Question about profiling reports

[Teofil Camarasu]

Hi Celeste,

It looks like by default only cost centers that account for at least 1% of
either allocations or time are shown. But all cost centers are shown when
the -pa flag is passed.

See:
https://downloads.haskell.org/ghc/latest/docs/users_guide/profiling.html?highlight=cost%20centre#rts-flag--pa
and
https://gitlab.haskell.org/ghc/ghc/-/blob/cf47b96f8b8100d9f9ad1b8121cbfb65fd7eaf2a/rts/ProfilerReport.c#L124

If you are processing these reports, it might be worth checking out the
eventlog interface via the ghc-events library (if you haven't already).

Cheers,
Teo


On Tue, Jul 30, 2024 at 3:58 PM Celeste Hollenbeck <
celeste.hollenbeck at gmail.com> wrote:

> Hi GHC Team,
>
> I have a question for a research project. I'm looking at GHC's profiler,
> and the documentation says a profiling report displays "a break-down by
> cost centre of the most costly functions in the program". Here's an example
> of the report that I'm talking about:
>
>
> MAIN        MAIN                    102           0    0.0    0.0   100.0
>  100.0
>  CAF        GHC.IO.Handle.FD        128           0    0.0    0.0     0.0
>    0.0
>  CAF        GHC.IO.Encoding.Iconv   120           0    0.0    0.0     0.0
>    0.0
>  CAF        GHC.Conc.Signal         110           0    0.0    0.0     0.0
>    0.0
>  CAF        Main                    108           0    0.0    0.0   100.0
>  100.0
>   main      Main                    204           1    0.0    0.0   100.0
>  100.0
>    fib      Main                    205     2692537  100.0  100.0   100.0
>  100.0
>
> This example is under Section 8.1 of the GHC User's Guide
>
> https://downloads.haskell.org/ghc/latest/docs/users_guide/profiling.html#:~:text=GHC's%20profiling%20system%20assigns%20costs,to%20the%20enclosing%20cost%20centre.
>
> It looks like the numbers often add up to less than 100% for the %time,
> but I don't see any documentation on a threshold for what makes a cost
> centre "costly"—so I assume that "costly" means that it takes up any time
> whatsoever, and any cost centres that take up any time at all are included
> in the report? So perhaps the numbers under %time don't add up to 100% all
> the time because of rounding error or perhaps garbage collection? Or
> something else that is not profiled?
>
> Is that correct?
>
> Thanks,
>
> Celeste
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