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POHow to spawn subthreads from threads in openMP (C++)
 

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  1. POHow to spawn subthreads from threads in openMP (C++)
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    <p>I am trying to build a tree with a fixed number of children as well as a fixed depth. I do not fully understand the underlying mechanics of openMP. Tree construction begins upon calling <code>build(root_node, 0)</code>. Now let's suppose that <code>maxDepth</code> is given an arbitrary number and that <code>maxChildren</code> is equal to <code>n</code>. When <code>build(root_node, 0)</code> is called, <code>n</code> threads are launched. I was under the impression that each of these <code>n</code> threads would create <code>n</code> threads. However, careful observation of <code>top</code> revealed that there are never more than <code>n</code> threads. I can saturate my cores only when <code>maxChildren</code> is equal or higher than the number of cores I have. It seems that the <code>parallel</code> blocks in subsequent levels in the recursion have no effet, limiting the number of available threads for subsequent use to what was needed in the initial call to <code>build</code>.</p> <p>Why does it behave this way? Has the recursion any part in this? And most importantly, what can I do to remedy this? Thanks in advance.</p> <pre><code>void build(Node* pNode, unsigned int depth) { if (depth &gt;= maxDepth) return; std::list&lt;Node*&gt; children; std::list&lt;Node*&gt;::iterator it; // This loop cannot be parallelized because each call to select_next_node // is dependent on the previous one for (unsigned i = 0; i &lt; maxChildren; ++i) { Node* const p_candidate_node = select_next_node(...); if (is_valid(p_candidate_node)) children.push_back(p_candidate_node); } #pragma omp parallel private(it) for (it = children.begin(); it != children.end(); ++it) #pragma omp single nowait build(*it, depth + 1); } </code></pre>
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