On Tuesday, 24 March 2020 20:26:10 UTC, Sean Liao wrote:
>
> see https://github.com/golang/go/issues/27217
>
Thanks, will do.
>
>
> 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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