Re: [Haskell-cafe] Is Haskell capable of matching C in string processing performance?

22 Jan 2010

      The following snippet of code ran ~ 33% faster than yours on my computer (GHC 6.10.4, OSX):

import qualified Data.ByteString.Char8 as S
import qualified Data.ByteString.Lazy as L
import System.IO

null_str = S.pack "null"

main = withBinaryFile "out2.json" WriteMode $ \h -> do
  hPutStr h "["
  L.hPutStr h . L.fromChunks  . replicate 5000000 $ null_str
  hPutStr h "]"

The following snippet ran in roughly the same speed (which should not be surprising, as it is essentially the same code):

import Control.Monad
import qualified Data.ByteString.Char8 as S
import System.IO

null_str = S.pack "null"

main = withBinaryFile "out3.json" WriteMode $ \h -> do
  hPutStr h "["
  replicateM_ 5000000 (S.hPutStr h null_str)
  hPutStr h "]"

This C snippet ran ~ 6 times faster than the Haskell snippets:

#include 

int main() {
  int i;
  FILE* f = fopen("outc.json","w");
  fprintf(f,"[");
  for (i = 0; i < 5000000; ++i) fprintf(f,"null");
  fprintf(f,"]");
}

It seems to me this indicates that the big expense here is the call into the I/O system.

Cheers,
Greg

On Jan 21, 2010, at 10:09 PM, John Millikin wrote:
...
Recently I've been working on a library for generating large JSON[1]
documents quickly. Originally I started writing it in Haskell, but
quickly encountered performance problems. After exhausting my (meager)
supply of optimization ideas, I rewrote some of it in C, with dramatic
results. Namely, the C solution is
* 7.5 times faster than the fastest Haskell I could write (both using
raw pointer arrays)
* 14 times faster than a somewhat functional version (uses monads, but
no explicit IO)
* >30 times faster than fancy functional solutions with iteratees, streams, etc
I'm wondering if string processing is simply a Haskell weak point,
performance-wise. The problem involves many millions of very small
(<10 character, usually) strings -- the C solution can copy directly
from string literals into a fixed buffer and flush it occasionally,
while even the fastest Haskell version has a lot of overhead from
copying around arrays.
Dons suggested I was "doing it wrong", so I'm posting on -cafe in the
hopes that somebody can tell me how to get better performance without
resorting to C.
Here's the fastest Haskell version I could come up with. It discards
all error handling, validation, and correctness in the name of
performance, but still can't get anywhere near C:
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=16423
[1] http://json.org/
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Re: [Haskell-cafe] Is Haskell capable of matching C in string processing performance?

Gregory Crosswhite