The feature gate for the issue is #![feature(available_parallelism)].

This is a tracking issue for std::thread::available_parallelism; a portable API to determine how many threads to spawn in order to ensure a program can make use of all available parallelism available on a machine.

Public API

#![feature(available_parallelism)]
use std::thread;

fn main() -> std::io::Result<()> {
    let count = thread::available_parallelism()?.get();
    assert!(count >= 1);
    Ok(())
}

Tasks

• Resolve discussion on naming.
• Resolve discussion on function signature.
• Resolve discussion on terminology.
• Add support for externally-set limits on Linux (ref).
• Add support for externally-set limits on Windows (ref).
• Update documentation to remove mentions of "hardware" (ref).
• Smooth out the docs example.

Notes on Platform-specific behavior

available_parallelism is (for now) only considered a hint. The following platform limitations exist:

• On Windows the available concurrency may be undercounted when exceeding 64 threads Add std::thread::available_concurrency #74480 (comment)
• On Windows the available concurrency may be overcounted when limited by a process wide affinity mask or job object limitations Add std::thread::available_concurrency #74480 (comment)
• On Linux the available concurrency may be overcounted in the face of cgroup limits and task-wise affinity masks Add std::thread::available_concurrency #74480 (comment)

Implementation history

• Add std::thread::available_concurrency #74480
• Move available_concurrency implementation to sys #85182
• Make std::thread::available_concurrency support process-limited number of CPUs #89310
• Rename std::thread::available_conccurrency to std::thread::available_parallelism #89324
• Use cgroup quotas for calculating available_parallelism #92697
• Add cgroupv1 support to available_parallelism #97925