Profiling multithreading performance in a Haskell program — no speedups using parallel strategies

multithreading haskell parallel-processing profiling

Sorry that I couldn't provide code in a timely manner to assist respondents. It took me a while to untangle the exact location of the issue.

The problem was as follows: I was fmapping a function

f :: a -> S b

over the traversable data structure

structure :: T a

where S and T are two traversable functors.

Then, when using parTraversable, I was mistakenly writing

Compose (fmap f structure) `using` parTraversable rdeepseq

instead of

Compose $ fmap f structure `using` parTraversable rdeepseq

so I was wrongly using the Traversable instance for Compose T S to do the multithreading (using Data.Functor.Compose).

(This looks like it should've been easy to catch, but it took me a while to extract the above mistake from the code!)

This now looks much better!

Graph 1

Graph 2

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