Control.Parallel.Strategies.parMap CPU usage

[Christian Hoener zu Siederdissen]

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