[Haskell-cafe] GHC's parallel garbage collector -- what am I doing wrong?

[Bryan O'Sullivan]

The parallel GC currently doesn't behave well with concurrent programs that
uses multiple capabilities (aka OS threads), and the behaviour you see is
the known symptom of this.. I believe that Simon Marlow has some fixes in
hand that may go into 6.12.2.

Are you saying that you see two different classes of undesirable
performance, one with -qg and one without? How are your threads in your real
program communicating with each other? We've seen problems there when
there's a lot of contention for e.g. IORefs among thousands of threads.

On Mon, Mar 1, 2010 at 7:59 AM, Michael Lesniak <mlesniak at uni-kassel.de>wrote:

> Hello haskell-cafe,
>
> Sorry for this long post, but I can't think of a way to describe and
> explain
> the problem in a shorter way.
>
> I've (again) a very strange behaviour with the parallel GC and would be
> glad
> if someone could either reproduce (and explain) it or provide a solution. A
> similar but unrelated problem has been described in [1].
>
>
> EXAMPLE CODE
> The following demonstration program, which is a much smaller and
> single-threaded version of my real problem behaves as my real program.
> It does some number crunching by calculating pi to a definable precision:
>
> > -- File Pi.hs
> > -- you need the numbers package from hackage.
> > module Main where
> > import Data.Number.CReal
> > import System.Environment
> > import GHC.Conc
> >
> > main = do
> >     digits <- (read . head) `fmap` getArgs :: IO Int
> >     calcPi digits
> >
> > calcPi digits = showCReal (fromEnum digits) pi `pseq` return ()
>
> Compile it with
>
>  ghc --make -threaded -O2 Pi.hs -o pi
>
>
> BENCHMARKS
> On my two-core machine I get the following quite strange and
> unpredictable results:
>
> * Using one thread:
>
>    $ for i in `seq 1 5`;do time pi 5000 +RTS -N1;done
>
>    real        0m1.441s
>    user        0m1.390s
>    sys 0m0.020s
>
>    real        0m1.449s
>    user        0m1.390s
>    sys 0m0.000s
>
>    real        0m1.399s
>    user        0m1.370s
>    sys 0m0.010s
>
>    real        0m1.401s
>    user        0m1.380s
>    sys 0m0.000s
>
>    real        0m1.404s
>    user        0m1.380s
>    sys 0m0.000s
>
>
> * Using two threads, hence the parallel GC is used:
>
>    for i in `seq 1 5`;do time pi 5000 +RTS -N2;done
>
>    real        0m2.540s
>    user        0m2.490s
>    sys 0m0.010s
>
>    real        0m1.527s
>    user        0m1.530s
>    sys 0m0.010s
>
>    real        0m1.966s
>    user        0m1.900s
>    sys 0m0.010s
>
>    real        0m5.670s
>    user        0m5.620s
>    sys 0m0.010s
>
>    real        0m2.966s
>    user        0m2.910s
>    sys 0m0.020s
>
>
> * Using two threads, but disabling the parallel GC:
>
>    for i in `seq 1 5`;do time pi 5000 +RTS -N2 -qg;done
>
>    real        0m1.383s
>    user        0m1.380s
>    sys 0m0.010s
>
>    real        0m1.420s
>    user        0m1.360s
>    sys 0m0.010s
>
>    real        0m1.406s
>    user        0m1.360s
>    sys 0m0.010s
>
>    real        0m1.421s
>    user        0m1.380s
>    sys 0m0.000s
>
>    real        0m1.360s
>    user        0m1.360s
>    sys 0m0.000s
>
>
> THREADSCOPE
> I've additionally attached the threadscope profile of a really bad run,
> started with
>
>     $ time pi 5000 +RTS -N2 -ls
>
>    real        0m15.594s
>    user        0m15.490s
>    sys 0m0.010s
>
> as file pi.pdf
>
>
> FURTHER INFORMATION/QUESTION
> Just disabling the parallel GC leads to very bad performance in my original
> code, which forks threads with forkIO and does a lot of communications.
> Hence,
> using -qg is not a real option for me.
>
> Do I have overlooked some cruical aspect of this problem? If you've
> read this far, thank you for reading ... this far ;-)
>
> Cheers,
>  Michael
>
>
>
> [1] http://osdir.com/ml/haskell-cafe@haskell.org/2010-02/msg00850.html
>
>
> --
> Dipl.-Inf. Michael C. Lesniak
> University of Kassel
> Programming Languages / Methodologies Research Group
> Department of Computer Science and Electrical Engineering
>
> Wilhelmshöher Allee 73
> 34121 Kassel
>
> Phone: +49-(0)561-804-6269
>
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