Add call to check total number of CPUs?

[avl]

On linux, num_cpus::get() currently returns 1 if the calling thread has its sched_affinity set to only one thread.

I have a use case where a main thread is pinned to core 0, and worker threads are pinned to cores 1, 2, 3 etc.

In this use case, the main thread can no longer use num_cpus::get() to determine the number of cpus in the system. In my case, I had a problem where a thread pool-implementation erroneously decided to use a single worker thread, because it thought the machine had only one CPU, since the pool was initialized on a thread which was pinned to a specific core.

I get the idea of returning "usable cores", but I think there is also a use case for a call to return the total number of cpus (the set which could be used in a call to sched_setaffinity).

Would you be open to pull request to add such a call?

[seanmonstar]

Does the num_cpus::get_physical() serve your needs?

[avl]

Does the num_cpus::get_physical() serve your needs?

Ah, does it not take affinity into account?

Still, it doesn't really help because I am interested in the number of CPU threads, not the number of cores.

[kubo39]

I'm not sure what you want, cpuid may be helpful.

#[macro_use]
extern crate raw_cpuid;

fn main() {
    /// For Intel Core I7 and later, use `0x0B` to determine number of processors.
    ///
    /// https://www.scss.tcd.ie/Jeremy.Jones/CS4021/processor-identification-cpuid-instruction-note.pdf
    ///   5.1.12 Processor Topology
    let cpu_info = cpuid!(0x0B);
    let threads_per_core = cpu_info.ebx & 0xFFFF;
    println!("{}", threads_per_core);
}

[avl]

@kubo39 Cool! I suppose that is x86-only? And in a totally different crate? And what would the result be in a multi-CPU machine? Would I get just the number of cores on the physical CPU the current thread happens to execute on?

[kubo39]

@avl

I suppose that is x86-only?

AFAIK, such extension only exists on x86.

And in a totally different crate?

yes, this is raw-cpuid crate.

And what would the result be in a multi-CPU machine?

Perhaps It depends on cache level, please check the section 5.1.12 on https://www.scss.tcd.ie/Jeremy.Jones/CS4021/processor-identification-cpuid-instruction-note.pdf

Would I get just the number of cores on the physical CPU the current thread happens to execute on?

Ah, I may have misunderstood. You just want system-wide logical processor count?
If so, heim-rs may be preferable.

[avl]

@kubo39 Thanks a lot! That crate does indeed seem to do exactly what I want.