memory and cpu abnormal raise when use complex relations about flecs HOT 2 CLOSED

If you connect your application to the explorer, you'll see that this is due to a growth in the number of empty tables:

The reason this is happening is because of these three lines:

attack.add<CastOf>(e);
attack.add<Target>(monsters[rand()%monsters.size()]);
e.add<Damage>(monsters[rand()%monsters.size()]);

You have 1000 monsters, which means there are in total 1000 * 1000 * 1000 unique combinations of pairs. Unique combinations of components (and pairs) create tables in Flecs. So given enough time, your application will create 1 billion tables, which is a lot.

The memory growth can be solved by periodically cleaning up empty tables, which you can do by adding the following system to your application:

    world.system()
      .interval(0.1)
      .no_readonly()
      .iter([](flecs::iter& it){
        ecs_delete_empty_tables(it.world(), 0, 1, 1, 0, 0);
      });

That doesn't solve the high CPU issue. Here there are a few things you can improve:

• The system that prints the attack count creates a filter each frame, which is a bit wasteful. Filters are also not as fast as queries, so to speed this up you can do this instead:

    flecs::query<Attack> q = world.query<Attack>();

    world.set<Root>({0});
    world.system<Root>()
        .each([=](flecs::entity e, Root& root){
            // ...
            std::cout << "tick : attack count" << q.count() << std::endl;
    });

• The CastOf, Target and Attack relationships fragment your attack entities to the point where you only have a single attack entity per table. To fix this, you can turn these into union relationships (docs on unions):

    world.component<Attack>().add(flecs::Union);
    world.component<Damage>().add(flecs::Union);
    world.component<CastOf>().add(flecs::Union);

You could also consider making them regular components, like:

struct Attack {
  flecs::entity entity;
};

After these fixes, I can easily run the application at ~1000 FPS.

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Thank you very much!

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