no-std-async-experiments

Experiments in no_std cooperative multitasking

This repository contains two executors: futures-executor and generator-executor. The former uses the core::{future,task} machinery; the latter builds on top of the Generator trait.

Both executors support running up to N -- for simplicity, we currently hardcode N = 8 -- concurrent "infinite" tasks. Both executors will sleep (see WFI and WFE instructions) the microcontroller when no task can make progress. Interrupts, and other tasks, can wake up other tasks using a lightweight signaling mechanism (see the cortex-m-signal crate).

Each executor has a simple example (futures-app and generator-app) which you can run on your laptop / PC using QEMU. You can run the examples using these commands:

$ # install `qemu-system-arm`
$ sudo apt-get install qemu-system-arm

$ cd futures-app
$ cargo run --release
T2
T1
T1
T2
T1
T1
T2
T1
T1

Observations

The generator-executor has less levels of indirection and produces smaller binaries. I haven't measure execution time on a device but I expect it'll run faster than futures-executor.

I expect that async / await functionality will be possible to implement on top of generator-executor without any sort of TLS (Thread Local Storage) or static variable. (async fn uses future::from_generator which internally uses a thread-local variable for the waker).

TODO list

(but don't hold your breath)

• Prototype out of tree async / await functionality.

• Figure out how to make an interrupt wake up a task without context switching to the interrupt handler. (Can we use interrupt masking and NVIC.ISPR, somehow?)

• Support for finite tasks, but these need Box-ing.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.