Exploring Multicore Concurrency for Embedded Swift

[FranzBusch]

This is exciting to follow and amazing progress so far.

@Gonzalo_Larralde

• Explore a small family of executor implementations for platforms without libdispatch. The existing CooperativeGlobalExecutor already covers the single-threaded, cooperatively drained case. Two additional models seem useful: a fixed-worker executor for systems with a known number of cores or execution contexts, where a fixed number of long-lived workers consume jobs from shared queues; and a thread-backed executor for platforms that provide basic thread creation and synchronization but no higher-level scheduling library. Both need enough machinery for queueing, wakeups, priorities, and timers, but should remain substantially smaller than reimplementing or embedding libdispatch. The goal would be a few adaptable executor models spanning single-threaded, fixed-core, and general threaded environments.

• Explore a wasi-threads executor. This is platform-specific and likely involves shared memory, Wasm atomics, worker startup, and TLS. Some executor design lessons may overlap with bare metal, although the host-provided threading model is quite different.

For those two in particular, I would love if we could add them to GitHub - swiftlang/swift-platform-executors: This package provides platform-native executors for Swift Concurrency. · GitHub. The repo is intended to provide executors for Swift's supported platforms. Ideally, at some point in the future we would start using them as the default executors for Swift. I am mostly focused on the Linux executors here which are implemented without any Dispatch dependency using pthreads. Decoupling the Concurrency runtime entirely from Dispatch is a larger thing though and requires exposing more hooks as protocols that executors can implement such as the ongoing [Pitch 4] Delayed Enqueuing for Executors.