On 18/12/2009 18:31, Marcus D. Gabriel wrote:
Hello,
In Control.Parallel.Strategies, parList is defined as
parList strat [] = () parList strat (x:xs) = strat x `par` (parList strat xs)
with
parMap strat f xs = map f xs `using` parList strat.
I have recently found that if I define
forceParMap strat f xs = map f xs `using` forceParList strat
where
forceParList strat = foldl (\done -> (done>||) . strat) ()
then to date, forceParList via forceParMap gives faster results than parList via parMap. For example, in one experiment, parMap with parList run at 0.81 the time of the serial solution whereas forceParMap with forceParList run at 0.58 the time of the serial solution. This is to say, forceParList completed in 0.72 the time of parList. So,
1. Why is forceParList faster than parList? 2. Is this generic to the ghc runtime model or a particularity of the ghc implementation?
I'm not sure. Your forceParList looks equivalent to parList, unless I'm misreading it. I recommend trying out the new parallel package, here: http://hackage.haskell.org/package/parallel which has a new implementation of Strategies. The version of parList you quoted above has a space leak problem which may be affecting your results. Cheers, Simon