haskell@kudling.de schrieb:
Hi,
i have compared a C++ implementation with a Haskell implementation of the Monte Carlo Pi approximation:
http://lennart.kudling.de/haskellPi/
The Haskell version is 100 times slower and i wonder whether i do something obvious wrong.
Hi, Nice benchmark, but I think the culprit is the random number generator. For simplicity, here is a 1-1 translation of the C++ code:
main = do args <- getArgs let samples = read $ head args randomNumbers <- genRandoms print (monteCarlo samples randomNumbers) genRandoms :: IO [Double] genRandoms = liftM randoms getStdGen monteCarlo :: Int -> [Double] -> Double monteCarlo samples = go 0 samples where go cnt 0 _ = (4.0 :: Double) * (fromIntegral cnt) / (fromIntegral > samples) go cnt k (x:y:rs) | sqrt (x*x + y*y) <= 1.0 = go (cnt+1) (k-1) rs | otherwise = go cnt (k-1) rs
The profiling shows that most time is spend in genRandoms: genRandoms Main 199 1 96.5 98.3 96.5 98.3 When replacing genRandoms by (repeat 0.5), the haskell code is only a little bit slower than the C++ code *with* random number generation. Of course, the random number generation in C++ takes time too:
montecarlo(0) $ pprof --text ./monte_carlo /tmp/profile Total: 6691 samples 3928 58.7% 58.7% 5965 89.1% _main 2134 31.9% 90.6% 2134 31.9% _do_rand 435 6.5% 97.1% 435 6.5% _rand 194 2.9% 100.0% 194 2.9% 0x00003072 0 0.0% 100.0% 6691 100.0% __mh_execute_header 0 0.0% 100.0% 6691 100.0% start
But the factor 100 is clearly due to genRandoms. cheers, benedikt
Profiling says that the majority of the time is spend in "main". But i have no idea where.
Can someone give me a hint?
Thanks, Lenny
individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc
MAIN MAIN 1 0 0.0 0.0 100.0 100.0 main Main 254 1 88.1 90.8 100.0 100.0 monteCarloPi Main 255 1 0.6 1.1 11.9 9.2 pairs Main 257 10000000 0.7 1.4 0.7 1.4 countHits Main 256 10000001 4.2 2.9 10.6 6.7 accumulateHit Main 258 27852236 3.0 2.3 6.4 3.8 isInCircle Main 259 30000000 3.3 1.5 3.3 1.5 CAF:lit_r1A7 Main 248 1 0.0 0.0 0.0 0.0 isInCircle Main 260 0 0.0 0.0 0.0 0.0