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

[Tobias Olausson]

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 at gmail.com