avoid race in OpenMP (in a parallel for loop)

Question

I am writing an OpenMP program in C. I have this shared array "data" which is being updated by all threads. i want to ensure that every thread has completed the reading part and stored the value in temp before the next statement data[j] = temp is executed.

I tried putting #pragma omp barrier between the two statements but the compiler throws an error. Please help.

#pragma omp parallel for shared(data)

for (j = 0; j < numints; j++){

     if (j >= max_j)

     {

              temp = data[j] + data[j - max_j];
             data[j] = temp; 
     }

}

barrier directive cannot appear within parallel for construct(on IBM compiler) — bcubed
– bcubed, Commented Oct 17, 2010 at 5:46

Jonathan Dursi · Accepted Answer · 2010-10-18 11:40:51Z

As you've seen, barrier won't work; critical is rather heavy-weight for this particular operation. Atomic is lighter weight than critical; you could always do

if (j >= max_j)
{
    #pragma omp atomic
    data[j] += data[j-max_j]; 
}

but you should always be wary of having any such construct (atomic, critical) inside a loop -- it kills performance, because it kills parallelism (that is, after all, their entire purpose).

It would help to know what you're trying to accomplish with this bit of code, because even once the data races in the updates are eliminated, the final result in (say) data[maxints-1] will depend on what order data[maxints-1-max_j],data[maxints-1-2*max_j].. were updated in, which is explicitly not guaranteed by OpenMPs parallel for. (You can use the ordered construct, but that's barely better than not using a parallel for at all).

If maxints < 2*max_j, then this is easy; you can just do

#pragma omp parallel for shared(data)
for (j = max_j; j < numints; j++){
    data[j] += data[j-max_j];
}

and you don't need any synchronization at all, because every thread is only updating one data[j] and none depend on any others. But I get the impression (a) that it isn't, and (b) this is a snippet of a larger piece of code...

Anycorn · Accepted Answer · 2010-10-17 06:22:43Z

1

barrier is for synchronization. you want to use critical section: http://msdn.microsoft.com/en-us/library/b38674ky%28VS.80%29.aspx.

answered Oct 17, 2010 at 6:22

Anycorn

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