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POWhat's the best way to demonstrate the effect of affinity setting?
 

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  1. POWhat's the best way to demonstrate the effect of affinity setting?
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    <p>Once I noticed that Windows doesn't keep computation-intensive threads on a specific core - it keeps switching cores instead. So I speculated that the job would be done faster, if the thread would keep access to the same data caches. And really, I was able to observe a stable ~1% speed improvement after setting the thread's affinity mask to a single core (in a ppmd (de)compression thread). But then I tried to build a simple demo for this effect, and more or less failed - that is, it works as expected on my system (Q9450):</p> <pre> buflog=21 bufsize=2097152 (cache flush) first run = 6.938s time with default affinity = 6.782s time with first core only = 6.578s speed gain is 3.01% </pre> <p>but people I asked weren't exactly able to reproduce the effect. Any suggestions?</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;windows.h&gt; int buflog=21, bufsize, bufmask; char* a; char* b; volatile int r = 0; __declspec(noinline) int benchmark( char* a ) { int t0 = GetTickCount(); int i,h=1,s=0; for( i=0; i&lt;1000000000; i++ ) { h = h*200002979 + 1; s += ((int&amp;)a[h&amp;bufmask]) + ((int&amp;)a[h&amp;(bufmask&gt;&gt;2)]) + ((int&amp;)a[h&amp;(bufmask&gt;&gt;4)]); } r = s; t0 = GetTickCount() - t0; return t0; } DWORD WINAPI loadcore( LPVOID ) { SetThreadAffinityMask( GetCurrentThread(), 2 ); while(1) benchmark(b); } int main( int argc, char** argv ) { if( (argc&gt;1) &amp;&amp; (atoi(argv[1])&gt;16) ) buflog=atoi(argv[1]); bufsize=1&lt;&lt;buflog; bufmask=bufsize-1; a = new char[bufsize+4]; b = new char[bufsize+4]; printf( "buflog=%i bufsize=%i\n", buflog, bufsize ); CreateThread( 0, 0, &amp;loadcore, 0, 0, 0 ); printf( "(cache flush) first run = %.3fs\n", float(benchmark(a))/1000 ); float t1 = benchmark(a); t1/=1000; printf( "time with default affinity = %.3fs\n", t1 ); SetThreadAffinityMask( GetCurrentThread(), 1 ); float t2 = benchmark(a); t2/=1000; printf( "time with first core only = %.3fs\n", t2 ); printf( "speed gain is %4.2f%%\n", (t1-t2)*100/t1 ); return 0; } </code></pre> <p>P.S. I can post a link to compiled version if anybody needs that.</p>
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      1. POWhat's the best way to demonstrate the effect of affinity setting?
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    1. COLooks like the best way to demonstrate that affinity setting is useless :)
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      1. USAlex F
    2. COCommonly its really hard to squeeze more speed after some point, so an extra 1% with a single line of code is still a good thing imho.
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    3. CO@Shelwien: but only if it can be reproduced.
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      1. USjalf
 

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