Determine number of cores using Rust

Question

I want to spawn a certain number of tasks based on the cores that a machine has. Is there anything in Rust that can find the number of cores, or should I just run external commands and parse the output?

Winded

It would also be useful to have a standardized way for the number of threads to be configurable? A combination of environmental variables and an overriding command line option. – Pinky

Answer 1

There's a crate to do this: num_cpus.

References:

• crates.io page

• github.com source

Add this to your Cargo.toml:

[dependencies]
num_cpus = "1.0"

Then in your source:

extern crate num_cpus;
let num = num_cpus::get();

Grekin

And if you want the number of physical cores: num_cpus::get_physical() – Safranine

Answer 2

You can use std::thread::available_parallelism as of Rust 1.59.0:

use std::thread::available_parallelism;
let default_parallelism_approx = available_parallelism().unwrap().get();

It returns 12 on my machine with a Ryzen 5 4600H which is quite consistent with the number of logical processors it has.

Pampuch

Answer 3

You could use std::os::num_cpus. Example:

fn main() {
    println!("{}", std::os::num_cpus());
}

Kirkendall

@Arjan: Oh, you are right. default_sched_threads() is public, however, and should be usable, more or less, for the OP purposes. – Kirkendall

std::os::num_cpus is now gone, as is default_sched_threads, so the questions is now unanswered. The answers were good when presented, it is Rust that has changed. – Thence

docs.rs/num_cpus/1.4.0/num_cpus/index.html "Replaces the deprecated functionality of std::os::num_cpus" – Dilator

Answer 4

It is now possible to use:

std::os::num_cpus

pub fn num_cpus() -> uint

---

Version of Rust:

$ rustc --version
rustc 0.13.0-nightly (d91a015ab 2014-11-14 23:37:27 +0000)

Reference:

std::os::num_cpus

Bindery

std::os::num_cpus is now gone, as is default_sched_threads, so the questions is now unanswered. The answers were good when presented, it is Rust that has changed. – Thence