`specialized_material_pipeline_cache` grows forever, causing performance to greatly degrade over time

[Satellile]

Bevy 0.17.3

Features: file_watcher, mp3, serialize, dynamic_linking

Windows 10

This "performance leak" began occurring for me in Bevy 0.16, but persists to Bevy 0.17.3.

The Bug

Over time, specialized_material_pipeline_cache.len() grows, increasing with no limit.

Each level in my game has a ton of mesh entities, which are all despawned upon unloading the level. Leaving and reloading one level a handful of times can drop FPS from ~230 to below 60, even though the entity count remains the same. Performance continues dropping the more I do it.

It seems very similar to #21526, but the issue wasn't fixed for me with 0.17.3. When I print specialized_material_pipeline_cache.len() in the specialize_material_meshes system, I can witness it growing in a similar manner, and the functions that actually start taking up frametime is similarly specialize_material_meshes and queue_material_meshes.

My game uses two cameras, but removing the second camera didn't stop this from occurring.

Workaround

I'm using a local clone of Bevy 0.17.3, where I made a change to bevy_pbr\src\material.rs around line ~870:

Before:

        entity_specialization_ticks.remove(&MainEntity::from(entity));
        for view in views {
            if let Some(cache) =
                specialized_material_pipeline_cache.get_mut(&view.retained_view_entity)
            {
                cache.remove(&MainEntity::from(entity));
            }
            if let Some(cache) = specialized_prepass_material_pipeline_cache
                .as_mut()
                .and_then(|c| c.get_mut(&view.retained_view_entity))
            {
                cache.remove(&MainEntity::from(entity));
            }
           [...]
        }

After:

        for (_, cache) in specialized_material_pipeline_cache.iter_mut() {
            cache.remove(&MainEntity::from(entity));
        }

        entity_specialization_ticks.remove(&MainEntity::from(entity));
        for view in views {
            if let Some(cache) = specialized_prepass_material_pipeline_cache
                .as_mut()
                .and_then(|c| c.get_mut(&view.retained_view_entity))
            {
                cache.remove(&MainEntity::from(entity));
            }
           [...]
        }

This seems to have fully fixed the issue.

It caused me no immediately apparent performance loss.

However I have no idea what cold specialization is, nor do I know what exactly I've done. Just kinda seemed like the growth was a problem with it not being cleaned out so I thought iterating through it all and deleting everything unconditionally seemed good and wouldnt have consequences, um, so that's why I didn't make this a PR.

In my game, most of the times I'll despawn a mesh/entity is when I'm despawning almost everything and loading a new map, and I'm not sure if I ever modify a mesh/material after creating it, so perhaps that's why this workaround isn't causing me immediate issues.

[IceSentry]

Can you try and see if you can replicate it on bevy main? IIRC this has changed a bit since 0.17