Hi, Can anyone put me right here. I am trying to use a setup similar to parMap to spark each valuation in a list in parallel, where the resulting (evaluated) list is folded to produce a final single result. Having done the obligatory google, I modified a few common examples to give: pfoldl f acc xs = foldl' f acc (xs `using` parList rwhnf) This compiles and if I monitor my CPUs it starts to use both cores, but after approx 10 seconds, one core drops to low rate (I'm using a 2 core machine). The end result is that -N2 is actually a bit slower than -N1! I'm guessing I haven't grasped the concept properly - although as map is just 'foldl (+) 0' I'm at a loss to see why this approach wouldn't work given it is pretty similar to parMap - could anyone point out what I'm missing? If it's any use the context of the code is below. Many thanks! Phil. mc :: MonteCarloUserData -> [[Double]] -> Double mc userData rndss = existentialResult (pfoldl f existenAvg rndss) $ numOfSims userData where f = flip $ existentialCombine . payOff' . expiryValue payOff' = existentialPayOff userData expiryValue = foldl' (existentialEvolve userData) (stock userData) existenAvg = averager userData