[Haskell-beginners] Performance of parallel mergesort

[Antoine Latter]

Hi Jon,

You're more likely to get a useful response over on the
glasgow-haskell-users mailing list, as it looks like this is a GHC
specific bug: http://www.haskell.org/mailman/listinfo/glasgow-haskell-users

Running this at -N6 doesn't want to complete for me, so I'm not sure
if it would segfault. I'm on a 64-bit Intel Mac OS X using GHC 6.12,
dual core.

With such a small example as this your best bet would be to file a bug report:

http://hackage.haskell.org/trac/ghc/

You'll need to use the login guest/guest to file a new ticket.

Antoine

On Wed, Dec 23, 2009 at 9:14 PM, Jon Harrop <jon at ffconsultancy.com> wrote:
>
> I took the parallel merge sort described here:
>
>  http://martin.sulzmann.googlepages.com/AMP-Spring2008-intro.pdf
>
> and added a simple test:
>
> main :: IO ()
> main = do
>    putStrLn $ show $ sum $ mergesort $ [sin x | x <- [1.0 .. 1000000.0]]
>
> Compiled it with GHC 6.8.2 from Debian testing and ran it with various
> +RTS -N<n> arguments to measure its performance and obtained the following
> results on an 8 core:
>
> $ ghc --make -O2 -threaded mergesort.hs -o mergesort
> [1 of 1] Compiling Main             ( mergesort.hs, mergesort.o )
> Linking mergesort ...
> $ time ./mergesort +RTS -N1
> -0.117109518058233
>
> real    0m9.723s
> user    0m9.461s
> sys     0m0.232s
> $ time ./mergesort +RTS -N2
> -0.117109518058233
>
> real    0m13.574s
> user    0m15.225s
> sys     0m0.140s
> $ time ./mergesort +RTS -N3
> -0.117109518058233
>
> real    0m13.185s
> user    0m15.529s
> sys     0m0.184s
> $ time ./mergesort +RTS -N4
> -0.117109518058233
>
> real    0m13.251s
> user    0m15.829s
> sys     0m0.336s
> $ time ./mergesort +RTS -N5
> -0.117109518058233
>
> real    0m13.093s
> user    0m16.929s
> sys     0m0.164s
> $ time ./mergesort +RTS -N6
> Segmentation fault
>
> real    0m5.711s
> user    0m7.408s
> sys     0m0.136s
>
> That segfault must be due to a bug in GHC so I thought perhaps a newer version
> of GHC might fix the segfault but I installed GHC 6.10.4 from Sid and now I
> get:
>
> $ ghc --make -O2 -threaded mergesort.hs -o mergesort
> Binary: Int64 truncated to fit in 32 bit Int
> ghc: panic! (the 'impossible' happened)
>  (GHC version 6.10.4 for i386-unknown-linux):
>        Prelude.chr: bad argument
>
> Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
>
> I'll try the haskell-platform package next. Are these known problems?
>
> --
> Dr Jon Harrop, Flying Frog Consultancy Ltd.
> http://www.ffconsultancy.com/?e
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