How to yield/resume OpenMP untied tasks correctly?

问题

I wrote a small C program to assess OpenMP's capability to yield to another task when idle time in a task occurs (e.g. wait for communicated data):

#include <stdio.h>
#include <sys/time.h>
#include <omp.h>

#define NTASKS 10

double wallClockTime(void) {

    struct timeval t;

    gettimeofday(&t, NULL);

    return (double)(t.tv_sec + t.tv_usec/1000000.);
}

void printStatus(char *status, int taskNum, int threadNum) {

#pragma omp critical(printStatus)
    {
        int i;
        for (i = 0; i < taskNum; i++) printf("    ");
        printf(" %s%i \n", status, threadNum);
    }
}

void task(int taskNum) {

    // "r"un task
    printStatus("r", taskNum, omp_get_thread_num());
    sleep(1);

    // "s"leeping task that can yield
    printStatus("s", taskNum, omp_get_thread_num());
    double idleStartTime = wallClockTime();
    while (wallClockTime() < idleStartTime + 1) {
#pragma omp taskyield
    }

    // "c"ontinue task
    printStatus("c", taskNum, omp_get_thread_num());
    sleep(1);
}


int main(int argc, char* argv[]) {

#pragma omp parallel
#pragma omp single nowait
    {
        int i;

        printf("thread %d is master\n\n", omp_get_thread_num());
        for (i = 0; i < NTASKS; i++) printf(" %02d ", i);
        printf("\n");

        for (i = 0; i < NTASKS; i++) {

#pragma omp task untied
            task(i);
        }
    }

    return 0;
}

I used Intel C compiler 17.0.4. Here is the output from a run with 3 threads:

thread 0 is master

 00  01  02  03  04  05  06  07  08  09 
 r1 
                                     r0 
     r2 
 s1 
                                     s0 
     s2 
                                 r0 
 c1 
     c2 
                                 s0 
                             r0 
         r1 
             r2 
                             s0 
                         r0 
         s1 
             s2 
                         s0 
                     r0 
         c1 
             c2 
                     s0 
                 r0 
                 s0 
                 c0 
                     c0 
                         c0 
                             c0 
                                 c0 
                                     c0

Thread 1 and 2 do not yield at all, but they stick to their assigned task instead. I would also expect threads 1 and 2 to continue on the suspended untied tasks 04 ... 09, but these are only handled by master thread 0 while the other threads are idle.

Do the tasks have to be issued or yielded in a different way, or is Intel's OpenMP runtime not (yet) capable to handle this? Btw., GNU gcc 4.9.2 does not yield from tasks at all.

回答1:

I think your code is just fine and this is an implementation issue. In fact in the LLVM OpenMP implementation - which is very much related to Intel's - pushed a commit two weeks ago that fixes your issue. In my tests, clang's current libiomp5.so (built from trunk) was compatible with icc 17.0.4 just by setting LD_LIBRARY_PATH and produces the desired result.

thread 0 is master

 00  01  02  03  04  05  06  07  08  09 
                                     r0 
 r2 
     r1 
                                     s0 
                                 r0 
 s2 
         r2 
     s1 
             r1 
                                 s0 
                             r0 
         s2 
                 r2 
             s1 
                     r1 
                             s0 
                         r0 
                 s2 
                     s1 
                 c2 
                         s0 
                     c1 
                         c0 
         c2 
             c1 
                             c0 
 c2 
     c1 
                                 c0 
                                     c0

I can also confirm that gcc does not yield at all, but haven't looked in detail.

I have no idea if and when the change might be merged into library shipped by Intel.

Update: You are right that the behavior is still not optimal. From briefly looking through the code it seems that libiomp supports the notion of tied tasks, but does not requeue the task during a taskwait but instead just executes another task and retains the context of the suspended task on the stack. I suspect a proper support would require more heavy compiler support (continuations of a sort) rather than just generating library calls.

Again, you are doing everything right, but the compiler / runtime is not sophisticated enough to support what the standard allows (the behavior is fully standards compliant). Also note that for the described current behavior of libiomp, the tasks don't even need to be untied, since they are only queue so far. There doesn't seem to be an easy way to get what you want short of splitting up / chaining tasks.

来源：https://stackoverflow.com/questions/47658571/how-to-yield-resume-openmp-untied-tasks-correctly

标签

task

openmp