OS-level thread::Builder priority and affinity extensions · Issue #195 · rust-lang/libs-team (original) (raw)

Proposal

Supersedes #71. This proposal is based on the discussion there and on Zulip about that proposal.

cc @AzureMarker @joshtriplett as they were involved in the earlier discussion / proposal as well.

Problem statement

Exposing OS thread scheduling options has been requested for quite a long time. Some third-party crates have been created to supplement the std functionality, but they seem to mostly work by either duplicating std APIs or only work for the current thread after spawning.

This proposal aims to enable std::os extension traits that can modify thread::Builder to set some of these properties before a thread is spawned, without committing to a higher-level cross-platform API (but ideally leaving room for one to be designed and implemented in the future).

Motivation, use-cases

Setting thread affinity and priority has a variety of motivations. For platforms that use a cooperative thread scheduler, setting CPU affinity and priority may be necessary to ensure that threads are not starved or cause deadlocks. High-performance / realtime applications may need fine grained control over their threads to meet performance requirements.

In principle, I believe this proposal should enable implementation of the necessary APIs to exert low-level OS-specific control over threads, which should pave the way for a more general-purpose cross-platform solution for these use cases.

Solution sketches

I have taken some of the changes from previous attempts and implemented them in a fork here, trying to show the minimum viable API surface:

The new public API surface area is fairly small but can be expanded on a per-platform basis as support is added / desired:

std::os::horizon::thread

/// Horizon-specific extension trait for thread::Builder. pub trait BuilderExt { /// Set the priority of the thread to be spawned. /// /// See https://www.3dbrew.org/wiki/Multi-threading#Threads for details /// about the meaning / valid values of the priority parameter. fn priority(self, priority: libc::c_int) -> Self; }

impl BuilderExt for crate::thread::Builder {}

std::os::linux::thread

/// Linux-specific extensions for thread::Builder. pub trait BuilderExt { /// Set the CPU affinity (which cores to run on) for the thread to be spawned. /// /// See https://man7.org/linux/man-pages/man3/CPU_SET.3.html for more details /// about how to construct the cpu_set parameter. fn affinity(self, cpu_set: libc::cpu_set_t) -> Self; }

impl BuilderExt for crate::thread::Builder {}

Draft of an implementation here:

rust-lang/rust@master...ian-h-chamberlain:rust:feature/thread-schedule-os-ext