asyncly VS samsara

One of the main benefits of async/await in Rust is that it can work in situations where you don't even have threads or dynamic memory. You can absolutely use it to write very concise code that's waiting on an interrupt on your microcontroller to have read some data coming in over I2C from some buffer. It's a higher level abstraction that allows your code to use concurrency (mostly) without having tons of interactions with the underlying runtime.

Every major piece of software that I have worked on has implemented this in one form or another (even in non-modern C++ where you don't have any coroutine concepts, Apple's grand central dispatch,). If you don't then your business logic will either be very imperformantly block on IO, have a gazillion of threads that make development/debugging a living hell, or be littered with implementation details of the underlying runtime or a combination of all 3.

If you don't use existing abstractions in the language (or through some library), you will end up building them yourselves, which is hard and probably overall inferior to widely used ones (if there are any). I have done so in the past, see https://github.com/goto-opensource/asyncly.

Keep in mind that these is a really basic building block where you can bring your own runtime and hook coroutines into it, not something that is at all usable out of the box. This is exacerbated by the fact that the C++ standard library is still lacking support for non-blocking futures/promises.

To see how it can be used for actual asynchronous operations on a thread pool, take a look at asyncly, which I co-authored:

https://github.com/LogMeIn/asyncly/blob/master/Test/Unit/fut...

> IME it's the other way around, per-object individual lifetimes is a rare special case

It depends on your application domain. But in most cases where objects have "individual lifetimes" you can still use reference counting, which has lower latency and memory overhead than tracing GC and interacts well with manual memory management. Tracing GC can then be "plugged in" for very specific cases, preferably using a high performance concurrent implementation much like https://github.com/chc4/samsara (for Rust) or https://github.com/pebal/sgcl (for C++).

> Just for example: "it needs a GC" could be the heart of such an argument

Rust can actually support high-performance concurrent GC, see https://github.com/chc4/samsara for an experimental implementation. But unlike other languages it gives you the option of not using it.

The compiler support you need is quite limited. Here's an implementation of cycle collection in Rust: https://github.com/chc4/samsara It's made possible because Rust can tell apart read-only and read-write references (except for interior mutable objects, but these are known to the compiler and references to them can be treated as read-write). This avoids a global stop-the-world for the entire program.

Cascading deletes are rare in practice, and if anything they are inherent to deterministic deletion, which is often a desirable property. When they're possible, one can often use arena allocation to avoid the issue altogether, since arenas are managed as a single object.

There are concurrent GC implementations for Rust, e.g. Samsara https://redvice.org/2023/samsara-garbage-collector/ https://github.com/chc4/samsara that avoid blocking, except to a minimal extent in rare cases of contention. That fits pretty well with the pattern of "doing a bit of GC every frame".

There are a number of efforts along these lines, the most interesting is probably Samsara https://github.com/chc4/samsara https://redvice.org/2023/samsara-garbage-collector/ which implements a concurrent, thread-safe GC with no global "stop the world" phase.

Nice blog post! I also wrote a concurrent reference counted cycle collector in Rust (https://github.com/chc4/samsara) though never published it to crates.io. It's neat to see the different choices that people made implementing similar goals, and dumpster works pretty differently from how I did it. I hit the same problems wrt concurrent mutation of the graph when trying to count in-degree of nodes, or adding references during a collection - I didn't even think of doing generational references and just have a RwLock...