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    <p>I'm not sure if this is the only problem here, but it's definitely the biggest one - your code would very quickly trigger the HW stream prefetchers, making you almost always hit in L1 or L2 latencies.</p> <p>More details can be found here - <a href="http://software.intel.com/en-us/articles/optimizing-application-performance-on-intel-coret-microarchitecture-using-hardware-implemented-prefetchers" rel="noreferrer">http://software.intel.com/en-us/articles/optimizing-application-performance-on-intel-coret-microarchitecture-using-hardware-implemented-prefetchers</a></p> <p>For your benchmark You should either disable them (through BIOS or any other means), or at least make your steps longer by replacing <code>j*16</code> (* 4 bytes per int = 64B, one cache line - a classic unit stride for the stream detector), with <code>j*64</code> (4 cache lines). The reason being - the prefetcher can issue 2 prefetches per stream request, so it runs ahead of your code when you do unit strides, may still get a bit ahead of you when your code is jumping over 2 lines, but become mostly useless with longer jumps (3 isn't good because of your modulu, you need a divider of step_size)</p> <p>Update the questions with the new results and we can figure out if there's anything else here.</p> <hr> <p><em>EDIT1</em>: Ok, I ran the fixed code and got - </p> <pre><code>1 time: 1.321001 4 time: 1.321998 8 time: 1.336288 16 time: 1.324994 24 time: 1.319742 32 time: 1.330685 64 time: 1.536644 128 time: 1.536933 256 time: 1.669329 384 time: 1.592145 512 time: 2.036315 1024 time: 2.214269 2048 time: 2.407584 3072 time: 2.259108 4096 time: 2.584872 5120 time: 2.203696 6144 time: 2.335194 7168 time: 2.322517 8192 time: 5.554941 9216 time: 2.230817 </code></pre> <p>It makes much more sense if you ignore a few columns - you jump after the 32k (L1 size), but instead of jumping after 256k (L2 size), we get too good of a result for 384, and jump only at 512k. Last jump is at 8M (my LLC size), but 9k is broken again. </p> <p>This allows us to spot the next error - ANDing with size mask only makes sense when it's a power of 2, otherwise you don't wrap around, but instead repeat some of the last addresses again (which ends up in optimistic results since it's fresh in the cache).</p> <p>Try replacing the <code>... &amp; size_mask</code> with <code>% steps[i]/sizeof(int)</code>, the modulu is more expensive but if you want to have these sizes you need it (or alternatively, a running index that gets zeroed whenever it exceeds the current size)</p>
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    1. COThank you!. I use j * 64 instead, but result remains confusing. Please see my update.
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    2. CO@KanLiu - updated my answer
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    3. COgreat, it works! Thanks a lot.
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