parList implementation question
Marcus D. Gabriel
marcus at gabriel.name
Fri Dec 18 13:31:03 EST 2009
In Control.Parallel.Strategies, parList is defined as
parList strat  = ()
parList strat (x:xs) = strat x `par` (parList strat xs)
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
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?
Thanks in advance for the clarification,
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