Haskell parMap performance?

Question

I tried to scan parMap vs map using a very simple example:

import Control.Parallel.Strategies import Criterion.Main sq x = x^2 a = whnf sum $ map sq [1..1000000] b = whnf sum $ parMap rseq sq [1..1000000] main = defaultMain [ bench "1" a, bench "2" b ] 

My results seem to indicate zero acceleration from parMap, and I was wondering why this could be?

 benchmarking 1 Warning: Couldn't open /dev/urandom Warning: using system clock for seed instead (quality will be lower) time 177.7 ms (165.5 ms .. 186.1 ms) 0.997 R² (0.992 R² .. 1.000 R²) mean 185.1 ms (179.9 ms .. 194.1 ms) std dev 8.265 ms (602.3 us .. 10.57 ms) variance introduced by outliers: 14% (moderately inflated) benchmarking 2 time 182.7 ms (165.4 ms .. 199.5 ms) 0.993 R² (0.976 R² .. 1.000 R²) mean 189.4 ms (181.1 ms .. 195.3 ms) std dev 8.242 ms (5.896 ms .. 10.16 ms) variance introduced by outliers: 14% (moderately inflated) 

Answer 1

The problem is that parMap cripples parallel computation for each individual list item. It does not group the list at all, as you seem to think from your comments, which will require the use of the parListChunk strategy.

So, parMap has a lot of overhead, so the fact that each spark simply squares a single number means its cost is overloaded due to this overhead.