Forcing apps to collect GC stats?

[Simon Marlow]

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