I recently ran into a problem writing a program that I hoped I could speed up by running over multiple CPUs. I want non-haskell users to be able to run the tool and take advantage of multiple CPUs. *But* there is a serious slowdown when the RTS is run with -N and some of the CPUs are already busy.

This is already mentioned in the GHC docs, but the problem I experienced was more serious than I expected causing a slowdown of around 2x compared to running with a single CPU.

I reproduced the problem with the following code from the haskell wiki:

{-# LANGUAGE BangPatterns #-}

import Data.Digest.Pure.MD5

import qualified Data.ByteString.Lazy as L

import System.Environment

import Control.Concurrent

import Control.Monad (replicateM_)

main = do

files <- getArgs

str <- newEmptyMVar

mapM_ (forkIO . hashAndPrint str) files

printNrResults (length files) str

printNrResults i var = replicateM_ i (takeMVar var >>= putStrLn)

hashAndPrint str f = do

bs <- L.readFile f

let !h = show $ md5 bs

putMVar str (f ++ ": " ++ h)

When run on 4 idle CPU cores, I get the following wall clock times:

./run +RTS -N1 : 20.4 sec

./run +RTS -N2 : 11.0 sec

./run +RTS -N4 : 6.7 sec

When run on the same 4 core machine, but with 2 cores already busy:

./run +RTS -N1 : 23.5 sec

./run +RTS -N2 : 14.1 sec

./run +RTS -N4 : 57.8 sec <---- Blowout...

This is quite a problem in practice when running on a shared server. Is there anything that can be done to address this?

(I wrote up a few more details here: http://thunking.drp.id.au/2012/06/slowdown-with-ghc-when-using-multiple.html)

Thanks,

--
David Powell