Today, we sort all entities added to all phases, even the phases that
don't strictly need sorting, such as the opaque and shadow phases. This
results in a performance loss because our `PhaseItem`s are rather large
in memory, so sorting is slow. Additionally, determining the boundaries
of batches is an O(n) process.
This commit makes Bevy instead applicable place phase items into *bins*
keyed by *bin keys*, which have the invariant that everything in the
same bin is potentially batchable. This makes determining batch
boundaries O(1), because everything in the same bin can be batched.
Instead of sorting each entity, we now sort only the bin keys. This
drops the sorting time to near-zero on workloads with few bins like
`many_cubes --no-frustum-culling`. Memory usage is improved too, with
batch boundaries and dynamic indices now implicit instead of explicit.
The improved memory usage results in a significant win even on
unbatchable workloads like `many_cubes --no-frustum-culling
--vary-material-data-per-instance`, presumably due to cache effects.
Not all phases can be binned; some, such as transparent and transmissive
phases, must still be sorted. To handle this, this commit splits
`PhaseItem` into `BinnedPhaseItem` and `SortedPhaseItem`. Most of the
logic that today deals with `PhaseItem`s has been moved to
`SortedPhaseItem`. `BinnedPhaseItem` has the new logic.
Frame time results (in ms/frame) are as follows:
| Benchmark | `binning` | `main` | Speedup |
| ------------------------ | --------- | ------- | ------- |
| `many_cubes -nfc -vpi` | 232.179 | 312.123 | 34.43% |
| `many_cubes -nfc` | 25.874 | 30.117 | 16.40% |
| `many_foxes` | 3.276 | 3.515 | 7.30% |
(`-nfc` is short for `--no-frustum-culling`; `-vpi` is short for
`--vary-per-instance`.)
---
## Changelog
### Changed
* Render phases have been split into binned and sorted phases. Binned
phases, such as the common opaque phase, achieve improved CPU
performance by avoiding the sorting step.
## Migration Guide
- `PhaseItem` has been split into `BinnedPhaseItem` and
`SortedPhaseItem`. If your code has custom `PhaseItem`s, you will need
to migrate them to one of these two types. `SortedPhaseItem` requires
the fewest code changes, but you may want to pick `BinnedPhaseItem` if
your phase doesn't require sorting, as that enables higher performance.
## Tracy graphs
`many-cubes --no-frustum-culling`, `main` branch:
<img width="1064" alt="Screenshot 2024-03-12 180037"
src="https://github.com/bevyengine/bevy/assets/157897/e1180ce8-8e89-46d2-85e3-f59f72109a55">
`many-cubes --no-frustum-culling`, this branch:
<img width="1064" alt="Screenshot 2024-03-12 180011"
src="https://github.com/bevyengine/bevy/assets/157897/0899f036-6075-44c5-a972-44d95895f46c">
You can see that `batch_and_prepare_binned_render_phase` is a much
smaller fraction of the time. Zooming in on that function, with yellow
being this branch and red being `main`, we see:
<img width="1064" alt="Screenshot 2024-03-12 175832"
src="https://github.com/bevyengine/bevy/assets/157897/0dfc8d3f-49f4-496e-8825-a66e64d356d0">
The binning happens in `queue_material_meshes`. Again with yellow being
this branch and red being `main`:
<img width="1064" alt="Screenshot 2024-03-12 175755"
src="https://github.com/bevyengine/bevy/assets/157897/b9b20dc1-11c8-400c-a6cc-1c2e09c1bb96">
We can see that there is a small regression in `queue_material_meshes`
performance, but it's not nearly enough to outweigh the large gains in
`batch_and_prepare_binned_render_phase`.
---------
Co-authored-by: James Liu <contact@jamessliu.com>
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