Forcing apps to collect GC stats?

[Austin Seipp]

The 'safe' mark was just an oversight from me - I don't really think
it matters, considering it's only called once at startup anyway.
Similarly I imagine the oversight is negligible in the Criterion or
ekg case.

And also, take note the conditional is very specific; you want
COLLECT_GC_STATS only when NO_GC_STATS is the current setting - if you
unconditionally force COLLECT_GC_STATS, then things like `+RTS
-sstderr -RTS` will no longer work. I mention this because I messed it
up originally. :)

Given we already use this in GHC, and other users want it, it's
probably reasonable to put such an interface in `base` somewhere, e.g.

data GCStatisticsOption = NoStatistics | CollectStatistics |
VerboseStatistics ...

getGcStatistics :: IO GcStatisticsOption
setGcStatistics :: GcStatisticsOption -> IO ()
modifyGcStatistics :: (GcStatisticsOption -> GcStatisticsOption) -> IO ()

or something like that.

The special case inside GHC could then be handled with such an API I suppose.


On Thu, Jul 31, 2014 at 11:41 AM, Johan Tibell <johan.tibell at gmail.com> wrote:
> Indeed. I filed a bug earlier today to make use of this.
>
> On Thu, Jul 31, 2014 at 6:15 PM, Edward Kmett <ekmett at gmail.com> wrote:
>> Interesting.
>>
>> I suppose ekg could also (ab)use this.
>>
>> Johan?
>>
>> -Edward
>>
>>
>> On Thu, Jul 31, 2014 at 5:51 AM, Simon Marlow <marlowsd at gmail.com> wrote:
>>>
>>> Hey Bryan,
>>>
>>> Sorry for the delay.
>>>
>>>
>>> On 15/07/14 01:57, Bryan O'Sullivan wrote:
>>>>
>>>> I spent a bit of time over the weekend trying to figure out how to force
>>>> the RTS to collect GC statistics, but was unable to do so.
>>>>
>>>> I'm currently working on enriching criterion's ability to gather data,
>>>> among which I'd like to see GC statistics. If I try to obtain GC stats
>>>> using criterion when I'm not running the benchmark app with +RTS -T, I
>>>> get an exception.
>>>>
>>>> Is there a way to allow criterion to forcibly enable stats collection?
>>>> My efforts to do so have gotten me nowhere. It would be unfortunate if I
>>>> had to tell users of criterion that they should always run with +RTS -T
>>>> or add a -rtsopts clause, as they'll simply forget.
>>>>
>>>> And while I'm asking, why does GHC not simply collect GC stats by
>>>> default? Collecting them seems to have zero cost, from what I can see?
>>>
>>>
>>> So you can do this in the same way as GHC. See
>>>
>>>
>>> https://phabricator.haskell.org/diffusion/GHC/browse/master/ghc/hschooks.c;6fa6caad0cb4ba99b2c0b444b0583190e743dd63$18-28
>>>
>>> Which is imported into Haskell like this:
>>>
>>>
>>> https://phabricator.haskell.org/diffusion/GHC/browse/master/ghc/Main.hs;6fa6caad0cb4ba99b2c0b444b0583190e743dd63$847-848
>>>
>>> I'm not sure why it's marked "safe", but it doesn't hurt.
>>>
>>> This API is kind-of public, in the sense that we deliberately expose it
>>> via the Rts.h header, and I'll try not to break it gratuitously.
>>>
>>> Cheers,
>>> Simon
>>>
>>> _______________________________________________
>>> ghc-devs mailing list
>>> ghc-devs at haskell.org
>>> http://www.haskell.org/mailman/listinfo/ghc-devs
>>
>>
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://www.haskell.org/mailman/listinfo/ghc-devs
>



-- 
Regards,

Austin Seipp, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/