Hoang Truong wrote:
Hello everybody,
I am following "A Tutorial on Parallel and Concurrent Programming in
Haskell" and I have a problem with making Haskell to use my multi-cores
(Core 2 Quad CPU).
The Haskel version I used is GHC 6.10.1, for Haskell 98. I compile my
below program with command: ghc --make -threaded -debug thread0.hs, and
run with: thread0 +RTS -N4 while watching the cpu usage on another
terminal (by: mpstat -P ALL 1 100), but the program uses only one core
of my Ubuntu Linux.
Do any of you know why or has any suggestions? Below is my program:
Why do people still insist on using forkOS? You don't need forkOS unless
you need to call C libraries that use thread-local state. Otherwise, it
will just reduce your performance compared to forkIO. Admittedly the
documentation for forkOS has been misleading in the past, but I think the
current version is pretty clear:
http://www.haskell.org/ghc/docs/latest/html/libraries/base/Control-Concurrent.html#v%3AforkOS
import Control.Concurrent
import Control.Concurrent.MVar
fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)
dowork =
putStrLn ("fib 35 = " ++ (show (fib 35)))
Perhaps you were expecting "fib 35" to be repeatedly executed each time you
call dowork? Laziness means it only gets evaluated once.
Cheers,
Simon
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