[Haskell-cafe] I killed performance of my code with Eval and Strategies

[Janek S.]

Dear Haskellers,

I am reading Simon Marlow's tutorial on parallelism and I have problems with correctly using Eval 
monad and Strategies. I *thought* I understand them but after writing some code it turns out that 
obviously I don't because parallelized code is about 20 times slower. Here's a short example 
(code + criterion benchmarks):

{-# LANGUAGE BangPatterns #-}
module Main where

import Control.Parallel.Strategies
import Criterion.Main

main :: IO ()
main = defaultMain [
        bench "Seq" $ nf calculateSeq xs
      , bench "Par" $ nf calculatePar xs ]
    where xs = [1..16384]

calculateSeq :: [Double] -> [Double]
calculateSeq [] = []
calculateSeq (x:xs) = (sin . sqrt $ x) : xs

calculatePar :: [Double] -> [Double]
calculatePar xss = runEval $ go xss
    where
      go :: Strategy [Double]
      go [] = return []
      go xs = do
          lsh <- (rpar `dot` rdeepseq) $ calculateSeq as
          lst <- go bs
          return (lsh ++ lst)
          where
            !(as, bs) = splitAt 8192 xs

Compiling and running with:

ghc -O2 -Wall -threaded -rtsopts -fforce-recomp -eventlog evalleak.hs
./evalleak -oreport.html -g +RTS -N2 -ls -s

I get:

benchmarking Seq
mean: 100.5990 us, lb 100.1937 us, ub 101.1521 us, ci 0.950
std dev: 2.395003 us, lb 1.860923 us, ub 3.169562 us, ci 0.950

benchmarking Par
mean: 2.233127 ms, lb 2.169669 ms, ub 2.296155 ms, ci 0.950
std dev: 323.5201 us, lb 310.2844 us, ub 344.8252 us, ci 0.950

That's a hopeless result. Looking at the spark allocation everything looks fine:

SPARKS: 202 (202 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

But analyzing eventlog with ThreadScope I see that parallel function spends most of the time doing 
garbage collection, which suggests that I have a memory leak somewhere. I suspected that problem 
might be caused by appending two lists together in the parallel implementation, but replacing 
this with difference lists doesn't help. Changing granularity (e.g. splitAt 512) also brings no 
improvement. Can anyone point me to what am I doing wrong?

Janek

PS. This is of course not a real world code - I know that I'd be better of using unboxed data 
structures for doing computations on Doubles.