[Haskell-cafe] Streaming bytes and performance

[Peter Simons]

Don Stewart <dons00 at gmail.com> writes:

 > Here's the final program: [...]

Here is a version of the program that is just as fast:

  import Prelude hiding ( getContents, foldl )
  import Data.ByteString.Char8

  countSpace :: Int -> Char -> Int
  countSpace i c | c == ' ' || c == '\n' = i + 1
                 | otherwise             = i

  main :: IO ()
  main = getContents >>= print . foldl countSpace 0

Generally speaking, I/O performance is not about fancy low-level system
features, it's about having a proper evaluation order:

 | $ ghc --make -O2 -funbox-strict-fields test1 && time ./test1
 | 37627064
 |
 | real 0m0.381s
 | user 0m0.356s
 | sys  0m0.023s

Versus:

 | $ ghc --make -O2 -funbox-strict-fields test2 && time ./test2 <test.txt
 | Linking test2 ...
 | 37627064
 |
 | real 0m0.383s
 | user 0m0.316s
 | sys  0m0.065s

Using this input file stored in /dev/shm:

 | $ ls -l test.txt 
 | -rw-r--r-- 1 simons users 208745650 Mar 19 21:40 test.txt

Take care,
Peter