Unload render assets from RAM (#10520)

# Objective
- No point in keeping Meshes/Images in RAM once they're going to be sent
to the GPU, and kept in VRAM. This saves a _significant_ amount of
memory (several GBs) on scenes like bistro.
- References
  - #1782
  - #8624 

## Solution
- Augment RenderAsset with the capability to unload the underlying asset
after extracting to the render world.
- Mesh/Image now have a cpu_persistent_access field. If this field is
RenderAssetPersistencePolicy::Unload, the asset will be unloaded from
Assets<T>.
- A new AssetEvent is sent upon dropping the last strong handle for the
asset, which signals to the RenderAsset to remove the GPU version of the
asset.

---

## Changelog
- Added `AssetEvent::NoLongerUsed` and
`AssetEvent::is_no_longer_used()`. This event is sent when the last
strong handle of an asset is dropped.
- Rewrote the API for `RenderAsset` to allow for unloading the asset
data from the CPU.
- Added `RenderAssetPersistencePolicy`.
- Added `Mesh::cpu_persistent_access` for memory savings when the asset
is not needed except for on the GPU.
- Added `Image::cpu_persistent_access` for memory savings when the asset
is not needed except for on the GPU.
- Added `ImageLoaderSettings::cpu_persistent_access`.
- Added `ExrTextureLoaderSettings`.
- Added `HdrTextureLoaderSettings`.

## Migration Guide
- Asset loaders (GLTF, etc) now load meshes and textures without
`cpu_persistent_access`. These assets will be removed from
`Assets<Mesh>` and `Assets<Image>` once `RenderAssets<Mesh>` and
`RenderAssets<Image>` contain the GPU versions of these assets, in order
to reduce memory usage. If you require access to the asset data from the
CPU in future frames after the GLTF asset has been loaded, modify all
dependent `Mesh` and `Image` assets and set `cpu_persistent_access` to
`RenderAssetPersistencePolicy::Keep`.
- `Mesh` now requires a new `cpu_persistent_access` field. Set it to
`RenderAssetPersistencePolicy::Keep` to mimic the previous behavior.
- `Image` now requires a new `cpu_persistent_access` field. Set it to
`RenderAssetPersistencePolicy::Keep` to mimic the previous behavior.
- `MorphTargetImage::new()` now requires a new `cpu_persistent_access`
parameter. Set it to `RenderAssetPersistencePolicy::Keep` to mimic the
previous behavior.
- `DynamicTextureAtlasBuilder::add_texture()` now requires that the
`TextureAtlas` you pass has an `Image` with `cpu_persistent_access:
RenderAssetPersistencePolicy::Keep`. Ensure you construct the image
properly for the texture atlas.
- The `RenderAsset` trait has significantly changed, and requires
adapting your existing implementations.
  - The trait now requires `Clone`.
- The `ExtractedAsset` associated type has been removed (the type itself
is now extracted).
  - The signature of `prepare_asset()` is slightly different
- A new `persistence_policy()` method is now required (return
RenderAssetPersistencePolicy::Unload to match the previous behavior).
- Match on the new `NoLongerUsed` variant for exhaustive matches of
`AssetEvent`.

crates/bevy_asset/src/assets.rs

@@ -1,5 +1,7 @@
-use crate::{self as bevy_asset, LoadState};
-use crate::{Asset, AssetEvent, AssetHandleProvider, AssetId, AssetServer, Handle, UntypedHandle};
+use crate::{self as bevy_asset};
+use crate::{
+    Asset, AssetEvent, AssetHandleProvider, AssetId, AssetServer, Handle, LoadState, UntypedHandle,
+};
 use bevy_ecs::{
     prelude::EventWriter,
     system::{Res, ResMut, Resource},
@@ -484,9 +486,7 @@ impl<A: Asset> Assets<A> {
     }
 
