On Tue, Apr 22, 2008 at 11:32 AM, Ben <[EMAIL PROTECTED]> wrote: > Hello Haskellers, > > I'm running ghc 6.8.2 on vista 64. Consider the following program, > which is compiled with -02 -prof -auto-all: > > module Main where > > import System.IO (openFile, IOMode(..), hPutStr) > > testlst = let ls = [(i, [(j, (fromIntegral j)::Float) | j <- > [1..5]::[Int]]) | i <- [1..500000]::[Int]] > in ls > > main2 = do > h <- openFile "bardump" WriteMode > mapM_ ((hPutStr h) . show) testlst > > > main = do > h <- openFile "bardump2" WriteMode > mapM ((hPutStr h) . show) testlst > return () > > main and main2 are different in only that mapM_ versus mapM_ are used. > But the mapM version runs about 20x slower! I'm running with +RTS -p > -hc -RTS and I see that the amount of memory allocated is about the > same, and I think the resident memory is about the same too. But the > mapM_ version runs in about 8.7 seconds, and the mapM version takes > 167 seconds.
My first guess is that the garbage collector is not running at all in the mapM_ version, but is working it's ass off in the mapM version cleaning up the list that will never be used. > You may ask, why use mapM if you're discarding the values? > Unfortunately in my real app I need the values, which are more > interesting than IO (). If you need the values, then you've got to pay that price I suppose. If you need the values, I'm going to take a stab that in your real app you use a lot of memory because of this (because presumably you're keeping the values around), whereas you're just seeing a speed hit on this small test program. Luke _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
