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Is there a way to check the number of CPU threads in use with sycl?

I'm not sure this question is correctly formulated, I'm still learning. I was wondering if there is a way, when I run a sycl program with a cpu_selector to get if I'm using it as single core or multi core

Solution

  • This is sort of possible using sycl::info::device::max_compute_units. Here's a minimal example:

    #include <CL/sycl.hpp>
#include <iostream>

int main(){
  sycl::device d = sycl::cpu_selector().select_device();
  std::cout << d.get_info<sycl::info::device::max_compute_units>() << std::endl;
}

    On my machine (which has 8 physical cores & 16 hardware threads), this returns 16.

    I think it is generally true that OpenCL considers hardware threads to be 'compute units', but I can't confirm this. Furthermore, it'll only tell you how many hardware threads the OpenCL backend considers are present, not how many you are using.

    Edit for follow up question:

    I think the OpenCL backend will always use all available resources. However, if you're working on a CPU, you can limit available threads using taskset. For example, if I set the mask to use only thread 0:

    taskset 0x00000001 ./a.out

    I get the answer 1.