     /// A system that synchronizes the state of assets in this collection with the [`AssetServer`]. This manages
-    /// [`Handle`] drop events and adds queued [`AssetEvent`] values to their [`Events`] resource.
-    ///
-    /// [`Events`]: bevy_ecs::event::Events
+    /// [`Handle`] drop events.
     pub fn track_assets(mut assets: ResMut<Self>, asset_server: Res<AssetServer>) {
         let assets = &mut *assets;
         // note that we must hold this lock for the entire duration of this function to ensure
@@ -496,24 +496,28 @@ impl<A: Asset> Assets<A> {
         let mut infos = asset_server.data.infos.write();
         let mut not_ready = Vec::new();
         while let Ok(drop_event) = assets.handle_provider.drop_receiver.try_recv() {
-            let id = drop_event.id;
+            let id = drop_event.id.typed();
+
+            assets.queued_events.push(AssetEvent::Unused { id });
+
             if drop_event.asset_server_managed {
-                let untyped = id.untyped(TypeId::of::<A>());
-                if let Some(info) = infos.get(untyped) {
+                let untyped_id = drop_event.id.untyped(TypeId::of::<A>());
+                if let Some(info) = infos.get(untyped_id) {
                     if info.load_state == LoadState::Loading
                         || info.load_state == LoadState::NotLoaded
                     {
                         not_ready.push(drop_event);
                         continue;
                     }
                 }
-                if infos.process_handle_drop(untyped) {
-                    assets.remove_dropped(id.typed());
+                if infos.process_handle_drop(untyped_id) {
+                    assets.remove_dropped(id);
                 }
             } else {
-                assets.remove_dropped(id.typed());
+                assets.remove_dropped(id);
             }
         }
+
         // TODO: this is _extremely_ inefficient find a better fix
         // This will also loop failed assets indefinitely. Is that ok?
         for event in not_ready {

crates/bevy_asset/src/event.rs

@@ -11,6 +11,8 @@ pub enum AssetEvent<A: Asset> {
     Modified { id: AssetId<A> },
     /// Emitted whenever an [`Asset`] is removed.
     Removed { id: AssetId<A> },
+    /// Emitted when the last [`super::Handle::Strong`] of an [`Asset`] is dropped.
+    Unused { id: AssetId<A> },
     /// Emitted whenever an [`Asset`] has been fully loaded (including its dependencies and all "recursive dependencies").
     LoadedWithDependencies { id: AssetId<A> },
 }
@@ -35,6 +37,11 @@ impl<A: Asset> AssetEvent<A> {
     pub fn is_removed(&self, asset_id: impl Into<AssetId<A>>) -> bool {
         matches!(self, AssetEvent::Removed { id } if *id == asset_id.into())
     }
+
+    /// Returns `true` if this event is [`AssetEvent::Unused`] and matches the given `id`.
+    pub fn is_unused(&self, asset_id: impl Into<AssetId<A>>) -> bool {
+        matches!(self, AssetEvent::Unused { id } if *id == asset_id.into())
+    }
 }
 
 impl<A: Asset> Clone for AssetEvent<A> {
@@ -51,6 +58,7 @@ impl<A: Asset> Debug for AssetEvent<A> {
             Self::Added { id } => f.debug_struct("Added").field("id", id).finish(),
             Self::Modified { id } => f.debug_struct("Modified").field("id", id).finish(),
             Self::Removed { id } => f.debug_struct("Removed").field("id", id).finish(),
+            Self::Unused { id } => f.debug_struct("Unused").field("id", id).finish(),
             Self::LoadedWithDependencies { id } => f
                 .debug_struct("LoadedWithDependencies")
                 .field("id", id)
@@ -65,6 +73,7 @@ impl<A: Asset> PartialEq for AssetEvent<A> {
             (Self::Added { id: l_id }, Self::Added { id: r_id })
             | (Self::Modified { id: l_id }, Self::Modified { id: r_id })
             | (Self::Removed { id: l_id }, Self::Removed { id: r_id })
+            | (Self::Unused { id: l_id }, Self::Unused { id: r_id })
             | (
                 Self::LoadedWithDependencies { id: l_id },
                 Self::LoadedWithDependencies { id: r_id },

crates/bevy_asset/src/handle.rs

@@ -124,7 +124,7 @@ impl std::fmt::Debug for StrongHandle {
 #[reflect(Component)]
 pub enum Handle<A: Asset> {
     /// A "strong" reference to a live (or loading) [`Asset`]. If a [`Handle`] is [`Handle::Strong`], the [`Asset`] will be kept
-    /// alive until the [`Handle`] is dropped. Strong handles also provide access to additional asset metadata.  
+    /// alive until the [`Handle`] is dropped. Strong handles also provide access to additional asset metadata.
     Strong(Arc<StrongHandle>),
     /// A "weak" reference to an [`Asset`]. If a [`Handle`] is [`Handle::Weak`], it does not necessarily reference a live [`Asset`],
     /// nor will it keep assets alive.
@@ -189,7 +189,7 @@ impl<A: Asset> Handle<A> {
 
