.8ms ± 1%  -13.31% (p=0.016 n=4+5)
JSONEncode  32.1ms ± 1%  31.8ms ± 1%     ~    (p=0.286 n=4+5)

Note that the JSONEncode result is reported as statistically insignificant instead of a -0.93% delta.

Package Files


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