In the past, we had a method on ExecFuture called:

limitPerCore($factor = 1)

Maybe we should restore that? A simple implementation would be:

public function limitPerCore($factor = 1) {
  $cores_on_this_machine = 4;
  return $this->limit((int)ceil($factor * $cores_on_this_machine));
}

A more advanced implementation could try to figure out the number of cores instead of guessing that it's 4.

The logic here is that when you limit(), the "right" number is almost always one process per core, or some factor thereof.

I've found that Linux is a lot better at parallising this kind of work than Windows. On Linux with 4 cores I can quite comfortably set the limit to 50 and all the execs are finished within 15 seconds (most in 9 seconds). Windows on the other hand struggles with parallising workload. So I don't think setting it to either a fixed number or based on CPU cores will really work; the kernel's ability to parallise tasks has a huge impact on what number is appropriate.

Is there a way to put this config in arcrc? I don't intend on this setting being set per-project.

Doesn't that make putting this in project config worse? I'm completely fine with an implementation like:

if (phutil_is_windows()) {
  // lol windows
  return 2;
}
return $actual_core_count;