sgcl VS samsara

The SGCL repository contains the source code for this benchmark that uses the tracked pointers: https://github.com/pebal/sgcl/blob/main/examples/treap/treap...

You can look at the SGCL garbage collector for C++: https://github.com/pebal/sgcl. It works in a separate thread, is locks-free and never stops the world.

It will never be called gc_ptr because C++ programmers have an allergy to the term GC. However, an attempt was made to implement a similar solution. Take a look at tracked_ptr: https://github.com/pebal/sgcl

SGCL is a real-time garbage collector for C++ without any pauses.

https://github.com/pebal/sgcl

Removed GC support was useless. You can have GC pointers in C++.

Mutators are threads that allocate memory and manipulate pointers, they can work completely independently of the GC. A mutator needs only to tag an object when copies or moves a pointer to this object. The GC detects this tag and marks the object as alive. Here is a working implementation for C++: https://github.com/pebal/sgcl

I don't know Rust but you can have a pauseless GC in C++. You just need to provide asynchronous access to root pointers.

> 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...