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POWrite a program to get CPU cache sizes and levels
 

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    <p>I want to write a program to get my cache size(L1, L2, L3). I know the general idea of it.</p> <ol> <li>Allocate a big array</li> <li>Access part of it of different size each time.</li> </ol> <p>So I wrote a little program. Here's my code:</p> <pre><code>#include &lt;cstdio&gt; #include &lt;time.h&gt; #include &lt;sys/mman.h&gt; const int KB = 1024; const int MB = 1024 * KB; const int data_size = 32 * MB; const int repeats = 64 * MB; const int steps = 8 * MB; const int times = 8; long long clock_time() { struct timespec tp; clock_gettime(CLOCK_REALTIME, &amp;tp); return (long long)(tp.tv_nsec + (long long)tp.tv_sec * 1000000000ll); } int main() { // allocate memory and lock void* map = mmap(NULL, (size_t)data_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, 0, 0); if (map == MAP_FAILED) { return 0; } int* data = (int*)map; // write all to avoid paging on demand for (int i = 0;i&lt; data_size / sizeof(int);i++) { data[i]++; } int steps[] = { 1*KB, 4*KB, 8*KB, 16*KB, 24 * KB, 32*KB, 64*KB, 128*KB, 128*KB*2, 128*KB*3, 512*KB, 1 * MB, 2 * MB, 3 * MB, 4 * MB, 5 * MB, 6 * MB, 7 * MB, 8 * MB, 9 * MB}; for (int i = 0; i &lt;= sizeof(steps) / sizeof(int) - 1; i++) { double totalTime = 0; for (int k = 0; k &lt; times; k++) { int size_mask = steps[i] / sizeof(int) - 1; long long start = clock_time(); for (int j = 0; j &lt; repeats; j++) { ++data[ (j * 16) &amp; size_mask ]; } long long end = clock_time(); totalTime += (end - start) / 1000000000.0; } printf("%d time: %lf\n", steps[i] / KB, totalTime); } munmap(map, (size_t)data_size); return 0; } </code></pre> <p>However, the result is so weird:</p> <pre><code>1 time: 1.989998 4 time: 1.992945 8 time: 1.997071 16 time: 1.993442 24 time: 1.994212 32 time: 2.002103 64 time: 1.959601 128 time: 1.957994 256 time: 1.975517 384 time: 1.975143 512 time: 2.209696 1024 time: 2.437783 2048 time: 7.006168 3072 time: 5.306975 4096 time: 5.943510 5120 time: 2.396078 6144 time: 4.404022 7168 time: 4.900366 8192 time: 8.998624 9216 time: 6.574195 </code></pre> <p>My CPU is Intel(R) Core(TM) i3-2350M. L1 Cache: 32K (for data), L2 Cache 256K, L3 Cache 3072K. Seems like it doesn't follow any rule. I can't get information of cache size or cache level from that. Could anybody give some help? Thanks in advance.</p> <p><strong>Update:</strong> Follow @Leeor advice, I use <code>j*64</code> instead of <code>j*16</code>. New results:</p> <pre><code>1 time: 1.996282 4 time: 2.002579 8 time: 2.002240 16 time: 1.993198 24 time: 1.995733 32 time: 2.000463 64 time: 1.968637 128 time: 1.956138 256 time: 1.978266 384 time: 1.991912 512 time: 2.192371 1024 time: 2.262387 2048 time: 3.019435 3072 time: 2.359423 4096 time: 5.874426 5120 time: 2.324901 6144 time: 4.135550 7168 time: 3.851972 8192 time: 7.417762 9216 time: 2.272929 10240 time: 3.441985 11264 time: 3.094753 </code></pre> <p>Two peaks, 4096K and 8192K. Still weird.</p>
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