On Tue, Apr 22, 2008 at 11:32 AM, Ben 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