[Haskell-cafe] poor performance when generating random text

[Dmitry Vyal]

Hello anyone

I've written a snippet which generates a file full of random strings. 
When compiled with -O2 on ghc-7.6, the generation speed is about 2Mb per 
second which is on par with interpreted php. That's the fact I find 
rather disappointing. Maybe I've missed something trivial? Any 
suggestions and explanations are welcome. :)

% cat ext_sort.hs
import qualified Data.Text as T
import System.Random
import Control.Exception
import Control.Monad

import System.IO
import qualified Data.Text.IO as TI

gen_string g = let (len, g') = randomR (50, 450) g
                in T.unfoldrN len rand_text (len, g')
  where rand_text (0,_) = Nothing
        rand_text (k,g) = let (c, g') = randomR ('a','z') g
                          in Just (c, ((k-1), g'))

write_corpus file = bracket (openFile file WriteMode) hClose $ \h -> do
   let size = 100000
   sequence $ replicate size $ do
     g <- newStdGen
     let text = gen_string g
     TI.hPutStrLn h text

main = do
   putStrLn "generating text corpus"
   write_corpus "test.txt"



% cat ext_sort.prof
         Wed Oct 17 10:59 2012 Time and Allocation Profiling Report (Final)

            ext_sort +RTS -p -RTS

         total time  =       32.56 secs   (32558 ticks @ 1000 us, 1 
processor)
         total alloc = 12,742,917,332 bytes  (excludes profiling overheads)

COST CENTRE                MODULE  %time %alloc

gen_string.rand_text.(...) Main     70.7   69.8
gen_string                 Main     17.6   15.8
gen_string.rand_text       Main      5.4   13.3
write_corpus.\             Main      4.3    0.8


individual     inherited
COST CENTRE                       MODULE no.     entries  %time %alloc   
%time %alloc

MAIN MAIN                                67           0    0.0    0.0 
100.0  100.0
  main                             Main 135           0    0.0    0.0   
100.0  100.0
   write_corpus                    Main 137           0    0.0    0.0   
100.0  100.0
    write_corpus.\                 Main 138           1    4.3    0.8   
100.0  100.0
     write_corpus.\.text           Main 140      100000    0.0    0.0    
95.7   99.2
      gen_string                   Main 141      100000   17.6   15.8    
95.7   99.2
       gen_string.g'               Main 147      100000    0.0    
0.0     0.0    0.0
       gen_string.rand_text        Main 144    25109743    5.4   13.3    
77.5   83.2
        gen_string.rand_text.g'    Main 148    24909743    0.6    
0.0     0.6    0.0
        gen_string.rand_text.(...) Main 146    25009743   70.7   69.8    
70.7   69.8
        gen_string.rand_text.c     Main 145    25009743    0.8    
0.0     0.8    0.0
       gen_string.len              Main 143      100000    0.0    
0.0     0.0    0.0
       gen_string.(...)            Main 142      100000    0.6    
0.3     0.6    0.3