Getting wrong results with OpenMP

Question

I wrote a programm that multiplies a vector by a matrix. The matrix has periodically repeated cells, so I use a temporary variable to sum vector elements before multiplication. The period is the same for adjacent rows. I create a separate temp variable for each thread. sizeof(InnerVector) == 400 and I don't want to allocate memory for it on every iterration (= 600 times).

Code looks something like this:

tempsSize = omp_get_max_threads();
InnerVector temps = new InnerVector[tempsSize];

for(int k = 0; k < tempsSize; k++)
    InnerVector_init(temps[k]);

for(int jmin = 1, jmax = 2; jmax < matrixSize/2; jmin *= 2, jmax *= 2)
{
    int period = getPeriod(jmax);

    #pragma omp parallel
    {
        int threadNum = omp_get_thread_num();
        // printf("\n threadNum = %i", threadNum);

        #pragma omp for
        for(int j = jmin; j < jmax; j++)
        {
            InnerVector_reset(temps[threadNum]);   
            for(int i = jmin; i < jmax; i++)
            {
                InnerMatrix cell = getCell(i, j);
                if(temps[threadNum].IsZero)
                    for(int k = j; k < matrixSize; k += period)
                        InnerVector_add(temps[threadNum], temps[threadNum], v[k]);
                InnerVector_add_mul(v_res[i], cell, temps[threadNum]);
            }
        }
    }
}

The code looks to be correct but I get wrong result. In fact, I get different results for different runs... sometimes result is correct.

When I compile in debug mode the result is always correct. When I uncomment the row with "printf" the result is always correct.

p.s. I use Visual Studio 2010.

InnerVector* v; // array of type "InnerVector" which I need to multiply. — Shamil
– Shamil, Commented May 1, 2011 at 15:28

Alexey Kukanov · Accepted Answer · 2011-05-01 07:02:13Z

3

I suspect there might be a data race in
InnerVector_add_mul(v_res[i], cell, temps[threadNum]);

Since v_res appears to be a resulting vector, and i changes from jmin to jmax in each iteration of the parallelized loop, it can happen that multiple threads write to v_res[i] for the same value of i, with unpredictable result.

answered May 1, 2011 at 7:02

Alexey Kukanov

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2 Comments

Shamil Over a year ago

To get it correct I should switch "for j" and "for i". But this way I'll lost "calculate temp one time for several i" optimization...

Jacob Over a year ago

@Shamil: If this answer is correct, please accept it by clicking the check-mark under the vote counter.

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