     /// Converts this [`Handle`] to an "untyped" / "generic-less" [`UntypedHandle`], which stores the [`Asset`] type information
     /// _inside_ [`UntypedHandle`]. This will return [`UntypedHandle::Strong`] for [`Handle::Strong`] and [`UntypedHandle::Weak`] for
-    /// [`Handle::Weak`].  
+    /// [`Handle::Weak`].
     #[inline]
     pub fn untyped(self) -> UntypedHandle {
         self.into()

crates/bevy_asset/src/lib.rs

@@ -100,7 +100,7 @@ pub enum AssetMode {
     ///
     /// When developing an app, you should enable the `asset_processor` cargo feature, which will run the asset processor at startup. This should generally
     /// be used in combination with the `file_watcher` cargo feature, which enables hot-reloading of assets that have changed. When both features are enabled,
-    /// changes to "original/source assets" will be detected, the asset will be re-processed, and then the final processed asset will be hot-reloaded in the app.  
+    /// changes to "original/source assets" will be detected, the asset will be re-processed, and then the final processed asset will be hot-reloaded in the app.
     ///
     /// [`AssetMeta`]: meta::AssetMeta
     /// [`AssetSource`]: io::AssetSource
@@ -872,13 +872,23 @@ mod tests {
                 id: id_results.d_id,
             },
             AssetEvent::Modified { id: a_id },
+            AssetEvent::Unused { id: a_id },
             AssetEvent::Removed { id: a_id },
+            AssetEvent::Unused {
+                id: id_results.b_id,
+            },
             AssetEvent::Removed {
                 id: id_results.b_id,
             },
+            AssetEvent::Unused {
+                id: id_results.c_id,
+            },
             AssetEvent::Removed {
                 id: id_results.c_id,
             },
+            AssetEvent::Unused {
+                id: id_results.d_id,
+            },
             AssetEvent::Removed {
                 id: id_results.d_id,
             },
@@ -1062,7 +1072,11 @@ mod tests {
         // remove event is emitted
         app.update();
         let events = std::mem::take(&mut app.world.resource_mut::<StoredEvents>().0);
-        let expected_events = vec![AssetEvent::Added { id }, AssetEvent::Removed { id }];
+        let expected_events = vec![
+            AssetEvent::Added { id },
+            AssetEvent::Unused { id },
+            AssetEvent::Removed { id },
+        ];
         assert_eq!(events, expected_events);
 
         let dep_handle = app.world.resource::<AssetServer>().load(dep_path);

crates/bevy_core_pipeline/src/tonemapping/mod.rs

@@ -6,7 +6,7 @@ use bevy_reflect::Reflect;
 use bevy_render::camera::Camera;
 use bevy_render::extract_component::{ExtractComponent, ExtractComponentPlugin};
 use bevy_render::extract_resource::{ExtractResource, ExtractResourcePlugin};
-use bevy_render::render_asset::RenderAssets;
+use bevy_render::render_asset::{RenderAssetPersistencePolicy, RenderAssets};
 use bevy_render::render_resource::binding_types::{
     sampler, texture_2d, texture_3d, uniform_buffer,
 };
@@ -356,6 +356,7 @@ fn setup_tonemapping_lut_image(bytes: &[u8], image_type: ImageType) -> Image {
         CompressedImageFormats::NONE,
         false,
         image_sampler,
+        RenderAssetPersistencePolicy::Unload,
     )
     .unwrap()
 }
@@ -381,5 +382,6 @@ pub fn lut_placeholder() -> Image {
         },
         sampler: ImageSampler::Default,
         texture_view_descriptor: None,
+        cpu_persistent_access: RenderAssetPersistencePolicy::Unload,
     }
 }

crates/bevy_gizmos/src/lib.rs

@@ -51,7 +51,10 @@ use bevy_render::{
     color::Color,
     extract_component::{ComponentUniforms, DynamicUniformIndex, UniformComponentPlugin},
     primitives::Aabb,
-    render_asset::{PrepareAssetError, RenderAsset, RenderAssetPlugin, RenderAssets},
+    render_asset::{
+        PrepareAssetError, RenderAsset, RenderAssetPersistencePolicy, RenderAssetPlugin,
+        RenderAssets,
+    },
     render_phase::{PhaseItem, RenderCommand, RenderCommandResult, TrackedRenderPass},
     render_resource::{
         binding_types::uniform_buffer, BindGroup, BindGroupEntries, BindGroupLayout,
@@ -365,28 +368,25 @@ struct GpuLineGizmo {
 }
 
 impl RenderAsset for LineGizmo {
-    type ExtractedAsset = LineGizmo;
-
     type PreparedAsset = GpuLineGizmo;
-
     type Param = SRes<RenderDevice>;
 
