[Haskell-cafe] I/O performance drop in ghc 6.12.1

[Peter Simons]

Hi,

I just updated to GHC 6.12.1, and I noticed a significant drop in I/O
performance that I can't explain. The following code is a simple
re-implementation of cat(1), i.e. it just echos all data from standard
input to standard output:

> module Main ( main ) where
>
> import System.IO
> import Foreign ( allocaBytes )
>
> bufsize :: Int
> bufsize = 4 * 1024
>
> catBuf :: Handle -> Handle -> IO ()
> catBuf hIn hOut = allocaBytes bufsize input
>   where
>   input ptr    = hGetBuf hIn ptr bufsize >>= output ptr
>   output  _  0 = return ()
>   output ptr n = hPutBuf hOut ptr n >> input ptr
>
> main :: IO ()
> main = do
>   mapM_ (\h -> hSetBuffering h NoBuffering) [ stdin, stdout ]
>   catBuf stdin stdout

That program used to have exactly the same performance as /bin/cat, but
now it no longer does:

 | $ dd if=/dev/urandom of=test.data bs=1M count=512
 |
 | $ time /bin/cat <test.data  >/dev/null
 |
 | real    0m1.939s
 | user    0m0.003s
 | sys     0m1.923s
 |
 | $ time ./cat-hgetbuf <test.data  >/dev/null
 |
 | real    0m4.327s
 | user    0m1.967s
 | sys     0m2.317s

I've tested different variants of the program that were built with -O,
-O2, and -O2 -funbox-strict-fields, respectively, but it doesn't seem to
make a difference.

Is there something I'm missing? Any suggestion would be kindly
appreciated.

Take care,
Peter