Getting some strange benchmark results

[Agniva De Sarker]

Hi everyone,

I have a piece of code which is behaving a bit strangely in benchmarks. Here is a simplified version of it.

The only difference is that in one case, the destination is a different slice. On the other case, its the same slice as source.

func BenchmarkStringCreate(b *testing.B) {
    for n := 0; n < b.N; n++ {
        DecodeStringCreate("ab")
    }
}

func BenchmarkStringNoCreate(b *testing.B) {
    for n := 0; n < b.N; n++ {
        DecodeStringNoCreate("ab")
    }
}

func DecodeStringCreate(str string) []byte {
    dst := make([]byte, 2)
    src := []byte(str)
    Decode(dst, src)
    return dst
}

func DecodeStringNoCreate(str string) []byte {
    src := []byte(str)
    Decode(src, src)
    return src
}

func Decode(dst, src []byte) {
    for i := 0; i < 1; i++ {
        dst[0] = src[0]
    }
}

BenchmarkStringCreate-4         50000000            37.2 ns/op           2 B/op           1 allocs/op
BenchmarkStringNoCreate-4       30000000            44.3 ns/op           8 B/op           1 allocs/op
PASS
ok      stdtest    3.279s

It seems very strange to me that on the case where I am not allocating any slice, allocates 8 bytes of memory, whereas, when I create the slice of 2, it clearly allocates only 2 bytes.

Now here is the fun part, if I change the Decode function to remove the for loop, all allocations go down to zero.

func Decode(dst, src []byte) {
  dst[0] = src[0]
}


BenchmarkStringCreate-4 100000000 10.2 ns/op 0 B/op 0 allocs/op
BenchmarkStringNoCreate-4 200000000 9.85 ns/op 0 B/op 0 allocs/op
PASS
ok stdtest 4.000s

How come removing the for loop changes the no. of allocations ? And the loop always runs only 1 iteration anyway. I have no clue as to what is happening here.

Any help will be appreciated. Thanks.


[kpr...@atlassian.com]

From the recent GopherCon, golang's profiler has a tool for inspecting allocations that might solve your problem.

https://youtu.be/2557w0qsDV0?list=PLq2Nv-Sh8EbZEjZdPLaQt1qh_ohZFMDj8&t=526

[Agniva De Sarker]

Thanks

On Aug 18, 2017 1:42 PM, <kpr...@atlassian.com> wrote:

From the recent GopherCon, golang's profiler has a tool for inspecting allocations that might solve your problem.

https://youtu.be/2557w0qsDV0?list=PLq2Nv-Sh8EbZEjZdPLaQt1qh_ohZFMDj8&t=526

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