[Haskell-cafe] Problem getting code from AFP08 parallel tutorial to run in parallel

[Don Stewart]

Which version of GHC are you using?

This particular example triggers a "boundary condition" in ghc 6.10
where, with only one spark, GHC doesn't fire up the extra cpu. Try it
with 6.8.x to see that in action.

Simon Marlow may be able to comment more.

-- Don

olivier.boudry:
> Hi all,
> 
> I'm reading the following tutorial:
> http://research.microsoft.com/~simonpj/papers/parallel/AFP08-notes.pdf
> "A Tutorial on Parallel and Concurrent
> Programming in Haskell" and have problems getting the expected speed
> improvement from running two tasks in parallel. With any version of
> the code present in pages 1-7 of the tutorial I keep getting the CPU
> stick to 50%.
> 
> I did not forget to compile the code with `-threaded` and run it with
> `+RTS -N2` and it runs on a dual core machine on which I already used
> the Control.Parallel.Strategies.parMap function and got 100% CPU
> usage.
> 
> The first version of the parallel function in the tutorial (page 6) is:
> 
>     parSumFibEuler :: Int −> Int −> Int
>     parSumFibEuler a b
>       = f 'par' (f + e)
>       where
>         f = fib a
>         e = sumEuler b
> 
> In the tutorial, swapping f and e on the 3rd line does the job, but in
> my case it doesn't change anything.
> 
>     C:\Temp\haskell>ghc --make -threaded SumEulerP6.hs
>     [1 of 1] Compiling Main             ( SumEulerP6.hs, SumEulerP6.o )
>     Linking SumEulerP6.exe ...
> 
>     C:\Temp\haskell>SumEulerP6 +RTS -N1
>     sum: 119201850
>     time: 36.890625 seconds
> 
>     C:\Temp\haskell>SumEulerP6 +RTS -N2
>     sum: 119201850
>     time: 36.859375 seconds
> 
> Next page of the tutorial the tasks are explicitly sequenced so the
> code does not depend on the ordering of the two `+` operands:
> 
>     parSumFibEuler :: Int -> Int -> Int
>     parSumFibEuler a b
>       = f `par` (e `pseq` (f + e))
>         where
>           f = fib a
>           e = sumEuler b
> 
> With once again a disappointing result:
> 
>     C:\Temp\haskell>ghc --make -threaded SumEulerP7.hs
>     [1 of 1] Compiling Main             ( SumEulerP7.hs, SumEulerP7.o )
>     Linking SumEulerP7.exe ...
> 
>     C:\Temp\haskell>SumEulerP7 +RTS -N1
>     sum: 119201850
>     time: 36.875 seconds
> 
>     C:\Temp\haskell>SumEulerP7 +RTS -N2
>     sum: 119201850
>     time: 36.75 seconds
> 
> I tried this on a Windows XP Dell Dual Core with GHC 6.10.1 and on a
> iMac Dual Core with GHC 6.8.3 and got the same result on both. I'm
> probably missing something really stupid but I'm lacking the "parallel
> thinking" skills to understand how to look at the problem and resolve
> it.
> 
> Any pointers?
> 
> Thanks,
> 
> Olivier.

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