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[Haskell-cafe] Parallel foldl doesn't work correctly

Subject: [Haskell-cafe] Parallel foldl doesn't work correctly
From: Philip Beadling
Date: Sat, 12 Dec 2009 02:57:22 +0000
Hi,

Can anyone put me right here.  I am trying to use a setup similar to
parMap to spark each valuation in a list in parallel, where the
resulting (evaluated) list is folded to produce a final single result.

Having done the obligatory google, I modified a few common examples to
give:

pfoldl f acc xs =  foldl' f acc (xs `using` parList rwhnf)


This compiles and if I monitor my CPUs it starts to use both cores, but
after approx 10 seconds, one core drops to low rate (I'm using a 2 core
machine).

The end result is that -N2 is actually a bit slower than -N1!

I'm guessing I haven't grasped the concept properly - although as map is
just 'foldl (+) 0' I'm at a loss to see why this approach wouldn't work
given it is pretty similar to parMap - could anyone point out what I'm
missing?

If it's any use the context of the code is below.

Many thanks!


Phil.


mc :: MonteCarloUserData -> [[Double]] -> Double
mc userData rndss = 
  existentialResult (pfoldl f existenAvg rndss) $ numOfSims userData
    where f           = flip $ existentialCombine . payOff' .
expiryValue 
          payOff'     = existentialPayOff userData
          expiryValue = foldl' (existentialEvolve userData) (stock
userData)
          existenAvg  = averager userData

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