parList implementation question
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? Thanks in advance for the clarification, - Marcus
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
Thank you Simon, I will obtain parallel 2.2.0.1 and work with it. Actually, the reason I asked my question was because I did not think forceParList should yield better performance than parList (unless it was becasue of the foldl?). I read the release notes for 6.12.1 about the work done on the ghc parallel framework and did just a little bit more experimenting. You might find this of interesting. ghc 6.10.4 with parallel 1.1.0.1 (as reported before): Serial algorithm: 1.00 unit of time Parallel algorithm with parList: 0.81 units of time Parallel algorithm with forceParList: 0.58 units of time ghc 6.12.1 with parallel 1.1.0.1: Serial algorithm: 1.00 unit of time Parallel algorithm with parList: 0.58 units of time* Parallel algorithm with forceParList: 0.58 units of time* Interesting. Well, from my perspective, 6.12.1 is certainly an improvement here. Cheers, - Marcus Simon Marlow wrote:
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
Thanks Simon. Parallel 2.2.0.1 was straight forward. I just replaced rnf with rdeepseq and my original use of parMap worked like a charm giving twice the performance for my dual-core system as I original expected and now find. Thanks, - Marcus Marcus D. Gabriel wrote:
Thank you Simon, I will obtain parallel 2.2.0.1 and work with it. Actually, the reason I asked my question was because I did not think forceParList should yield better performance than parList (unless it was becasue of the foldl?).
I read the release notes for 6.12.1 about the work done on the ghc parallel framework and did just a little bit more experimenting. You might find this of interesting.
ghc 6.10.4 with parallel 1.1.0.1 (as reported before): Serial algorithm: 1.00 unit of time Parallel algorithm with parList: 0.81 units of time Parallel algorithm with forceParList: 0.58 units of time
ghc 6.12.1 with parallel 1.1.0.1: Serial algorithm: 1.00 unit of time Parallel algorithm with parList: 0.58 units of time* Parallel algorithm with forceParList: 0.58 units of time*
Interesting. Well, from my perspective, 6.12.1 is certainly an improvement here.
Cheers, - Marcus
Simon Marlow wrote:
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
participants (2)
-
Marcus D. Gabriel -
Simon Marlow