[Python-Dev] Disabling cyclic GC in timeit module

[Maciej Fijalkowski]

On Sat, Oct 8, 2011 at 1:47 AM, Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Sat, 8 Oct 2011 00:13:40 +0200
> Maciej Fijalkowski <fijall at gmail.com> wrote:
>> On Fri, Oct 7, 2011 at 11:47 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> > On Fri, Oct 7, 2011 at 4:50 PM, Maciej Fijalkowski <fijall at gmail.com> wrote:
>> >> Hi
>> >>
>> >> Can we disable by default disabling the cyclic gc in timeit module?
>> >> Often posts on pypy-dev or on pypy bugs contain usage of timeit module
>> >> which might change the performance significantly. A good example is
>> >> json benchmarks - you would rather not disable cyclic GC when running
>> >> a web app, so encoding/decoding json in benchmark with the cyclic GC
>> >> disabled does not make sense.
>> >>
>> >> What do you think?
>> >
>> > No, it's disabled by default for a reason (to avoid irrelevant noise
>> > in microbenchmarks), and other cases don't trump those original use
>> > cases.
>>
>> People don't use it only for microbenchmarks though. Also, you can't
>> call noise a thing that adds something every now and then I think.
>>
>> Er. How is disabling the GC for microbenchmarks any good by the way?
>
> In CPython, looking for reference cycles is a parasitic task that
> interferes with what you are trying to measure. It is not critical in
> any way, and you can schedule it much less often if it takes too much
> CPU, without any really adverse consequences. timeit takes the safe way
> and disables it completely.
>
> In PyPy, it doesn't seem gc.disable() should do anything, since you'd
> lose all automatic memory management if the GC was disabled.
>

it disables finalizers but this is besides the point. the point is
that people use timeit module to compute absolute time it takes for
CPython to do things, among other things comparing it to PyPy. While I
do agree that in microbenchmarks you don't loose much by just
disabling it, it does affect larger applications. So answering the
question like "how much time will take json encoding in my
application" should take cyclic GC time into account.

Cheers,
fijal