CODEWITHSUNDEEP
c++multithreadingintel-mkl
WalkerShaw
WalkerShaw

Reputation: 11

Parallel SpMV using MKL slows down with increasing thread number

I'm calling intel MKL for csr format SpMV. To accelerate, I'm using multiple threads by mkl_set_num_threads. However, when the threads increases, the performance slows down. Any idea what's going on?

Though the documents say the thread number specified by mkl_set_num_threads only limits the maximum threads used, and less threads might be used, I expect the performance at least stays the same when using more threads. Here is my code:

for(int j = 1; j <= max_threads; ++j){

    mkl_set_num_threads(j);

    mkl_scsrmv(&transa, &m, &k, &alpha, matdescra, val, col_idx, row_idx, &row_idx[1], x, &beta, y); 
    start = clock();

    for(int i = 0; i < 100; ++i) { 
            mkl_scsrmv(&transa, &m, &k, &alpha, matdescra, val, col_idx, row_idx, &row_idx[1], x, &beta, y);
    }
    end = clock();
    elapsed = end - start;
    cout << "the float CSR spmv performance is " << (double)nnz  * ( (double)CLOCKS_PER_SEC / 10000000 ) / (double)elapsed  << " Gflops using " << j << " threads" << endl;
}

And here is the result:

Upvotes: 1

Views: 223

Answers (1)

lljbash
lljbash

Reputation: 96

clock() returns the total CPU time used by the process. You should measure real-world time instead.

Upvotes: 1

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