[Openmp-dev] Question about implementation of task dependencies

[Churbanov, Andrey via Openmp-dev]

Jonas,

The OpenMP specification says (section 2.13.9 depend Clause):

For the out and inout dependence-types, if the storage location of at least one of the list items is
the same as the storage location of a list item appearing in an in, out, or inout dependence-type
list of a task construct from which a sibling task was previously generated, then the generated task
will be a dependent task of that sibling task.

So in your example task #3 must depend on task #2, as well as on task #1. The runtime behavior looks correct.
The only independent sequence is when task with depend(in:dep) follows another task with depend(in:dep).
All other combinations produce dependencies.

Regards,
Andrey

-----Original Message-----
From: Openmp-dev [mailto:openmp-dev-bounces at lists.llvm.org] On Behalf Of Hahnfeld, Jonas via Openmp-dev
Sent: Wednesday, August 3, 2016 2:37 PM
To: LLVM-OpenMP (openmp-dev at lists.llvm.org) <openmp-dev at lists.llvm.org>
Cc: Duran, Alejandro <alejandro.duran at intel.com>
Subject: [Openmp-dev] Question about implementation of task dependencies

Dear all,

I'm currently doing some tests with task dependencies and have found a behavior which I would have at least not expected. I'm possibly missing something and there is not much documentation in the code so maybe the Intel folks or someone else can enlighten me here.

With the following code snippet, the runtime will currently add a dependence from task #3 to task #2:

    #pragma omp parallel num_threads(4)
    #pragma omp single
    {
        int dep;
        #pragma omp task depend(out: dep)
        {
            sleep(1);
            printf("task #1\n");
        }
        #pragma omp task depend(in: dep)
        {
            sleep(1);
            printf("task #2\n");
        }
        #pragma omp task depend(out: dep)
        {
            sleep(1);
            printf("task #3\n");
        }
    }

As out means the same as inout per the current standard, I would expect a dependence to task #1 which specifies out on the same variable. (Note: The dependence from #2 to task #1 is not questioned.)

I have attached a patch which then fulfills my expectations. However I don't want to officially put it to review until I fully understand the reasoning behind the current code.

Thanks,
Jonas


--
Jonas Hahnfeld, MATSE-Auszubildender

IT Center
Group: High Performance Computing
Division: Computational Science and Engineering RWTH Aachen University Seffenter Weg 23 D 52074  Aachen (Germany) Hahnfeld at itc.rwth-aachen.de www.itc.rwth-aachen.de



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