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TIL: Reading benchmark results from Go

jonasbn
Computer programmer, runner, LEGO builder, powernapper, yakshaver and father of 2 boys all squeezed in the few hours available.
・4 min read

Go has benchmarking capabilities as part of it's standard toolchain.

If you want to do a benchmark of your current Go programming project:

$ go test -bench . -count 3
goos: darwin
goarch: amd64
pkg: twofer
BenchmarkShareWith-8  14216751  81.7 ns/op
BenchmarkShareWith-8  13949208  81.1 ns/op
BenchmarkShareWith-8  14090535  82.4 ns/op
PASS
ok      twofer    3.769s

NB: I have condensed the output a bit for readability, by collapsing excessive whitespace.

The above example is lifted from Exercism.io and the two-fer exercise.

NB: All examples from Exercism.io are under copyright and are available under the MIT License.

In order to do benchmarking, it requires that the test suite has a benchmarking function, which is just a variation of a test.

I can recommend this older piece on writing benchmark tests. In essence benchmark. So you write execution scenarios as you would tests. For Go you put these with your tests but name them with the prefix Benchmark instead of Test.

This is based on the example lifted from the Exercism.io exercise mentioned above:

func BenchmarkShareWith(b *testing.B) {
        for i := 0; i < b.N; i++ {

                for _, test := range tests {
                        ShareWith(test.name)
                }

        }
}

Lets break down the invocation, it runs our test suite and benchmarks it, with 3 iterations, specified by the -count parameter.

As you can read from the example output.

  • 3 iterations are run
  • First column is the test run, which points to a function in the two_fer_test.go
  • Second column is the amount of times the test was run
  • Third column is the measured time and yes ns is nanoseconds

We both specify a number of iterations, but the test suite does an internal count, I have quoted the explanation from the official documentation:

The benchmark function must run the target code b.N times. During benchmark execution, b.N is
adjusted until the benchmark function lasts long enough to be timed reliably.

Please see the official documentation for more information.

Now if you want even more detail, you can also get numbers on memory allocation using the -benchmem flag. There are plenty of flags, check the official documentation for a listing.

$ go test -bench . -benchmem -count 3
goos: darwin
goarch: amd64
pkg: twofer
BenchmarkShareWith-8  14013910  82.4 ns/op  0 B/op  0 allocs/op
BenchmarkShareWith-8  14102596  81.7 ns/op  0 B/op  0 allocs/op
BenchmarkShareWith-8  14164332  81.1 ns/op  0 B/op  0 allocs/op
PASS
ok      twofer    3.873s

NB: I have condensed the output a bit for readability, by collapsing excessive whitespace.

This runs our test suite again and benchmarks it, with 3 iterations, but outputs even more information.

Since I was unable to understand the output, I had to ask around and I received the following explanation. Thanks to mzi from the Gopher Slack for the explanation.

B/op is how many bytes were allocated per iteration, and allocs/op is how many allocations were
made. (per iteration)

The first 3 columns are the ones from the basic benchmark. So with the --benchmem flag, two more columns are added:

  • Fourth column is the number of additional bytes allocated per iteration
  • Fifth column is the number of addtional allocations made per iteration

My interpretation of this is a that these are the additional allocations made by running the executable, providing the user with information on memory consumption in addition to the time consumption reported first.

Do note this is my understanding of the numbers, I have not been able to confirm this completely, so any additional pointers would be most welcome.

And finally thanks to superstas at Exercism.io for pushing me into benchmarking my first real Go solution.

To describe more of what I learned, lets examine my solution from Exercism.io.

package twofer

func ShareWith(name string) string {
    if name == "" {
        name = "you"
    }

    return "One for " + name + ", one for me."
}

NB: I have condensed the output a bit for readability, by removing inline documentation.

This was the optimized solution, as pointed out by superstas could be made simpler and less expernsive resource wise.

The solution proposal he was giving feedback on was this one:

package twofer

import "fmt"

func ShareWith(name string) string {
    if name == "" {
        name = "you"
    }

    return fmt.Sprintf("One for %s, one for me.", name)
}

NB: I have condensed the output a bit for readability, by removing inline documentation.

If we benchmark this you can compare the numbers:

$ go test -bench . -benchmem -count 3
goos: darwin
goarch: amd64
pkg: twofer
BenchmarkShareWith-8  2691262  443 ns/op  144 B/op  6 allocs/op
BenchmarkShareWith-8  2716256  443 ns/op  144 B/op  6 allocs/op
BenchmarkShareWith-8  2702143  447 ns/op  144 B/op  6 allocs/op
PASS
ok      twofer    5.207s

NB: I have condensed the output a bit for readability, by collapsing excessive whitespace.

As pointed out by superstas use of Go's fmt package comes at a certain price and in this case a basic concatenation would suffice as demonstrated in the example with the optimized solution.

With Go benchmarking at the tip of your fingers, you can gain insights on your code, read the documentation and start experimenting and perhaps even optimizing.

References

This was lifted from my "Today I Learned (TIL) collection"

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