std thread hardware_concurrency() should not be used

[jbd]

Hello,

std::thread::hardware_concurrency() returns, when possible, the underlying hardware capability to run threads, which might not corresponds to the actual number of cores available to the process (through the use of taskset, batch system like slurm, etc...). The consequence is that mkdwarfs might run in a non optimal way. For example, if I run taskset -c 1 mkdwarfs on my 20 cores machines, it will run 20 workers on only one core.

The immediate workaround is to use the -N option to set the number of workers, but I think a more sane behavior would be to use sched_getaffinity as in opencv/opencv#16268. Gromacs did something similar (https://github.com/gromacs/gromacs/blob/1e6873fadf16d5f5be861e6f9ef5f9923a12e540/src/gromacs/hardware/hardwaretopology.cpp#L1221).

What do you think ?

Thank you.

[jbd]

In fact, the solution from folly/folly/system/HardwareConcurrency.cpp is great !

#include <folly/system/HardwareConcurrency.h>

#include <thread>

#include <folly/portability/Sched.h>

namespace folly {

unsigned int hardware_concurrency() noexcept {
#if defined(__linux__) && !defined(__ANDROID__)
  cpu_set_t cpuset;
  if (!sched_getaffinity(0, sizeof(cpuset), &cpuset)) {
    auto count = CPU_COUNT(&cpuset);
    if (count != 0) {
      return count;
    }
  }
#endif

  return std::thread::hardware_concurrency();
}

} // namespace folly

[mhx]

Hey, thanks for the report! Switching to the folly implementation shouldn't be too much trouble. I'll consider that for the next release.

[mhx]

Fixed in e4971b4.