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    <p>Don't nest it too deep. Usually, it would be enough to identify a good point for parallelization and get away with just one directive.</p> <p>Some comments and probably the root of your problem:</p> <pre><code> #pragma omp parallel default(shared) // Here you open several threads ... { #pragma omp for for (int nY = nYTop; nY &lt;= nYBottom; nY++) { #pragma omp parallel shared(nY, nYBottom) // Same here ... { #pragma omp for for (int nX = nXLeft; nX &lt;= nXRight; nX++) { </code></pre> <p>(Conceptually) you are opening many threads, in each of them you open many threads again in the for loop. For each thread in the for loop, you open many threads again, and for each of those, you open again many in another for loop.</p> <p>That's <code>(thread (thread)*)+</code> in pattern matching words; there should just be <code>thread+</code></p> <p>Just do a single parallel for. Don't be to fine-grained, parallelize on the outer loop, each thread should run as long as possible:</p> <pre><code>#pragma omp parallel for for (int nY = nYTop; nY &lt;= nYBottom; nY++) { for (int nX = nXLeft; nX &lt;= nXRight; nX++) { } } </code></pre> <p>Avoid data and cache sharing between the threads (another reason why the threads shouldn't be too fine grained on your data).</p> <p>If that's running stable and shows good speed up, you can fine tune it with different scheduling algorithms as per your OpenMP reference card.</p> <p>And put your variable declarations to where you really need them. Do not overwrite what is read by sibling threads.</p>
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    1. POThe correct usage of nested #pragma omp for directives
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    1. USParag S. Chandakkar
    1. USSebastian Mach
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    1. CO`#pragma omp parallel default(shared) // Here you open several threads ... { #pragma omp for // <- syntax error:` you are wrong phresnel. This is totally valid way to do a parallel for loop. That pragma tells the parallel environment to be parallelized now. The combination of `omp parallel` and following `omp for` is especially useful if you need thread local allocations / initializations, although here it is not needed and doing a combined `omp parallel` for will certainly do it.
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    2. COand yeah to much nesting GoldenLee. I would go for the outer loop, only. You are just open another set of threads within a thread...
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    3. CO@phresnel: Thank you for your detailed explanation on OpenMP usage. And sorry for my late reply for it's too late (actually I went to sleep). I have a test on the basis of your suggestion. But the result let me down. It's still in endless running! Why? Would you please give me further advice? Thank you!
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      1. USGoldenLee
 

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