ByteString I/O Performance

[Donald Bruce Stewart]

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

That's a useful benchmark.  Thanks for looking into this.

-- Don