Greetings,
when using parMap (or parList and demanding) I see a curious pattern in CPU
usage.
Running "parMap rnf fib [1..100]" gives the following pattern of used CPUs:
4,3,2,1,4,3,2,1,...
The fib function requires roughly two times the time if we go from fib(n) to fib(n+1), meaning that
calculating the next element in the list always takes longer than the current. What I would like is
a version of parMap that directly takes a free CPU and lets it calculate the next result, giving the
usage pattern 4,4,4,4,...
Below you find the simple Haskell program, which gives these results, please compile with "ghc
--make -threaded -O2 Para.hs" and run on a machine with at least two cores and "./Para +RTS -N2" or
better.
I am not filing a bug yet as I would prefer to be told that I did it wrong and
here is a better way: ...
Thanks,
Christian
(Please assume that later on, "fib" will be replaced by something meaningful ;)
# ghc --version
# The Glorious Glasgow Haskell Compilation System, version 6.10.1
module Main where
import Control.Parallel.Strategies
-- parallel computation of fibonacci numbers in slow
fib :: Int -> Int
fib n
| n < 1 = error "n < 1"
| n == 1 = 1
| n == 2 = 1
| otherwise = fib (n-1) + fib(n-2)
fibs = parMap rnf fib $ [1..100]
-- fibs = let fs = map fib $ [1..100] in fs `demanding` (parList rnf fs)
main = do
mapM_ (putStrLn . show) $ zip [1..] fibs
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