Re: [Haskell-beginners] Performance of parallel mergesort

On Thu, Dec 24, 2009 at 12:16 AM, Antoine Latter wrote:

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

Here's a paste-bin link for the code in question: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=14871#a14871

On Wed, Dec 23, 2009 at 9:14 PM, Jon Harrop 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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners