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POParallelize recursive function with OpenMP v.2.0
 

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  1. POParallelize recursive function with OpenMP v.2.0
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    <p>I'm trying to parallelize parts of a project that relies on a lot of recursive algorithms.</p> <p>Most of them are some form of binary tree creation or traversal and processing.</p> <p>I'm stuck using GCC v. 4.1.2 on RedHat and the VC++ compiler on Windows (both don't support OpenMP 3.0 with its convenient <code>task</code> construct). I found <a href="https://stackoverflow.com/questions/8023135/c-openmp-parallel-quicksort">this question</a> which seems to get the job done with nested parallel sections and some throttling to prevent an exorbitant number of threads.</p> <p>My question: any way to avoid this approach? Some of the functions are called at each timestep, the overhead of repeatedly creating and destroying a thread team is unacceptable.</p> <p>Here's the basic structure of a recursive function I've been using, in line with the linked question:</p> <pre class="lang-cpp prettyprint-override"><code>extern int threads; omp_set_nested(1); omp_set_num_threads(2); void cell::updateRecursive() { // do stuff for cell for this timestep #pragma omp flush(threads) if (threads&gt;=omp_get_num_procs()) { child0-&gt;updateRecursive(); child1-&gt;updateRecursive(); // no new threads } else { #pragma omp atomic threads++; #pragma omp flush(threads) #pragma omp parallel sections nowait { #pragma omp seciton child0-&gt;updateRecursive(); #pragma omp section child1-&gt;updateRecursive(); } #pragma omp atomic threads--; } } </code></pre> <p>This would be sufficient if this function weren't called so often. I'd like a way to recurse that can use an existing team of threads to perform work, rather than creating one while traversing the tree.</p> <p>Would this be possible at all without <code>task</code>? I've experimented with simply using <code>sections</code>, but apparently they can't be nested to use an existing team of threads.</p>
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    1. PTQuestion
    1. USCommunity
    1. USRobbie Rosati
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    1. COThere are options (I thought it was the default) to have threads not be destroyed but wait and be recycled when needed again.
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      1. POParallelize recursive function with OpenMP v.2.0
      1. USMarc Glisse
    2. COI tried explicitly setting `OMP_WAIT_POLICY=active` but unfortunately that [isn't recognized by VC++](http://msdn.microsoft.com/en-us/library/6sfk977f%28v=vs.110%29).
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      1. POParallelize recursive function with OpenMP v.2.0
      1. USRobbie Rosati
    3. COMicrosoft's OpenMP runtime uses thread pools. `libgomp` also implements its own thread pool on top of POSIX threads. Still there is a lot of overhead in managing the regions themselves. Perhaps you should look into other programming models like TBB.
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      1. POParallelize recursive function with OpenMP v.2.0
      1. USHristo Iliev
 

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