[Haskell-cafe] mapM vs mapM_ performance

[Luke Palmer]

On Tue, Apr 22, 2008 at 11:32 AM, Ben <midfield at gmail.com> 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