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POSpecific thread order in C using GCC and OMP
 

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  1. POSpecific thread order in C using GCC and OMP
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    <p>I need to make 4 teams with 4 threads each one with contiguous processors. </p> <p>The result I'm expecting is, for example:</p> <pre><code>Team 0 Thread 0 Processor: 0 Team 0 Thread 1 Processor: 1 Team 0 Thread 2 Processor: 2 Team 0 Thread 3 Processor: 3 Team 1 Thread 0 Processor: 4 Team 1 Thread 1 Processor: 5 Team 1 Thread 2 Processor: 6 Team 1 Thread 3 Processor: 7 Team 2 Thread 0 Processor: 8 Team 2 Thread 1 Processor: 9 Team 2 Thread 2 Processor: 10 Team 2 Thread 3 Processor: 11 Team 3 Thread 0 Processor: 12 Team 3 Thread 1 Processor: 13 Team 3 Thread 2 Processor: 14 Team 3 Thread 3 Processor: 15 </code></pre> <p>I can handle Processor Affinity in GCC using the GOMP_CPU_AFFINITY variable.</p> <p>I'm using:</p> <pre><code>#pragma omp parallel num_threads(4) </code></pre> <p>twice in order to get 2 fork levels.</p> <p>At the moment I'm having this order in GOMP_CPU_AFFINITY:</p> <pre><code>0 4 8 12 1 2 3 5 6 7 9 10 11 13 14 15 </code></pre> <p>So the first fork, the "fathers fork", gets:</p> <pre><code>Team 0 Thread 0 Processor: 0 Team 1 Thread 0 Processor: 4 Team 2 Thread 0 Processor: 8 Team 3 Thread 0 Processor: 12 </code></pre> <p>The problem I'm having is that the second group of forks make without any order so, for example I could have this situation (I'm using #pragma omp atomic so only one 'father' can ask for more processors at any time):</p> <pre><code>Team 0 Thread 0 Processor: 0 Team 0 Thread 1 Processor: 5 Team 0 Thread 2 Processor: 6 Team 0 Thread 3 Processor: 7 Team 1 Thread 0 Processor: 4 Team 1 Thread 1 Processor: 13 Team 1 Thread 2 Processor: 14 Team 1 Thread 3 Processor: 15 Team 2 Thread 0 Processor: 8 Team 2 Thread 1 Processor: 1 Team 2 Thread 2 Processor: 2 Team 2 Thread 3 Processor: 3 Team 3 Thread 0 Processor: 12 Team 3 Thread 1 Processor: 9 Team 3 Thread 2 Processor: 10 Team 3 Thread 3 Processor: 11 </code></pre> <p>The question is: Is there any way to make this second petition in order? </p> <p>I think I would have to make some sinchronization method with locks or something... </p> <p>Thanks in advance!</p> <ul> <li>Javier</li> </ul>
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    1. USJavi Ortiz
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    1. COWhy, why, why, why? Threads are supposed to work asynchronously. This question comes up more often than 'floating point compares do not work'.
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      1. USMartin James
    2. CO@Martin: He's not talking about synchronizing anything, he's talking about assignment of thread IDs to processor IDs (which might be important on a NUMA system)
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      1. USBen Voigt
    3. CO@MartinJames: What is wrong with assigning threads to specific processor cores? Ensuring that threads which share data operate on the same processor to minimize false sharing impacts can be vitally important for performance on ccNUMA systems afterall.
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      1. USGrizzly
 

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