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.