使用数组元素进行OpenMP并行化

Question

I've been playing around with OpenMP, and am trying to see if I can get a speedup in a particular bit of C++ code.

    #pragma omp parallel for
    for (Index j=alignedSize; j<size; ++j)
    {
      res[j] = cj.pmadd(lhs0(j), pfirst(ptmp0), res[j]);
      res[j] = cj.pmadd(lhs1(j), pfirst(ptmp1), res[j]);
      res[j] = cj.pmadd(lhs2(j), pfirst(ptmp2), res[j]);
      res[j] = cj.pmadd(lhs3(j), pfirst(ptmp3), res[j]);
    }

I'm a complete newbie with OpenMP so be gentle with me, but could someone shed some light on why this code ends up doubling the execution time rather than speeding it up?

I'm running with 4 cores, just in case that matters.

Answer 1

What is the size of a res entry? If its less than the size of a cache line then its likely false sharing .

Answer 2

典型cpu的最低要求是128个字节的块，然后您需要统一的最后一级缓存。