Hello, I've written a smaller example which reproduces the unusual behaviour. Should I open a GHC-Ticket, too? -- A small working example which describes the problems (I have) with GHC -- 6.10.4, Ubuntu Karmic 9.10, explicit threading and core usage. -- -- See http://www.haskell.org/pipermail/haskell-cafe/2009-November/069144.html -- for the general description of the problem. -- -- For comparsion: -- Compilation on both machines with -- -- ghc --make -O2 -threaded Example.hs -o e -Wall -- -- -- 1. Machine B: (Quadcore, Ubuntu 9.04) -- a. With 1 thread: -- time e +RTS -N1 -RTS 16 -- e +RTS -N1 -RTS 16 11,00s user 5,00s system 100% cpu 16,004 total -- -- b. With 2 threads: -- time e +RTS -N2 -RTS 16 -- e +RTS -N2 -RTS 16 11,44s user 4,58s system 197% cpu 8,102 total -- -- -- 2. Machine C: (Dualcore, Ubuntu 9.10) -- a. With 1 thread: -- time e +RTS -N1 -RTS 16 -- -- real 0m16.414s -- user 0m11.360s -- sys 0m4.650s -- -- b. With 2 threads: -- time e +RTS -N2 -RTS 16 -- -- real 0m18.484s -- user 0m14.320s -- sys 0m5.940s -- ------------------------------------------------------------------------------- module Main where import GHC.Conc import Control.Concurrent import Control.Monad import System.Posix.Clock import System.Environment ------------------------------------------------------------------------------- main :: IO () main = do -- Configuration args <- getArgs let threads = numCapabilities -- number of threads determined by -N<...> taskDur = 1.0 -- seconds each task takes taskNum = (read . head) args -- Number of tasks is 1st parameter -- Generate a channel for the tasks to do and fill it with uniform and -- independent tasks. The other channel receives a message for each task -- which is finished. queue <- newChan finished <- newChan writeList2Chan queue (replicate taskNum taskDur) -- Fork threads replicateM_ threads (forkIO (thread queue finished)) -- Wait until the queue is empty replicateM_ taskNum (readChan finished) ------------------------------------------------------------------------------- thread :: Chan Double -> Chan Int -> IO () thread queue finished = forever $ do task <- readChan queue workFor task writeChan finished 1 ------------------------------------------------------------------------------- -- | Generates work for @s@ seconds. workFor :: Double -> IO () workFor s = do now <- getTime ThreadCPUTime repeat (time2Double now + s) where repeat fs = do now <- nSqrt 10000 `pseq` getTime ThreadCPUTime let f = time2Double now unless (f >= fs) $ repeat fs time2Double t = fromIntegral (sec t) + (fromIntegral (nsec t) / 1000000000) -- Calculates the sqrt of 2^1000. The parameter n is to ensure -- that GHC does not optimize it away. -- (In fact, I'm not sure this is needed...) nSqrt n = let sqs = map (\_ -> iterate sqrt (2^1000) !! 50) [1..n] in foldr seq 1 sqs