[Haskell-cafe] SMP parallelism increasing GC time dramatically

Simon Marlow marlowsd at gmail.com
Mon Oct 10 17:25:22 CEST 2011

On 10/10/2011 15:44, Tom Thorne wrote:
> thanks! I just tried setting -A32M and this seems to fix the parallel GC
> problems, I now get a speedup with parallel GC on and performance is the
> same as passing -qg. I had tried -H before and it only made things
> worse, but -A seems to do the trick.
> I'm still having problems with segmentation faults though. Depending on
> how I apply parMap, and whether I use monad-par or control.parallel,
> they seem to come and go arbitrarily. In a successful run that lasted
> about 30s in total with control.parallel, +RTS -s reports:
> SPARKS: 422712 (394377 converted, 0 pruned)
> am I creating too many sparks?

Please report the bug.  General rule: if in doubt, report it.  We'll 
decide whether it's a bug or not.  If it's a segfault, and you're not 
doing any FFI or unsafe stuff, and your hardware isn't faulty, then it's 
definitely a bug.


> On Mon, Oct 10, 2011 at 3:07 PM, Gregory Collins
> <greg at gregorycollins.net <mailto:greg at gregorycollins.net>> wrote:
>     On Mon, Oct 10, 2011 at 3:55 PM, Tom Thorne
>     <thomas.thorne21 at gmail.com <mailto:thomas.thorne21 at gmail.com>> wrote:
>      >
>      > Yes I will try to run threadscope on it, I tried it before and
>     the event log output produced about 1.8GB, and then crashed.
>      > Is there any way to tell the RTS to perform GC less often? My
>     code doesn't use too much memory and I'm using fairly hefty machines
>     (e.g one with 48 cores and 128GB of RAM) and so perhaps the
>     default/heuristic settings aren't optimal.
>     Increasing "-A" and "-H" in the RTS options should help with this.
>     G
>     --
>     Gregory Collins <greg at gregorycollins.net
>     <mailto:greg at gregorycollins.net>>
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