OpenMP in NDK always uses 1 thread

[Joydeep Mitra]

Hi,

I am trying to explore OpenMP in Android. However, my code always runs using 1 thread even if the max no of threads available is 6. I have tried to set the no. of threads to 6 but it still runs on only 1. I have also tried to set the environment variable to 6 but it still doesn't work. Can anyone provide some insight as to why this is happening? Here is the code I am using.

jdouble Java_edu_example_anative_myfirstnativeapp_MyNative_calcPipar(){
    int i,j;
    int    tid;
    double x;
    double pi, sum = 0.0;
    double start, delta;

    step = 1.0/(double) steps;

    sum = 0.0;

    __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, "Max Threads %d...\n", omp_get_max_threads());
    __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, "No of Processors %d...\n", omp_get_num_procs());
    omp_set_num_threads(omp_get_max_threads());
    omp_set_dynamic(0);
    start = omp_get_wtime();
#pragma omp parallel for private(x, tid) reduction(+:sum)
    tid = omp_get_thread_num();
    __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, "No. of Threads working %d \n", omp_get_num_threads());
    __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, "Thread %d working...\n", tid);
    for (i=0; i < steps; i++) {
        x = (i+0.5)*step;
        sum += 4.0 / (1.0+x*x);
    }

    delta = omp_get_wtime() - start;
// Out of the parallel region, finialize computation
    pi = step * sum;
    //printf("PI = %.16g computed in %.4g seconds\n", pi, delta);
    __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, "PI = %.16g computed in %.4g seconds\n", pi, delta);
    return pi;
}

[Ryan Prichard]

The code didn't compile:

 - LOG_TAG, step, and steps weren't declared.

 - The #pragma omp needs to immediately precede the for loop. Move the "for (i=0; i < steps; i++) {" line up.

Once I make those fixes, I see this output:

11-27 04:59:13.173 10356 10356 D test    : Max Threads 8...

11-27 04:59:13.173 10356 10356 D test    : No of Processors 8...

11-27 04:59:13.179 10356 10356 D test    : No. of Threads working 8 

11-27 04:59:13.179 10356 10358 D test    : No. of Threads working 8 

11-27 04:59:13.179 10356 10356 D test    : Thread 0 working...

11-27 04:59:13.179 10356 10358 D test    : Thread 2 working...

...

11-27 04:59:13.179 10356 10358 D test    : No. of Threads working 8 

11-27 04:59:13.179 10356 10358 D test    : Thread 2 working...

11-27 04:59:13.179 10356 10359 D test    : No. of Threads working 8 

11-27 04:59:13.179 10356 10359 D test    : Thread 3 working...

11-27 04:59:13.179 10356 10359 D test    : No. of Threads working 8 

11-27 04:59:13.179 10356 10359 D test    : Thread 3 working...

11-27 04:59:13.179 10356 10359 D test    : No. of Threads working 8 

11-27 04:59:13.179 10356 10359 D test    : Thread 3 working...

11-27 04:59:13.179 10356 10359 D test    : No. of Threads working 8 

11-27 04:59:13.179 10356 10359 D test    : Thread 3 working...

...

11-27 04:59:13.194 10356 10361 D test    : No. of Threads working 8 

11-27 04:59:13.194 10356 10361 D test    : Thread 5 working...

11-27 04:59:13.194 10356 10361 D test    : No. of Threads working 8 

11-27 04:59:13.194 10356 10361 D test    : Thread 5 working...

11-27 04:59:13.197 10356 10356 D test    : PI = 3.141600986923125 computed in 0.02364 seconds

-Ryan

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[Kenneth Geisshirt]

Which NDK version do you use? I think you need r15 and use clang as compiler.

- Kenneth

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[Arthur Araújo]

Hi,

Did you get it working? I'm having a similar problem trying to create OpenMP threads in Android. I'm using Clang 3.8 on Visual Studio 2015 (Android NDK r11c). Basically, I got it all compiled and running on mobile, but there is no forking going on, I always end up with one thread only.

Arthur