coreyoconnor:
> I have the need to regularly write tiny programs that analyze output
> logs. The output logs don't have a consistent formatting so I
> typically choose Perl for these tasks.
>
> The latest instance of having to write such a program was simple
> enough I figured I'd try my hand at using Haskell instead. The short
> story is that I could quickly write a Haskell program that achieved
> the goal. Yay! However, the performance was ~8x slower than a
> comparable Perl implementation. With a lot of effort I got the Haskell
> version to only 2x slower. A lot of the optimization was done with
> guesses that the performance difference was due to how each line was
> being read from the file. I couldn't determine much using GHC's
> profiler.
{-# OPTIONS -fbang-patterns #-}
import qualified Data.ByteString.Char8 as S
import Data.ByteString.Lex.Double
import Debug.Trace
main = print . go 0 =<< S.getContents
where
go !n !s = case readDouble str of
Nothing -> n
Just (k,t) -> let delta = 1.0 / k in go (n+delta) t
where
(_, xs) = S.break ((==) ':') s
str = S.drop 2 xs
It uses the bytestring-lexing package on Hackage to read the Doubles
out,
$ ghc --make Fast.hs -O2 -fvia-C -optc-O2
$ time ./Fast < test.out
3155.626666664377
./Fast < test.out 0.07s user 0.01s system 97% cpu 0.078 total
So that's twice as fast as the perl entry on my box,
$ time perl Sum.pl < test.out
Duration (sec): 3155.62666666438
perl Sum.pl < test.out 0.15s user 0.03s system 100% cpu 0.180 total
Note that the safe Double lexer uses a bit of copying, so
we can in fact do better still with a non-copying Double parser,
but that's only for the hardcore.
-- Don
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