Improve the performance of EntityComponentManager::Each

[adlarkin]

Desired behavior

As discussed in #678 (comment), it seems like adding more components to an ecm.Each call reduces performance. Considering how often we call this method in ign-gazebo, it would be nice to optimize ecm.Each so that performance is the same regardless of the number of components used.

Alternatives considered

TBD

Implementation suggestion

See #711 (comment)

Additional context

Could be related to #429

[adlarkin]

After some further investigation, I believe that the views are responsible for most of the performance in ecm.Each. The issue is that the views aren't really working as a true "cache". Once we have the view, we have to find the components in the view that correspond to the current entity, every single time we want to use an entity (view.Component<ComponentTypeTs>(entity, this)...): https://github.com/ignitionrobotics/ign-gazebo/blob/ignition-gazebo5_5.0.0/include/ignition/gazebo/detail/EntityComponentManager.hh#L386-L394

In reality, we shouldn't have to do this, because the view should already have these components cached for every entity that has them. I think it could be good to change how components are saved in a view. Currently, we store components in a view using a std::map, with a std::pair being the key of this map: https://github.com/ignitionrobotics/ign-gazebo/blob/ignition-gazebo3_3.8.0/include/ignition/gazebo/detail/View.hh#L129-L131

I tried to put a band-aid on the drawbacks of using a std::map with a key of std::pair in #752, but in reality, I believe that a better solution would be to store something like a table of component data in a view. This table would have the columns be the components, and the rows be entities - then, when it's time to use this view with an entity, the entity would simply index the proper row of data in the view's table, instead of having to find each entity's component individually.

If we were to take this approach, I believe that it would be crucial to create the following mappings (these mappings need to be fast):

• Entity ID to its row index in a view's table (we can't just assume that the Entity ID is the row ID in a view's table)
• A set of components that make up a view to the ID of the view. Right now, it looks like we have no mapping or hash function for this, which means that comparing two sets of components (when searching for views) could be slow:
  • https://github.com/ignitionrobotics/ign-gazebo/blob/ignition-gazebo3_3.8.0/src/EntityComponentManager.cc#L110-L111
  • https://github.com/ignitionrobotics/ign-gazebo/blob/ignition-gazebo3_3.8.0/include/ignition/gazebo/detail/View.hh#L37-L38

A part of this proposed change/enhancement may involve changing how we store component IDs. Right now, we scope component IDs relative to their type (I'm not sure if this is an issue or not; I just wanted to point this out as something that we may need to consider when making changes): https://github.com/ignitionrobotics/ign-gazebo/blob/ignition-gazebo5_5.0.0/include/ignition/gazebo/Types.hh#L78-L86

[chapulina]

Done in #856 🎉