-    fn extract_asset(&self) -> Self::ExtractedAsset {
-        self.clone()
+    fn persistence_policy(&self) -> RenderAssetPersistencePolicy {
+        RenderAssetPersistencePolicy::Unload
     }
 
     fn prepare_asset(
-        line_gizmo: Self::ExtractedAsset,
+        self,
         render_device: &mut SystemParamItem<Self::Param>,
-    ) -> Result<Self::PreparedAsset, PrepareAssetError<Self::ExtractedAsset>> {
-        let position_buffer_data = cast_slice(&line_gizmo.positions);
+    ) -> Result<Self::PreparedAsset, PrepareAssetError<Self>> {
+        let position_buffer_data = cast_slice(&self.positions);
         let position_buffer = render_device.create_buffer_with_data(&BufferInitDescriptor {
             usage: BufferUsages::VERTEX,
             label: Some("LineGizmo Position Buffer"),
             contents: position_buffer_data,
         });
 
-        let color_buffer_data = cast_slice(&line_gizmo.colors);
+        let color_buffer_data = cast_slice(&self.colors);
         let color_buffer = render_device.create_buffer_with_data(&BufferInitDescriptor {
             usage: BufferUsages::VERTEX,
             label: Some("LineGizmo Color Buffer"),
@@ -396,8 +396,8 @@ impl RenderAsset for LineGizmo {
         Ok(GpuLineGizmo {
             position_buffer,
             color_buffer,
-            vertex_count: line_gizmo.positions.len() as u32,
-            strip: line_gizmo.strip,
+            vertex_count: self.positions.len() as u32,
+            strip: self.strip,
         })
     }
 }

crates/bevy_gltf/src/loader.rs

@@ -22,6 +22,7 @@ use bevy_render::{
     },
     prelude::SpatialBundle,
     primitives::Aabb,
+    render_asset::RenderAssetPersistencePolicy,
     render_resource::{Face, PrimitiveTopology},
     texture::{
         CompressedImageFormats, Image, ImageAddressMode, ImageFilterMode, ImageLoaderSettings,
@@ -120,7 +121,7 @@ pub struct GltfLoader {
 ///     |s: &mut GltfLoaderSettings| {
 ///         s.load_cameras = false;
 ///     }
-/// );    
+/// );
 /// ```
 #[derive(Serialize, Deserialize)]
 pub struct GltfLoaderSettings {
@@ -389,7 +390,7 @@ async fn load_gltf<'a, 'b, 'c>(
             let primitive_label = primitive_label(&gltf_mesh, &primitive);
             let primitive_topology = get_primitive_topology(primitive.mode())?;
 
-            let mut mesh = Mesh::new(primitive_topology);
+            let mut mesh = Mesh::new(primitive_topology, RenderAssetPersistencePolicy::Unload);
 
             // Read vertex attributes
             for (semantic, accessor) in primitive.attributes() {
@@ -433,6 +434,7 @@ async fn load_gltf<'a, 'b, 'c>(
                     let morph_target_image = MorphTargetImage::new(
                         morph_target_reader.map(PrimitiveMorphAttributesIter),
                         mesh.count_vertices(),
+                        RenderAssetPersistencePolicy::Unload,
                     )?;
                     let handle =
                         load_context.add_labeled_asset(morph_targets_label, morph_target_image.0);
@@ -724,6 +726,7 @@ async fn load_image<'a, 'b>(
                 supported_compressed_formats,
                 is_srgb,
                 ImageSampler::Descriptor(sampler_descriptor),
+                RenderAssetPersistencePolicy::Unload,
             )?;
             Ok(ImageOrPath::Image {
                 image,
@@ -745,6 +748,7 @@ async fn load_image<'a, 'b>(
                         supported_compressed_formats,
                         is_srgb,
                         ImageSampler::Descriptor(sampler_descriptor),
+                        RenderAssetPersistencePolicy::Unload,
                     )?,
                     label: texture_label(&gltf_texture),
                 })

crates/bevy_pbr/src/material.rs

@@ -822,6 +822,7 @@ pub fn extract_materials<M: Material>(
     let mut changed_assets = HashSet::default();
     let mut removed = Vec::new();
     for event in events.read() {
+        #[allow(clippy::match_same_arms)]
         match event {
             AssetEvent::Added { id } | AssetEvent::Modified { id } => {
                 changed_assets.insert(*id);
@@ -830,6 +831,7 @@ pub fn extract_materials<M: Material>(
                 changed_assets.remove(id);
                 removed.push(*id);
             }
+            AssetEvent::Unused { .. } => {}
             AssetEvent::LoadedWithDependencies { .. } => {
                 // TODO: handle this
             }