<div dir="ltr"><div>Greetings to all!<br><br></div><div>Currently I'm trying to do physical simulations using finite difference method. To accomplish this I need to repeat some manipulations on 2-dimensional grid several thousands or millions times, you know. Is it possible to do using repa?<br>
<br>I wrote this rough sketch that shows what I am into. Apparently, program is severely slow. I think reason is:<br>"Every time an element is requested from a delayed array it is calculated
anew, which means that delayed arrays are inefficient when the data is
needed multiple times"<br>(<a href="http://www.haskell.org/haskellwiki/Numeric_Haskell:_A_Repa_Tutorial">http://www.haskell.org/haskellwiki/Numeric_Haskell:_A_Repa_Tutorial</a>).<br><br>I started diving into <a href="http://www.cse.unsw.edu.au/%7Ebenl/papers/guiding/guiding-Haskell2012-sub.pdf" class="" title="http://www.cse.unsw.edu.au/~benl/papers/guiding/guiding-Haskell2012-sub.pdf" rel="nofollow">Guiding Parallel Array Fusion with Indexed Types</a> mentioned there. Is it a right place to find any answer to my question?<br>
<br></div><div>To summarize, I want to apply my function to Array several thousand times as fast as possible (using multi-threading and GPGPU). It seems I need a lot of memory and throughput. Do I have to call <span style="font-family:courier new,monospace">iterate</span> only <span id="result_box" class="" lang="en"><span class="">once at a time or in some kind of batches.</span></span> What are my possibilities?<br>
</div><div><br></div><div>Thanks in advance!<br></div><div><br>Example program:<br><br><span style="font-family:courier new,monospace">import System.Environment<br>import Data.Array.Repa as Repa<br><br>h = 50<br>w = 50<br>
<br>grid = fromListUnboxed (Z :. h :. w) $ take (h * w) [1, 1..]<br><br>main = do<br> args <- getArgs<br> let n = read $ head args<br> g <- computeP $ accumulate n grid :: IO (Array U DIM2 Int)<br> putStrLn "Ok"<br>
<br>accumulate :: Int -> Array U DIM2 Int -> Array D DIM2 Int<br>accumulate n g = repaIterate step n $ delay g<br><br></span><pre class=""><span style="font-family:courier new,monospace"><font><span class=""><span style="font-family:courier new,monospace"><font>step</font></span> :: (</span>DIM2 <span class="">-></span> Int<font><span class="">)</span> <span class="">-></span> DIM<font>2</font> <span class="">-></span> <font>Int</font></font></font><font><br>
step f (Z :. i :. j) = center + right + left + top + bottom<br></font></span></pre><span style="font-family:courier new,monospace"><font> where<br> left = f</font>' j (i - 1)<br> center = f' j i<br>
right = f' j (i + 1)<br> top = f' (j + 1) i<br> bottom = f' (j - 1) i<br> f' j i =<br> if j<0 || i<0 || i>=h || j>=w<br> then 0<br> else f (Z :. j :. i)<br><br>repaIterate :: ((DIM2 -> Int) -> DIM2 -> Int) -> Int -> Array D DIM2 Int -> Array D DIM2 Int<br>
repaIterate f n = (!! n) . iterate (\array -> traverse array id f)</span><br><br>Compilation:<br><br><span style="font-family:courier new,monospace">ghc -O2 -threaded -rtsopts --make repatest.hs</span><br><br></div><span style="font-family:courier new,monospace">time</span> output:<br>
<br><div><span style="font-family:courier new,monospace">./repatest 3 +RTS -N2 -H 0,02s user 0,02s system 109% cpu 0,033 total<br>./repatest 4 +RTS -N2 -H 0,09s user 0,02s system 126% cpu 0,089 total<br>./repatest 5 +RTS -N2 -H 0,30s user 0,02s system 155% cpu 0,200 total<br>
./repatest 6 +RTS -N2 -H 1,16s user 0,06s system 185% cpu 0,663 total<br>./repatest 7 +RTS -N2 -H 5,63s user 0,24s system 191% cpu 3,071 total<br>./repatest 8 +RTS -N2 -H 27,72s user 0,89s system 191% cpu 14,960 total<br>
./repatest 8 +RTS -N1 -H 26,23s user 0,19s system 100% cpu 26,388 total</span><br></div></div>