Skip to content
Snippets Groups Projects

Random.StdGen slowness

    • Closed Issue created by remit @trac-remit 
    Two (performance) problems in one:

{-# OPTIONS -fffi #-}
module Main (main) where

import Control.Monad
import Random

foreign import ccall unsafe "random" _crandom :: IO Int

randomInt :: (Int, Int) -> IO Int
randomInt (min,max) = do
    n <- _crandom
    return $ min + n `rem` range
    where
        range = max - min + 1

main = replicateM_ (5*10^6) $ do
    x <- randomRIO (0::Int,1000) :: IO Int
    x `seq` return ()
    return ()

First, without the "seq" at the end, hardly anything is
evaluated and we're building huge amounts of thunks.
Three ideas about this one:
- Blame the user :)
- data StdGen = StdGen !Int !Int
  Use strict fields in StdGen. Doesn't actually help
(at least in this example).
- Force evaluation of the StdGen in getStdRandom.
  Does help in this example, but also changes behaviour
of the library:
  x <- randomRIO undefined
  currently dies only when x (or the result of a later
randomRIO) is evaluated. This change causes it to die
immediately.

Second, even _with_ the "seq", replacing "randomRIO" by
"randomInt" speeds the thing up with a factor of about
30. (2 to 3.6, in a "real world" university practicum
exercise of 900 lines of code)
Even given the fact that they're not really doing the
same thing, this seems rather much :(
    Edited
    To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information

    Child items 0

    • No child items are currently assigned. Use child items to break down this issue into smaller parts.

    Activity

    Please register or sign in to reply