samsara

A slightly newer attempt which is concurrent - in fact, it aims to implement the algorithm discussed in OP, though there may be some subtle differences in implementation: https://github.com/chc4/samsara

I have often thought that programmers can actually just choose to make Rust easy by using a cyclic garbage collector such as Samsara. [1] If cyclic GC in Rust works as well as I think it can, it should be the best option for the majority of high level projects that need fast development with a trade-off of slightly lower efficiency. I suspect we'll see a "hockey stick" adoption curve once everyone figures this out. [1] https://github.com/chc4/samsara

There's also more recent implementations such as https://github.com/chc4/samsara (quite similar to https://github.com/pebal/sgcl , which is for C++. Both implement a concurrent tracing algorithm which allows for reduced latency compared to a naïve stop-the-world GC).

Interesting discussion. If you want to experiment with 'lightweight' traced GC for systems programming, you might like https://github.com/chc4/samsara (for Rust) or https://github.com/pebal/sgcl (for C++) both implementing a concurrent tracing algorithm which is a bit more involved than what's directly discussed in OP.

https://github.com/chc4/samsara is also worth looking into, implementing the same concurrent GC algorithm for Rust. A C++ implementation is available at https://github.com/pebal/sgcl .

> 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++).

Yes, garbage collection seems to be the only viable solution for dealing with spaghetti reference graphs in their full generality, including possible cycles. In that context it's worth trying a high-performance concurrent GC implementation such as https://github.com/chc4/samsara Samsara.

> 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 in practice is quite limited. There is an implementation of incremental 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 references to 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 allocation.

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". (Note that doing GC over large object graphs will nonetheless involve significant overhead, even with efficient implementations as seen here; GC is not at all a silver bullet, and should be avoided if at all possible. The actual point of GC is to enable computing over highly general, possibly cyclical object graphs - if that doesn't apply, other memory management strategies can be used instead.)

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, https://redvice.org/2023/samsara-garbage-collector/) 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...