So I'm probably missing something obvious again but the fact that the loops
were checking different/inverse conditions should not have cause the append
line to cost 300k ns/op.
I'm referring to the line `lcp = append(lcp, 0)` which appear at the start
of both of the functions, when I'm running the "slow" function as-is I get
timing of about 600k ns/op, but when I comment out the above append line I
get timing of about 300k ns/op, which mean that calling append took around
300k ns (or maybe it doesn't mean that?).
This is weird since the loops do not refer in any way with the lcp array
which is just returned later (both functions just do the aforementioned
append and at the end return the lcp slice which contain just 1 element
with 0 value).
I actually even pinpointed the code which make the append become slower and
it's part of the check of the inner for loop, its the "pos < previousSuffixLen"
part, removing this part make the code speed be roughly the same with or
without the append line.
just to be sure that this is not a sane speed for an append function, I
also benchmarked the function below, it got 21 ns/op.
func timeAppend(suffixArray []string) (lcp []int) {
lcp = append(lcp, 0)
return lcp
}
Thanks again in advance :)
On Monday, June 22, 2020 at 8:19:08 PM UTC+3 Yonatan Ben-Nes wrote:
> Oh the shame!!!!
>
> Thank you for pointing that out :)
>
> On Mon, 22 Jun 2020, 20:03 Martin Schnabel, <m...@mb0.org> wrote:
>
>> Let me quickly point out that your logic is inverted in the second code
>> snippet because you break on the condition. That is most likely the
>> reason for the difference. I guess the second example exits on the first
>> iteration.
>>
>> for pos := 0; true; pos++ {
>> if pos < suffixLen && pos < previousSuffixLen {
>> continue
>> }
>> break
>> }
>>
>> On 22.06.20 14:15, Yonatan Ben-Nes wrote:
>> > Hi,
>> >
>> > I'm encountering a weird issue which I fail to explain. It boils down
>> to
>> > 2 almost identical functions which give wildly different benchmark
>> results.
>> >
>> > The difference between the functions is that at the slow func there is
>> a
>> > for loop like this:
>> > for pos := 0; pos < suffixLen && pos < previousSuffixLen; pos++ {
>> > }
>> >
>> > While at the fast func the loop is like this:
>> > for pos := 0; true; pos++ {
>> > if pos < suffixLen && pos < previousSuffixLen {
>> > break
>> > }
>> > }
>> > * do note that the check at both versions is the same.
>> >
>> > Other than that the functions are identical but when I benchmark them I
>> get:
>> > $ go test -bench=Weird .
>> > goos: linux
>> > goarch: amd64
>> > BenchmarkWeirdFast-4 280394 3602 ns/op
>> > BenchmarkWeirdSlow-4 1866 618953 ns/op
>> >
>> > It's extra weird since getting the condition check into the loop should
>> > be the slower form, or at least that what I intuitively thought.
>> >
>> > Also, the extra time cost is divided between the loop section and an
>> > append function call which run immediately at the start of the function
>> > and is not connected in any way to the for loop.
>> > And it's extra weird as the append cost also get smaller if instead of
>> > reading the test text from a file I just declare a string variable with
>> > the same text for the benchmark (mind you that I b.ResetTimer() after
>> > doing either of those options).
>> >
>> > I attached the files to this message as they will make everything more
>> > clear and I also added comments describing what happen at the right
>> points.
>> >
>> > I'm using Ubuntu 18.04 and go is version 1.14.2, and I also checked it
>> > on a GCP compute engine and got the same results.
>> >
>> > Any thoughts will be much appreciated as I'm totally confused here! :)
>> >
>> > --
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>> >
>> https://groups.google.com/d/msgid/golang-nuts/3cf40b00-96ec-42a6-8093-7f1c27bfd93fn%40googlegroups.com
>>
>> > <
>> https://groups.google.com/d/msgid/golang-nuts/3cf40b00-96ec-42a6-8093-7f1c27bfd93fn%40googlegroups.com?utm_medium=email&utm_source=footer
>> >.
>>
>
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