Thank, Daniel
Multiple threads are in evidence in my system monitor, but I wonder why I'm getting two different answers, one twice the other. The first is the parallel solution and the second is the non.
Michael
===========
{-import Control.Parallel
nfib :: Int -> Intnfib n | n <= 1 = 1 | otherwise = par n1 (pseq n2 (n1 + n2 + 1)) where n1 = nfib (n-1) n2 = nfib (n-2)-}
nfib :: Int -> Intnfib n | n <= 1 = 1 | otherwise = nfib (n-1) + nfib (n-2)
main = do putStrLn $ show $ nfib 39
=============
[michael@hostname ~]$ ghc --make -threaded nfib.hs[1 of 1] Compiling Main ( nfib.hs, nfib.o )Linking nfib ...[michael@hostname ~]$ ./nfib +RTS -N3204668309[michael@hostname ~]$ ghc --make nfib.hs[1 of 1] Compiling Main ( nfib.hs, nfib.o )Linking nfib ...[michael@hostname ~]$ ./nfib102334155[michael@hostname ~]$
--- On Thu, 5/26/11, Daniel Fischer
How do I compile and run this parallel program? Michael =========================== import Control.Parallel nfib :: Int -> Int nfib n | n <= 1 = 1 | otherwise = par n1 (seq n2 (n1 + n2 + 1))
The 'seq' here should be a 'pseq' to ensure that n2 is (started to be) calculated before (n1 + n2 +1) is evaluated (iirc, currently, both work with ghc, but it's not guaranteed for seq).
where n1 = nfib (n-1) n2 = nfib (n-2) {-nfib :: Int -> Intnfib n | n <= 1 = 1 | otherwise = nfib (n-1) + nfib (n-2)-}
main = do putStrLn $ show $ nfib 39 =========================== [michael@hostname ~]$ ghc --make -threaded nfib.hs [michael@hostname ~]$ ./nfib +RTS -N3
In principle, that is the correct way to compile and run it (note that with ghc-7, you don't need the --make).
nfib: the flag -N3 requires the program to be built with -threaded
Have you previously compiled the programme without -threaded? Unless you snipped the compilation messages, their absence strongly indicates so. If that is the case, you fell victim to ghc's recompilation-avoidance which doesn't keep track of the flags a programme/module was compiled with, so it saw that nothing [as far as it knows] changed, hence didn't recompile. Then removing .o and .hi or passing -fforce-recomp will make it recompile and link with the threaded runtime.