Re: [Haskell-cafe] Generate random UArray in constant memory space.

Am Dienstag 09 Februar 2010 14:51:31 schrieb Vasyl Pasternak:

Sorry, maybe I should ask more clearer.

I've looked at dons article "Haskell as fast as C"[1], and tried to implement similar algorithm but for list of random numbers.

Please look at code:

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import Text.Printf import Control.Applicative import System.Environment import Data.Array.Vector

main = do [size] <- map read <$> getArgs let ints = enumFromToU 0 size :: UArr Int printf "%d\n" (sumU ints)

This code runs in constant space (on my pc ~25kb allocates on the heap) regardless of array size.

The random number thing runs in constant space, too. The difference is that the enumFromToU produces a tight loop with variables which never leave the registers, while in the random number thing at least the StdGens are allocated in the heap (the produced Ints may stay in the registers, too, I don't know). But enumFromToU and friends were written for that to happen, there are a ton of rewrite rules to help the compiler create tight loops. StdGen has no such thing, randomR has no {-# INLINE #-} pragma, so you have a call to randomR (well, to randomIValInteger, actually) for each element. Maybe if you had the source for the PRNG in the same file, it could be inlined to give an allocation-free loop.

So I tried to achieve similar with random list, just to replace `enumFromToU` with my own list generator.

So the question - is it possible to implement random list similary to enumFromToU?

[1]http://donsbot.wordpress.com/2008/06/04/haskell-as-fast-as-c-working- at-a-high-altitude-for-low-level-performance/

Thank you, Vasyl Pasternak

2010/2/9 Daniel Fischer :

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Am Dienstag 09 Februar 2010 13:18:23 schrieb Vasyl Pasternak:

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Hello Cafe,

I tried to generate memory-efficient list of random numbers, so I've used uvector library for this task. But it doesn't work, it still allocates more than 6Gb of memory for the random list of 10

million elements. Here is the code:

Hmm,

$ ghc -O2 --make ranVec [1 of 1] Compiling Main             ( ranVec.hs, ranVec.o ) Linking ranVec ... $ ./ranVec 10000000 +RTS -sstderr 5130   4,919,912,080 bytes allocated in the heap         883,256 bytes copied during GC          26,896 bytes maximum residency (1 sample(s))          25,620 bytes maximum slop               1 MB total memory in use (0 MB lost due to fragmentation)

maximum residency is just eight bytes more than for 100,000 or 1,000,000 numbers. I think that is constant space.

The ~5 GB total allocation is sequential, ten million new StdGens are produced and allocated, then immediately garbage collected. I see no problem (except that StdGen is slow, e.g. the Mersenne twister is much faster [and allocates less, but still linear in size]).

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import Text.Printf import System.Random import Control.Applicative import System.Environment import Data.Array.Vector

randomListU :: (Int, Int) -> StdGen -> Int -> (UArr Int) randomListU b g size = unfoldU size gen g   where gen g = let (x, g') = randomR b g                 in JustS (x :*: g')

main = do   [size] <- map read <$> getArgs   let ints = randomListU (-10, 10) (mkStdGen 1) size   printf "%d\n"  (sumU ints)

Could someone give a hint, how to implement this function in constant memory space?

Thank you in advance.

Best regards, Vasyl Pasternak