从openmp并行区域调用多线程MKL

问题描述

我有一个具有以下结构的代码

I have a code with following structure

#pragma omp parallel
{
    #omp for nowait
    {
        // first for loop
    }

    #omp for nowait
    {
        // first for loop
    }

    #pragma barrier

    <-- #pragma omp single/critical/atomic --> not sure
    dgemm_(....)

    #pragma omp for
    {
        // yet another for loop
    }

}

对于dgemm_，我链接了多线程mkl.我希望mkl使用所有可用的8个线程.最好的方法是什么?

For dgemm_, I link with multithreaded mkl. I want mkl to use all available 8 threads. What is the best way to do so?

推荐答案

这是嵌套并行性的一种情况.它由MKL支持，但是仅当您的可执行文件是使用Intel C/C ++编译器构建的时，它才有效.限制的原因是MKL使用Intel的OpenMP运行时，并且不同的OMP运行时不能很好地发挥作用.

This is a case of nested parallelism. It is supported by MKL, but it only works if your executable is built using the Intel C/C++ compiler. The reason for that restriction is that MKL uses Intel's OpenMP runtime and that different OMP runtimes do not play well with each other.

一旦解决，您应该通过将OMP_NESTED设置为TRUE来启用嵌套并行性，并通过将MKL_DYNAMIC设置为FALSE来禁用MKL对嵌套并行性的检测.如果共享要使用dgemm_处理的数据，则必须从single构造内调用后者.如果每个线程处理自己的私有数据，则您不需要任何同步结构，但是使用多线程MKL也不会给您带来任何好处.因此，我认为您的情况是前者.

Once that is sorted out, you should enable nested parallelism by setting OMP_NESTED to TRUE and disable MKL's detection of nested parallelism by setting MKL_DYNAMIC to FALSE. If the data to be processes with dgemm_ is shared, then you have to invoke the latter from within a single construct. If each thread processes its own private data, then you don't need any synchronisation constructs, but using multithreaded MKL won't give you any benefit too. Therefore I would assume that your case is the former.

总结一下:

#pragma omp single
dgemm_(...);

并运行:

$ MKL_DYNAMIC=FALSE MKL_NUM_THREADS=8 OMP_NUM_THREADS=8 OMP_NESTED=TRUE ./exe

您还可以通过适当的调用来设置参数:

You could also set the parameters with the appropriate calls:

mkl_set_dynamic(0);
mkl_set_num_threads(8);
omp_set_nested(1);

#pragma omp parallel num_threads(8) ...
{
   ...
}

尽管我更愿意使用环境变量.

though I would prefer to use environment variables instead.

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