see https://github.com/golang/go/issues/27217
On Tuesday, March 24, 2020 at 5:24:08 PM UTC+1, Orson Cart wrote:
>
> I posted this earlier but I realised that the code had a fundamental error
> in it. I've corrected here it but the underlying problem still exists.
>
> I've recently started using go test's benchmarks support and I'm
> particularly interested in understanding the benchmark timer functions.
> I've been getting results that I found surprising and I was wondering if
> anyone could explain what's going on here.
>
> The code below has three benchmarks that each invoke a single function
> (foo). The implementation of foo isn't important, it's just there to
> consume some time:
> - foo is called once per iteration in Benchmark1.
> - It's called twice per iteration in Benchmark2 so I'd expect Benchmark2's
> duration to be nominally twice that of Benchmark1.
> - It's also called twice per iteration in Benchmark3 but the first call is
> wrapped in b.StopTimer and b.startTimer calls. Because of this I'd have
> expected Benchmark3 to be about the same duration as Benchmark1
>
> Apologies for the length of the example but I didn't think it fair to ask
> the question and leave anything out.
>
> package demo_test
>
> import (
> "strconv"
> "testing"
> )
>
> var Foo1 []string
> var Foo2 []string
> var Count int = 32767
>
> func Benchmark1(b *testing.B) {
> for i := 0; i < b.N; i++{
> Foo1 = foo(Count)
> }
> }
>
> func Benchmark2(b *testing.B) {
> for i := 0; i < b.N; i++{
> Foo1 = foo(Count)
> Foo2 = foo(Count)
> }
> }
>
> func Benchmark3(b *testing.B) {
> for i := 0; i < b.N; i++{
> b.StopTimer()
> Foo1 = foo(Count)
> b.StartTimer()
> Foo2 = foo(Count)
> }
> }
>
> func foo(count int) []string{
> testData := []string{}
> for i:= 0; i < count; i++ {
> testData = append(testData, strconv.Itoa(i))
> }
>
> return testData
> }
>
>
> When the benchmarks are run the results are as follows:
>
> Benchmark1-4 351 3345215 ns/op
> Benchmark2-4 166 7206582 ns/op
> Benchmark3-4 334 3457907 ns/op
> PASS
> ok bar.com/benchmarks 6.881s
>
> OK benchmark3 is a little slower than Benchmark1 but that's not what's
> bothering me. It's this: if I now change Count to something much smaller
> the results are a surprise, at least to me. Here are the results when Count
> = 8:
>
> Benchmark1-4 2706196 442 ns/op
> Benchmark2-4 1357482 873 ns/op
> Benchmark3-4 840729 1387 ns/op
> PASS
> ok bar.com/benchmarks 23.547s
>
> The ratio of timings for Benchmark1 and Benchmark2 are roughly in line
> with expectations but I was surprised to see that the timings for
> Benchmark3 are now larger than those for Benchmark2.
>
> Can anyone explain this?
>
> TIA
> Orson
>
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