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    <p>There's no problem with having the parallel section start in another routine before the for, certainly since OpenMP 3.0 (2008) and maybe since OpenMP 2.5. With gcc4.4:</p> <p>outer.c:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;omp.h&gt; void update(int n, int iter); int main(int argc, char **argv) { int n=10; #pragma omp parallel num_threads(4) default(none) shared(n) for (int iter=0; iter&lt;3; iter++) { #pragma omp single printf("---iteration %d---\n", iter); update(n, iter); } return 0; } </code></pre> <p>inner.c:</p> <pre><code>#include &lt;omp.h&gt; #include &lt;stdio.h&gt; void update(int n, int iter) { int thread = omp_get_thread_num(); #pragma omp for for (int i=0;i&lt;n;i++) { int newthread=omp_get_thread_num(); printf("%3d: doing loop index %d.\n",newthread,i); } } </code></pre> <p>Building:</p> <pre><code>$ make gcc44 -g -fopenmp -std=c99 -c -o inner.o inner.c gcc44 -g -fopenmp -std=c99 -c -o outer.o outer.c gcc44 -o main outer.o inner.o -fopenmp -lgomp $ ./main ---iteration 0--- 2: doing loop index 6. 2: doing loop index 7. 2: doing loop index 8. 0: doing loop index 0. 0: doing loop index 1. 0: doing loop index 2. 1: doing loop index 3. 1: doing loop index 4. 1: doing loop index 5. 3: doing loop index 9. ---iteration 1--- 0: doing loop index 0. 0: doing loop index 1. 0: doing loop index 2. 1: doing loop index 3. 1: doing loop index 4. 1: doing loop index 5. 3: doing loop index 9. 2: doing loop index 6. 2: doing loop index 7. 2: doing loop index 8. ---iteration 2--- 0: doing loop index 0. 0: doing loop index 1. 0: doing loop index 2. 3: doing loop index 9. 2: doing loop index 6. 2: doing loop index 7. 2: doing loop index 8. 1: doing loop index 3. 1: doing loop index 4. 1: doing loop index 5. </code></pre> <p>But as per @jdv-Jan de Vaan, I'd be very surprised if in a up-to-date OpenMP implmentation this led to a significant performance improvement over having the parallel for in update, particularly if update is expensive enough. </p> <p>BTW, there are issues with just putting a parallel for around the i-loop in the Gauss-Seidel routine in update; you can see that the i steps aren't independant, and that will lead to race conditions. You will need to do something like Red-Black or Jacobi iteration instead... </p> <p><strong>Update:</strong></p> <p>The code sample provided is for a G-S iteration, not Jacobi, but I'll just assume that's a typo.</p> <p>If your question is actually about the reduce and not the orphaned for loop: yes, you sadly have to roll your own min/max reductions in OpenMP, but it's pretty straightforward, you just use the usual tricks.</p> <p><strong>Update 2</strong> -- yikes, locmax needs to be private, not shared.</p> <p>outer.c:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;omp.h&gt; int update(int n, int iter); int main(int argc, char **argv) { int n=10; int max, locmax; max = -999; #pragma omp parallel num_threads(4) default(none) shared(n, max) private(locmax) for (int iter=0; iter&lt;3; iter++) { #pragma omp single printf("---iteration %d---\n", iter); locmax = update(n, iter); #pragma omp critical { if (locmax &gt; max) max=locmax; } #pragma omp barrier #pragma omp flush #pragma omp single printf("---iteration %d's max value = %d---\n", iter, max); } return 0; } </code></pre> <p>inner.c:</p> <pre><code>#include &lt;omp.h&gt; #include &lt;stdio.h&gt; int update(int n, int iter) { int thread = omp_get_thread_num(); int max = -999; #pragma omp for for (int i=0;i&lt;n;i++) { printf("%3d: doing loop index %d.\n",thread,i); if (i+iter&gt;max) max = i+iter; } return max; } </code></pre> <p>and building:</p> <pre><code>$ make gcc44 -g -fopenmp -std=c99 -c -o inner.o inner.c gcc44 -g -fopenmp -std=c99 -c -o outer.o outer.c gcc44 -o main outer.o inner.o -fopenmp -lgomp bash-3.2$ ./main ---iteration 0--- 0: doing loop index 0. 0: doing loop index 1. 0: doing loop index 2. 2: doing loop index 6. 2: doing loop index 7. 2: doing loop index 8. 1: doing loop index 3. 1: doing loop index 4. 1: doing loop index 5. 3: doing loop index 9. ---iteration 0's max value = 9--- ---iteration 1--- 0: doing loop index 0. 0: doing loop index 1. 0: doing loop index 2. 3: doing loop index 9. 2: doing loop index 6. 2: doing loop index 7. 2: doing loop index 8. 1: doing loop index 3. 1: doing loop index 4. 1: doing loop index 5. ---iteration 1's max value = 10--- ---iteration 2--- 0: doing loop index 0. 0: doing loop index 1. 0: doing loop index 2. 1: doing loop index 3. 1: doing loop index 4. 1: doing loop index 5. 3: doing loop index 9. 2: doing loop index 6. 2: doing loop index 7. 2: doing loop index 8. ---iteration 2's max value = 11--- </code></pre>
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    1. COI am actually using Jacobi iteration, my main problem is how do I find the maximum difference from all the different threads in an iteration? I'm using C so I can't use reduce unfortunately.
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    2. CONo, you just have to write your own using the usual tricks.
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    3. CO@Jonathan - I don't believe you have to specify both -fopenmp and -lgomp on the last line. The -fopenmp option does several things including adding -lgomp to the ld command gcc issues.
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