4

The task is to speed up a summation by using parMap or parListChunk or better. It actually runs slower with parallelism code.

Edit: Facepalm.. I overlooked how to execute the application correctly.

Don't forget to add cores by 

./myHaskellApp paramaters +RTS -N4 -sstderr

Where N4 is number of cores.

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7
  • 1
    Are these also slower for really big lists?
    – gspr
    Commented Mar 6, 2012 at 7:55
  • Depending on how expensive euler is, 5 may be much too small a chunk size. What if you try something more like 2000? (Although considering the smallitude of 15000, euler is probably fairly expensive, in which case this wouldn't work)
    – luqui
    Commented Mar 6, 2012 at 8:15
  • The best tool to analyze your parallel program is haskell.org/haskellwiki/ThreadScope
    – Chris Kuklewicz
    Commented Mar 6, 2012 at 13:48
  • Considering that the list doesn't even exist for the non parallel case (it compiles down to a non-allocating loop) but for the parallel case you must allocate a large list, chop it into chunks, and spark work. Are you really surprised? If you post the full code I might give it a look later.
    – Thomas M. DuBuisson
    Commented Mar 6, 2012 at 16:43
  • I will try larger chunks later.
    – gorn
    Commented Mar 6, 2012 at 20:23

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6

Always make sure that you're actually running -threaded, optimzied with -O2, and you are using a reasonable number of cores (e.g. -N4). Furthermore, check your garbage collector statistics.

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