Hi,
I tried a get into concurrent Haskell using multiple cores. The program below
creates 2 task in different threads, executes them, synchronizes the threads
using MVar () and calculates the time needed.
import System.CPUTime
import Control.Concurrent
import Control.Concurrent.MVar
myTask1 = do
return $! fac 60000
print "Task1 done!"
where fac 0 = 1
fac n = n * fac (n-1)
myTask2 = do
return $! fac' 60000 1 1
print "Task2 done!"
where fac' n m p = if m>n then p else fac' n (m+1) (m*p)
main = do
mvar <- newEmptyMVar
pico1 <- getCPUTime
forkIO (myTask1 >> putMVar mvar ())
myTask2
takeMVar mvar
pico2 <- getCPUTime
print (pico2 - pico1)
I compiled the code using
$ ghc FirstFork.hs -threaded
and executed it by
$ main +RTS -N1 resp. $ main +RTS -N2
I use GHC 6.8.3 on Vista with an Intel Dual Core processor. Instead of getting
a speed up when using 2 cores I get as significant slow down, even though there
is no sharing in my code above (at least none I am aware of. BTW, that was
reason
that I use 2 different local factorial functions). On my computer the 1-core
version
takes about 8.3sec and the 2-core version 12.8sec. When I increase the numbers
from 60000 to 100000 the time difference gets even worse (30sec vs 51 sec). Can
anybody give me an idea what I am doing wrong?
Thanks,
Michael
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