Question about profiling reports

[Rodrigo Mesquita]

Hi Celeste,

You may already be aware, but there’s an alternative format to visualise profiles graphically.
Instead of `-p`, use `-pj` to produce the profiling report in JSON format, and then load the resulting .prof file into speedscope.app <https://www.speedscope.app/>.

Cheers,
Rodrigo Mesquita

> On 30 Jul 2024, at 15:57, 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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