Information regarding multithreading

[Bvsemmer]

Hello!

I am exploring Flecs C++ for a small pet project and I am trying to collect information specifically about multithreading. There is some information about threading systems. But I have some more questions that I cannot find clear information about:

• Does the scheduler have support for running whole systems in parallel? The focus seems mainly on parallelizing groups of entities within a system. Since Flecs knows the query access patterns, it could run systems that do not share mutable access to the same component types at the same time as well. Is this correct?
• In see some discussions here that ask about the safety of creating Entities from multiple threads on the same world. The answer seems to be that it's safe, but when looking at the code this seems to be only true when the EcsWorldMultiThreaded flag is raised. But I find very little info about this flag. Even a simple Google search does not bring me anything. Not even how I can create a world with this flag raised. Is this flag enough to make the world thread safe? Any other considerations I need to take regarding this?
• How are changes to components and entities deferred exactly? I assume it's not safe to detach components for example from workerthreads, are these deferred to somewhere later in the frame? How is synchronization happening exactly within Flecs?
• A question in the same vain: when will newly created entities (on worker threads) be part of queries? Within the same frame or only the beginning of next world update?

Thank you very much for any information about this!

[SanderMertens]

Does the scheduler have support for running whole systems in parallel?

No, the Flecs scheduler does not schedule individual systems. It is as you said: each multithreaded system is run on all threads, and entities get divided across threads. IME individual systems are a bad scheduling primitive since they are too small & add too much scheduling overhead in large applications.

In see some discussions here that ask about the safety of creating Entities from multiple threads on the same world

This is not possible at the moment. You can only create entities from single threaded systems.

How are changes to components and entities deferred exactly?

Mutations are added to a command queue per thread. During sync points all commands from all threads are merged. There is no synchronization between threads while systems are running.

A question in the same vain: when will newly created entities (on worker threads) be part of queries?

You can't create entities from multiple threads- but components created from threads will become available after the next sync point which by default is the end of the frame.

[Bvsemmer]

Thank you very much, Sander. These are clear answers. Can you tell a bit more about the sync points? When are these exactly?

So, I keep in mind that entities cannot be created from other threads but components can.

Is there by any chance any machinery in place to prepare a separate world in a workerthread and then merge it with the main world on the main thread?

[SanderMertens]

Can you tell a bit more about the sync points?

More info here: https://www.flecs.dev/flecs/md_docs_2Systems.html#sync-points

The explorer & logging level 1 will show you when sync points happen.

Is there by any chance any machinery in place to prepare a separate world in a workerthread and then merge it with the main world on the main thread?

There's no builtin functionality for copying data between worlds.