Parallel command buffers

[Uriopass]

I find the system local command buffers very useful, however I wish there were a concurrent alternative.

I have a lot of parallelizable queries in my application where I very rarely need to actually mutate the world, for example:

• Add/delete a new component to my entity
• Mutate some ressource
• Add/delete a component to another entity

I could put the command buffer behind a mutex, then use that mutex in the parallel query to submit commands, but I feel like this is not the most optimized way (concurrent queues could be better?) or the most ergonomic.

I also can't declare my own system local ressource (I might be wrong?) so I can't develop this myself.

Anyway, tell me what you think.

[Ratysz]

There is Local<T>.

If you don't want to implement FromResources for the concurrent buffer, you can use Resources::insert_local(), although it is a bit unwieldy:

let system = my_system.system();
resources.insert_local(system.id(), concurrent_buffer);

This will improve in Bevy 0.5.

[Uriopass]

That's not very practical, I would need to import both the parallel command buffer and the command buffer in all systems, and not forget to flush it. I guess it could also be a separate system that runs before the end of each stage but I'm not sure if it's possible?

[Ratysz]

It should be possible to hide that boilerplate behind a custom SystemParam - either implement that manually, or derive it. There is no live documentation of that, though, other than this PR. Be aware that it targets main branch, not 0.4 - should still work for now, though.

I guess it could also be a separate system that runs before the end of each stage but I'm not sure if it's possible?

It's possible, but less practical: to get another system's Local<T> you need to know its SystemId. If you store the concurrent buffers in a plain resource, you'll have to reimplement some form of the mechanism that drives Local<T> in order to differentiate between systems' buffers, which will likely lead to silly stuff like Local<SystemId>.

[Uriopass]

Using a custom system param seems like it could solve the problem. Just to be sure, is it possible to fill the commands with the parallel buffer when the system ends, so I guess it'd have to run on some destructor?
Also, I don't mind waiting for another bevy update, I'm considering switching over from legion but I want to be sure that I can get the same ergonomics.

[TheRawMeatball]

Hmm, as a quick reminder in case you haven't seen: Arc<parking_lot::Mutex<Commands>> already implements SystemParam

[Ratysz]

I guess it'd have to run on some destructor?

Most likely, yes. I haven't tried to implement something like that myself, though.

Hmm, as a quick reminder in case you haven't seen: Arc<parking_lot::Mutex> already implements SystemParam

This is not the same as a concurrent buffer. It can be used as a stand-in for one, but there are better ways.

[sdfgeoff]

Idle thinking:
I wonder about the feasibility of using a MPSC queue for commands is? Chuck it in a resource, get a non-mutable reference to the resource, clone the queue and submit your command. Some other system empties the queue into the Command buffer at the end of each frame

[Uriopass]

It doesn't have the same guarantees of the command buffer, but it should work, yes :-)