How do I write a parallel reduction using strategies in Haskell?

Question

In high-performance computing, sums, products, etc are often calculated using a \"parallel reduction\" that takes n elements and completes in O(log n) time

Answer 1

This seems like a good start:

parFold :: (a -> a -> a) -> [a] -> a
parFold f = go
  where
  strategy = parList rseq

  go [x] = x
  go xs = go (reduce xs `using` strategy)

  reduce (x:y:xs) = f x y : reduce xs
  reduce list     = list   -- empty or singleton list

It works, but parallelism is not so great. Replacing parList with something like parListChunks 1000 helps a bit, but speedup is still limited to under 1.5x on an 8-core machine.