How are you running the program? You have to explicitly tell the compiler/interpreter to use multiple system threads. Michael On Tue, May 5, 2009 at 10:19 PM, Jack Kennedy wrote:

In step 4 of Haskell in 5 Steps [ http://haskell.org/haskellwiki/Haskell_in_5_steps], a parallel program is given. I changed it very slightly so it would run a long time (see below).

It compiles and runs but my CPU meter is barely above 50%. I have a dual core processor. What in the world would keep this program from completely saturating the CPU?

import Control.Parallel

main = a `par` b `pseq` print (a + b) where a = ack 4 10 b = ack 4 10

fac 0 = 1 fac n = n * fac (n-1)

ack 0 n = n+1 ack m 0 = ack (m-1) 1 ack m n = ack (m-1) (ack m (n-1))

fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2)

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Hello Jack, according to the source code your program should not run for a long time. If you're using GHC, use the '-threaded' flag when compiling and also pass the parameters '+RTS -N2' to your program to let it run with two parallel threads. However, my experience is that the pure parallel constructs are currently not really suitable for real world programs. With my dual core processor I get the expected ~190% CPU time using explicit parallelism using concurrency (i.e. forkIO and MVars), whereas with pure constructs like the ones from Control.Parallel and Control.Parallel.Strategies I get only about 140%. Greets, Ertugrul. Jack Kennedy wrote:

In step 4 of Haskell in 5 Steps [ http://haskell.org/haskellwiki/Haskell_in_5_steps], a parallel program is given. I changed it very slightly so it would run a long time (see below).

It compiles and runs but my CPU meter is barely above 50%. I have a dual core processor. What in the world would keep this program from completely saturating the CPU?

import Control.Parallel

main = a `par` b `pseq` print (a + b) where a = ack 4 10 b = ack 4 10

fac 0 = 1 fac n = n * fac (n-1)

ack 0 n = n+1 ack m 0 = ack (m-1) 1 ack m n = ack (m-1) (ack m (n-1))

fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2)

-- nightmare = unsafePerformIO (getWrongWife >>= sex) http://blog.ertes.de/