On Sunday 15 August 2010 09:58:49, Travis Erdman wrote:
in the code below, finalmap seems fast enough ... but it has a space leak. otoh, finalmap'rnf runs in constant space, but its performance is terrible, at least 4x slower than finalmap.
The performance isn't so bad, actually. Consider that in each step it has to rnf the entire map. Even for small maps like this one, that's quite a bit of work (and mostly unnecessary, because almost everything is already in normal form).
this is a common problem i'm having ... foldl' isn't strict enough, but foldl'rnf kills performance.
Firstly, foldl'rnf is a good idea *only* for small structures or if in each step large parts of the structure are changed. If you change only small parts of a large structure, you're wasting a lot of work. Secondly, in this case your problem is a) the choice of a suboptimal data structure b) a bad choice of functions to manipulate the structure c) perhaps the lacking strictness of Data.IntMap. a) STUArray would be better. But that makes itself really felt only for larger n. b) that's the biggo c) Data.IntMap doesn't offer any strict versions of insertWith, insertWithKey, adjust et al, which would often make it far easier to avoid space leaks. Data.Map at least offers strict(er) versions of insertWith[Key].
And not only with IntMap as the cumulating data structure, but others as well.
any ideas on this one?
Sure. See below.
how can i get a fast fold in constant space?
thanks again,
travis
{-# LANGUAGE BangPatterns #-}
import System.Environment import Foreign (unsafePerformIO) import System.Random.Mersenne import Data.List import Control.DeepSeq import Control.Parallel.Strategies import qualified Data.IntMap as IntMap
mersennegen = unsafePerformIO $ newMTGen Nothing infrandoms = unfoldr ( Just . splitAt 3) $ map (\x -> abs (x `mod` n)) (unsafePerformIO $ (randoms mersennegen)::[Int])
Arrrgh! Use of unsafePerformIO in that way makes my head hurt. Pass things as arguments, please.
n = 200
foldl'rnf :: NFData a => (a -> b -> a) -> a -> [b] -> a foldl'rnf f z xs = lgo z xs where lgo z [] = z lgo !z (x:xs) = lgo (runEval (rdeepseq (f z x))) xs
startmap = IntMap.fromDistinctAscList $ zip [0..] [1..n]
That is almost certainly a bad idea. If the map contains a contiguous range of keys, an array is practically guaranteed to be more appropriate (uses less memory and is faster to boot).
finalmap x = foldl' g startmap (take x infrandoms)
Okay, that looks reasonable.
finalmap'rnf x = foldl'rnf g startmap (take x infrandoms)
As stated above, rnf'ing the entire map at each step is a baad idea. You can get reasonable performance while squashing the leak by splitting your list in chunks of size k and rnf'ing the map only after each chunk has been processed. finalmap'rnf x = foldl'rnf h startmap (takeWhile (not . null) (unfoldr (Just . splitAt 200) (take x infrandoms))) where h mp lst = foldl' g mp lst The chunk size should be approximately the size of the map, so that after each chunk - a large part of the map has been changed - no value has been changed too often. Then rnf'ing the entire map doesn't waste too much work (since only a small part of the map is already in NF) and no key maps to a too large thunk (which can cause a space leak with vanilla foldl', though here the problem is something else). But, here at least, changing the folded function does better.
g:: IntMap.IntMap Int -> [Int] -> IntMap.IntMap Int g !a [x,y,z] = IntMap.adjust (const $ y + (a IntMap.! z) `mod` n) x a
Oy gevalt! The bang on the map parameter is superfluous since we foldl' anyway, but that's no big deal. IntMap.adjust (const val) is not the best if the key at which the map is to be adjusted is guaranteed to be in the map. Then IntMap.insert is better. But the difference is small, and if the key is not algorithmically guaranteed to be present, adjust is cleaner. So the use of adjust is no big deal either, even if insert is faster here. What is bad is that you IntMap.adjust (const thunk). Well, that alone isn't so bad. What kills you is that the thunk refers to the map. Thus each modified map contains a reference to the previous version, the old contents can't be garbage collected, hello space leak (and finally getting the values involves hopping through old cells). If you modify a data structure, don't let the modified version contain any thunks referencing the old. That spells space leak. With g a [x,y,z] = let !w = y + (a IntMap.! z) `mod` n in IntMap.adjust (const w) x a you're fine with foldl'. Question by the way, did you really want y + ((a ! z) `mod` n) or rather (y + (a ! z)) `mod` n ?
main = do args <- getArgs print $ finalmap (read $ head args)