ByteString I/O Performance

[Peter Simons]

Duncan Coutts writes:

 | As I recall from when I profiled this for the ByteString paper
 | [...], the slowdown compared to a simple hGetBuf 'cat' was all
 | down to cache locality, because we're cycling between a range
 | of buffers rather than a single cache-hot buffer.

I believe you are right. The following implementation performs
just fine:

> import System.IO
> import qualified Data.ByteString.Base as Str
> import qualified Data.ByteString as Str
> import Data.ByteString ( ByteString )
>
> bufsize :: Int
> bufsize = 4 * 1024
>
> hGet :: Handle -> ByteString -> IO ByteString
> hGet h buf = do i <- Str.unsafeUseAsCStringLen buf (\(p,n) -> hGetBuf h p n)
>                 return (Str.unsafeTake i buf)
>
> catString :: Handle -> Handle -> IO ()
> catString hIn hOut = Str.create bufsize (\_ -> return ()) >>= input
>   where
>   input buf      = hGet hIn buf >>= output buf
>   output buf b
>     | Str.null b = return ()
>     | otherwise  = Str.hPut hOut b >> input buf
>
> main :: IO ()
> main = do
>   mapM_ (\h -> hSetBuffering h NoBuffering) [ stdin, stdout ]
>   catString stdin stdout

time /bin/cat <test.data  >/dev/null

real	0m2.093s
user	0m0.024s
sys	0m2.068s

time ./cat-bytestring <test.data  >/dev/null

real	0m2.753s
user	0m0.568s
sys	0m2.184s

Peter