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POWhy is my program slow when looping over exactly 8192 elements?
 

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  1. POWhy is my program slow when looping over exactly 8192 elements?
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    <p>Here is the extract from the program in question. The matrix <code>img[][]</code> has the size SIZE×SIZE, and is initialized at:</p> <p><code>img[j][i] = 2 * j + i</code></p> <p>Then, you make a matrix <code>res[][]</code>, and each field in here is made to be the average of the 9 fields around it in the img matrix. The border is left at 0 for simplicity.</p> <pre><code>for(i=1;i&lt;SIZE-1;i++) for(j=1;j&lt;SIZE-1;j++) { res[j][i]=0; for(k=-1;k&lt;2;k++) for(l=-1;l&lt;2;l++) res[j][i] += img[j+l][i+k]; res[j][i] /= 9; } </code></pre> <p>That's all there's to the program. For completeness' sake, here is what comes before. No code comes after. As you can see, it's just initialization.</p> <pre><code>#define SIZE 8192 float img[SIZE][SIZE]; // input image float res[SIZE][SIZE]; //result of mean filter int i,j,k,l; for(i=0;i&lt;SIZE;i++) for(j=0;j&lt;SIZE;j++) img[j][i] = (2*j+i)%8196; </code></pre> <p>Basically, this program is slow when SIZE is a multiple of 2048, e.g. the execution times:</p> <pre><code>SIZE = 8191: 3.44 secs SIZE = 8192: 7.20 secs SIZE = 8193: 3.18 secs </code></pre> <p>The compiler is GCC. From what I know, this is because of memory management, but I don't really know too much about that subject, which is why I'm asking here.</p> <p>Also how to fix this would be nice, but if someone could explain these execution times I'd already be happy enough.</p> <p>I already know of malloc/free, but the problem is not amount of memory used, it's merely execution time, so I don't know how that would help.</p>
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      1. POWhy is my program slow when looping over exactly 8192 elements?
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    1. COAre you sure that the issue is caused by the code you posted? Where else are `i` and `j` used?
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      1. POWhy is my program slow when looping over exactly 8192 elements?
      1. USDan Puzey
    2. COI don't understand how you can get faster results with 8193 instead of 8192. Did you tried that several times ?
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    3. CO@bokan it happens when the size is a multiple of the critical stride of the cache.
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      1. USLuchian Grigore
 

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