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    <p>Based on Floris's fast function I tried to see if I could find a way to find a faster solution with OpenMP. I came up with two functions <code>foo_v2</code> and <code>foo_v3</code> which are faster for larger arrays, <code>foo_v2</code> is faster independent of density and <code>foo_v3</code> is faster for sparser arrays. The function <code>foo_v2</code> essentially creates a 2D array with width <code>N*nthreads</code> as well as an array <code>countsa</code> which contains the counts for each thread. This is better explained with code. The following code would loop over all the elements written out to T.</p> <pre><code>for(int ithread=0; ithread&lt;nthreads; ithread++) { for(int i=0; i&lt;counta[ithread]; i++) { T[ithread*N/nthread + i] } } </code></pre> <p>The function<code>foo_v3</code>creates a 1D array as requested. In all cases<code>N</code>has to be pretty large to overcome the OpenMP overhead. The code below defaults to 256MB with a density of<code>M</code>about 10%. The OpenMP functions are both faster by over a factor of 2 on my 4 core Sandy Bridge system. If you put the density at 50%<code>foo_v2</code>is faster still by about about a factor of 2 but<code>foo_v3</code>is no longer faster. </p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;omp.h&gt; int foo_v1(int *M, int *T, const int N) { int count = 0; for(int i = 0; i&lt;N; i++) { T[count] = i; count += M[i]; } return count; } int foo_v2(int *M, int *T, int *&amp;counta, const int N) { int nthreads; #pragma omp parallel { nthreads = omp_get_num_threads(); const int ithread = omp_get_thread_num(); #pragma omp single counta = new int[nthreads]; int count_private = 0; #pragma omp for for(int i = 0; i&lt;N; i++) { T[ithread*N/nthreads + count_private] = i; count_private += M[i]; } counta[ithread] = count_private; } return nthreads; } int foo_v3(int *M, int *T, const int N) { int count = 0; int *counta = 0; #pragma omp parallel reduction(+:count) { const int nthreads = omp_get_num_threads(); const int ithread = omp_get_thread_num(); #pragma omp single { counta = new int[nthreads+1]; counta[0] = 0; } int *Tprivate = new int[N/nthreads]; int count_private = 0; #pragma omp for nowait for(int i = 0; i&lt;N; i++) { Tprivate[count_private] = i; count_private += M[i]; } counta[ithread+1] = count_private; count += count_private; #pragma omp barrier int offset = 0; for(int i=0; i&lt;(ithread+1); i++) { offset += counta[i]; } for(int i=0; i&lt;count_private; i++) { T[offset + i] = Tprivate[i]; } delete[] Tprivate; } delete[] counta; return count; } void compare(const int *T1, const int *T2, const int N, const int count, const int *counta, const int nthreads) { int diff = 0; int n = 0; for(int ithread=0; ithread&lt;nthreads; ithread++) { for(int i=0; i&lt;counta[ithread]; i++) { int i2 = N*ithread/nthreads+i; //printf("%d %d\n", T1[n], T2[i2]); int tmp = T1[n++] - T2[i2]; if(tmp&lt;0) tmp*=-1; diff += tmp; } } printf("diff %d\n", diff); } void compare_v2(const int *T1, const int *T2, const int count) { int diff = 0; int n = 0; for(int i=0; i&lt;count; i++) { int tmp = T1[i] - T2[i]; //if(tmp!=0) printf("%i %d %d\n", i, T1[i], T2[i]); if(tmp&lt;0) tmp*=-1; diff += tmp; } printf("diff %d\n", diff); } int main() { const int N = 1 &lt;&lt; 26; printf("%f MB\n", 4.0*N/1024/1024); int *M = new int[N]; int *T1 = new int[N]; int *T2 = new int[N]; int *T3 = new int[N]; int *counta; double dtime; for(int i=0; i&lt;N; i++) { M[i] = ((rand()%10)==0); } //int repeat = 10000; int repeat = 1; int count1, count2; int nthreads; dtime = omp_get_wtime(); for(int i=0; i&lt;repeat; i++) count1 = foo_v1(M, T1, N); dtime = omp_get_wtime() - dtime; printf("time v1 %f\n", dtime); dtime = omp_get_wtime(); for(int i=0; i&lt;repeat; i++) nthreads = foo_v2(M, T2, counta, N); dtime = omp_get_wtime() - dtime; printf("time v2 %f\n", dtime); compare(T1, T2, N, count1, counta, nthreads); dtime = omp_get_wtime(); for(int i=0; i&lt;repeat; i++) count2 = foo_v3(M, T3, N); dtime = omp_get_wtime() - dtime; printf("time v2 %f\n", dtime); printf("count1 %d, count2 %d\n", count1, count2); compare_v2(T1, T3, count1); } </code></pre>
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