smarm

SMARM — Smarm, Marks Actor Runtime Machinery. A proof-of-concept green-thread actor runtime for Rust.

Implements the core ideas in Achitecture.md: green-thread actors on a shared heap, scheduled cooperatively, communicating only by Send messages. Erlang's isolation model without Erlang's copying GC, Rust's zero-copy ownership transfers without async's function colouring.

The scheduler is multi-threaded — one OS thread per available CPU, all drawing from a shared run queue. The single-threaded run() entry point is kept as a convenience wrapper around runtime::init(Config::exact(1)).run(f).

What's here

Module	What it does
stack	mmap'd growable stack with guard page; SIGSEGV on overflow
context	#[naked] x86-64 context-switch shims, callee-saved regs only
preempt	Allocator-driven preemption; check!() macro for no-alloc loops
pid	(index, generation) PIDs; stale handles are detectable, not silent
actor	Trampoline + catch_unwind boundary at the actor entry point
scheduler	Run queue, slot table, spawn/join, parking, idle path
channel	Unbounded MPSC channel; recv parks the actor
mutex	Mutex<T> with mandatory timeout; FIFO waiters; parks the green thread
timer	Min-heap of (deadline, reason); Sleep and WaitTimeout reasons
io	block_on_io for blocking work; wait_readable/wait_writable + read/write via epoll
supervisor	Signal::Exit / Signal::Panic delivered to a parent actor's mailbox

Quick taste

use smarm::{run, spawn, channel};

run(|| {
    let (tx, rx) = channel::<i64>();
    let h = spawn(move || {
        for _ in 0..3 {
            let v = rx.recv().unwrap();
            println!("got {v}");
        }
    });
    for v in 1..=3i64 {
        tx.send(v).unwrap();
    }
    h.join().unwrap();
});

Layout

src/
  stack.rs context.rs preempt.rs pid.rs actor.rs
  scheduler.rs channel.rs mutex.rs timer.rs io.rs supervisor.rs
  lib.rs
tests/
  per-module integration tests
benches/
  primes.rs    fan-out/fan-in compute, vs tokio current_thread

Building and running

Standard Cargo. Requires Rust 1.95 or newer (the #[naked] attribute went stable in 1.88; we use a few unrelated post-1.88 features). x86-64 Linux only — ARM64 and macOS are on the deferred list because of the assembly shim and the epoll dependency.

cargo test                # all tests
cargo test --test mutex   # one module
cargo bench               # primes benchmark vs tokio

What's not here

See the Defer section of Architecture.md. restart-intensity caps, join! for handle groups, stack growth via remap, hierarchical timer wheel, fd-wait timeouts, Signal::Timeout. Each is mechanism we know how to add; none belongs in this iteration.

Docs

Document	What it covers
Architecture.md	Design intent, runtime model, and deferred work
smarm - Deep Dive.html	Generated walkthrough of the system; good starting point
BENCHMARKS_AND_TUNING.md	Where smarm wins and loses vs tokio, preemption knob recommendations
benchmarks.md	Raw benchmark results, methodology, and tuning experiment log

Contributing

This is a personal proof-of-concept. There's no PR workflow — if you fork it and do something interesting, just send me an email. I'd genuinely like to hear about it.

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_{The name is a recursive acronym. The M is for Marks, as in the BEAM — Bogdan/Björn's Erlang Abstract Machine, the virtual machine that runs Erlang and Elixir. smarm is not the BEAM. It just admires it from a safe distance.}