Question ECS Escaping from Structural Changes - Filtering Entities

In ECS, if you wanna do filtering based on some state, using tags is the way to go.
This can make the filtering of the entities really fast but it also has downsides, like Structural Changes.

When we talk about Structural Changes, people say, "make structural changes only on user input" or stuff like that. So it doesn't happen every frame.

But let's say you have an alive world or something like that, like you need to do Structural Change even if the player is not giving inputs. Then as you have more and more entities that has to do these Structural Changes time and time in order to work and filtered by the Systems, you start to have more and more Structural Changes happening on a every frame.

One approach would be using flags with boolean or enums instead of tags. But the problem with this is, if you have many different flags then your systems mostly gonna have to skip many entities that it has to do nothing with it.

What other approaches do have to this problem, without Structural Changes while also keeping ScheduleParallel?
Something like using flags but without it's downsides(like having no filter at all).
We can sacrifice CPU caching.
I was thinking about what can be done with the new Entities.WithFilter(NativeArray<Entitiy>).

 

read about per component enable flag that will be in ECS 1.0
It is way to go but only in 1.0

 

ok i did some tests
so turns out random memory access is slower than having many structural changes every frame
so using tags is the way to go

 

so using tags is the way to go
and then when the feature `Jes28` mentioned arrives https://forum.unity.com/threads/dots-skill-system-repo-available.894007/#post-5875804
you can convert addComponent/removeComponent for tags and data to enable/disable component

 

Interested in your test case.
Random access far out-performs structural changes.
Either 0.50 has drastically improved structural changes or something else is going on in your test. Do you have a dynamic buffer in the query? Could explain it.
The way to go until 1.0 hit is having bool fields and checking in your systems. This was the suggestion from Joachim when this was a hot topic.

 

You are right.
But.
Also, I still need to remove the Entity from the array. I first thought i can use RemoveAtSwapBack
But can't because there are multiple stuff to remove every frame and RemoveAtSwapBack mixes the list.
So I have to use RemoveAt which is not ideal.

So, iterating through a NativeList is not slow by itself but when we add other stuff to make it functional its slower than the structural changes method.

NativeList: ~7-8ms
Structural Changes: ~4-5ms
Using boolean: < 1ms

 

Do you remove from the NativeList in a burst method or bursted job? Utilizing NativeContainers in normal main thread is very slow and it's not intuitive to know about this performance characteristic.

 

I think it depends on how frequent the state changes, and if the state changes are permanent.

For example including a Dead tag could be more performant since Dead entities isn't likely to come back, and stay dead, so instead using a flag we could sort by it and it would be faster.

But something like AI state, which could be changed every frame, would cause a lot of structural changes and performance could be worse, so you check it each frame

There is no simple solution, either sort by type it or check the type