Re: [Haskell-cafe] Memory usage when passing arrays in state

Thank you Daniel. As I understood it DiffArrays are supposed to be faster than the regular Array due to the fact that it doesnt copy the entire Array, but just updates the position that changes, and keeps some kind of "changelog" on the array. But when looking at the statistics for my sample program, it seems that it allocates a lot more than what should be needed, which would indicate that maybe the array is copied anyway. At this point, the DiffArray/DiffUArray are the only functional arrays, right? I mean, I can add two and two together and see that it equals...four, and if the only functional array is sort of broken, that means so is my program. Are there any alternatives that are fast aswell? //Tobias 2009/3/4 Daniel Peebles :

This may be completely unrelated to your problem, but there's a ticket in the GHC trac saying that DiffArray is unusably slow: http://hackage.haskell.org/trac/ghc/ticket/2727 . It doesn't analyze the cause of the slowness, so it's quite possible that it may be related to GC as in your case.

Cheers, Dan

On Tue, Mar 3, 2009 at 7:44 PM, Tobias Olausson wrote:

...

Hello all. I am currently implementing an emulation of a CPU, in which the CPU's RAM is part of the internal state that is passed around in the program using a state monad. However, the program performs unexpectingly bad, and some profiling information makes us believe that the problem is the high memory usage of the program.

The program below is similar to our main program used when testing a sorting algorithm in this CPU:

module Main where

import Control.Monad.State.Lazy import Data.Word import Data.Array.Diff import Control.Concurrent (threadDelay)

data LoopState = LoopState    { intVal :: Integer    , diff   :: DiffUArray Word8 Word8    }

initState :: LoopState initState = LoopState 0 (array (0x00,0xFF) [(idx,0)|idx<-[0x00..0xFF]])

main :: IO () main = do    execStateT looper initState >>= putStrLn . show . intVal

looper :: StateT LoopState IO () looper = do    st <- get    let res = intVal st + 1        idx = fromIntegral res    put $ st { intVal = res, diff = (diff st) // [(idx,idx)] }    if res == 13000000        then return ()        else looper

Of course our program does more than updating a counter ;-) Compiling and running this program yields the following result:

[~]:[olaussot] >> ghc --make -O2 -o array ArrayPlay.hs [~]:[olaussot] >> ./array +RTS -sstderr ./array +RTS -sstderr 13000000     313,219,740 bytes allocated in the heap   1,009,986,984 bytes copied during GC     200,014,828 bytes maximum residency (8 sample(s))       4,946,648 bytes maximum slop             393 MB total memory in use (3 MB lost due to fragmentation)

 Generation 0:   590 collections,     0 parallel,  3.06s,  3.09s elapsed  Generation 1:     8 collections,     0 parallel,  3.56s,  4.21s elapsed

 INIT  time    0.00s  (  0.00s elapsed)  MUT   time    0.27s  (  0.27s elapsed)  GC    time    6.62s  (  7.30s elapsed)  EXIT  time    0.00s  (  0.00s elapsed)  Total time    6.89s  (  7.57s elapsed)

 %GC time      96.1%  (96.4% elapsed)

 Alloc rate    1,155,958,754 bytes per MUT second

 Productivity   3.9% of total user, 3.6% of total elapsed

Why does the program spend 96.1% of its total running time collecting garbage? Any tips to make this program perform better are appreciated. Please do tell if anything is unclear.

-- Tobias Olausson tobsan@gmail.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Tobias Olausson tobsan@gmail.com