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POWhy does GCD increase execution time?
 

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  1. POWhy does GCD increase execution time?
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    <p>I try to learn Grand Central Dispatch (GCD) and use the following code to test:</p> <p>With GCD:</p> <pre><code>#include &lt;dispatch/dispatch.h&gt; #include &lt;vector&gt; #include &lt;cstdlib&gt; #include &lt;iostream&gt; int main(int argc, char *argv[]) { const int N = atoi(argv[1]); __block std::vector&lt;int&gt; a(N, 0); dispatch_apply(N, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(size_t i) { a[i] = i; #ifdef DEBUG if ( i % atoi(argv[2]) == 0) std::cout &lt;&lt; a[i] &lt;&lt; std::endl; #endif }); return 0; } </code></pre> <p>Without GCD:</p> <pre><code>#include &lt;vector&gt; #include &lt;cstdlib&gt; #include &lt;iostream&gt; int main(int argc, char *argv[]) { const int N = atoi(argv[1]); std::vector&lt;int&gt; a(N, 0); for (int i = 0; i &lt; N; i++) { a[i] = i; #ifdef DEBUG if ( i % atoi(argv[2]) == 0) std::cout &lt;&lt; a[i] &lt;&lt; std::endl; #endif } return 0; } </code></pre> <p>The test result with GCD:</p> <pre><code>$ time ./testgcd 100000000 10000000 4.254 secs </code></pre> <p>The test without GCD:</p> <pre><code>$ time ./nogcd 100000000 10000000 1.462 secs </code></pre> <p>I thought that GCD should reduce execution time, but the results show the opposite. I am not sure if I misuse GCD. The OS environment is Mac OS X 10.8 with Xcode 4.5. The compiler is Clang++ 3.1. The hardware is Macbook Pro with i5 CPU, which has two cores. </p> <p>For comparison, I use OpenMP (also using GCC shipped with Xcode 4.5 on the same laptop):</p> <pre><code>#include &lt;vector&gt; #include &lt;cstdlib&gt; int main(int argc, char *argv[]) { const int N = atoi(argv[1]); std::vector &lt;int&gt; a(N, 0); #pragma omp parallel for for (int i = 0; i &lt; N; i++) a[i] = i; return 0; } </code></pre> <p>and w/wo (-fopenmp), I have two executables to test, </p> <p>with <code>-fopenmp</code> flag while compiling:</p> <pre><code>$ time ./testopenmp 100000000 1.280 secs </code></pre> <p>without <code>-fopenmp</code> flag while compiling: </p> <pre><code>$ time ./testnoopenmp 100000000 1.626 secs </code></pre> <p>With OpenMP, the executing time are reduced. </p>
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    1. USGong-Yi Liao
    1. USGong-Yi Liao
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      1. POWhy does GCD increase execution time?
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    1. COPresumably you've not defined `#DEBUG` for these benchmarks?
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      1. POWhy does GCD increase execution time?
      1. USOliver Charlesworth
    2. COThe DEBUG block is just for assuring the work flow is correct, I believe it has nothing to do with performance.
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      1. POWhy does GCD increase execution time?
      1. USGong-Yi Liao
 

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