# Objective
Fix a performance regression in the "[bevy vs
pixi](https://github.com/SUPERCILEX/bevy-vs-pixi)" benchmark.
This benchmark seems to have a slightly pathological distribution of `z`
values -- Sprites are spawned with a random `z` value with a child
sprite at `f32::EPSILON` relative to the parent.
See discussion here:
https://github.com/bevyengine/bevy/issues/8100#issuecomment-1726978633
## Solution
Use `radsort` for sorting `Transparent2d` `PhaseItem`s.
Use random `z` values in bevymark to stress the phase sort. Add an
`--ordered-z` option to `bevymark` that uses the old behavior.
## Benchmarks
mac m1 max
| benchmark | fps before | fps after | diff |
| - | - | - | - |
| bevymark --waves 120 --per-wave 1000 --random-z | 42.16 | 47.06 | 🟩
+11.6% |
| bevymark --waves 120 --per-wave 1000 | 52.50 | 52.29 | 🟥 -0.4% |
| bevymark --waves 120 --per-wave 1000 --mode mesh2d --random-z | 9.64 |
10.24 | 🟩 +6.2% |
| bevymark --waves 120 --per-wave 1000 --mode mesh2d | 15.83 | 15.59 | 🟥
-1.5% |
| bevy-vs-pixi | 39.71 | 59.88 | 🟩 +50.1% |
## Discussion
It's possible that `TransparentUi` should also change. We could probably
use `slice::sort_unstable_by_key` with the current sort key though, as
its items are always sorted and unique. I'd prefer to follow up later to
look into that.
Here's a survey of sorts used by other `PhaseItem`s
#### slice::sort_by_key
`Transparent2d`, `TransparentUi`
#### radsort
`Opaque3d`, `AlphaMask3d`, `Transparent3d`, `Opaque3dPrepass`,
`AlphaMask3dPrepass`, `Shadow`
I also tried `slice::sort_unstable_by_key` with a compound sort key
including `Entity`, but it didn't seem as promising and I didn't test it
as thoroughly.
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Robert Swain <robert.swain@gmail.com>
bdb063497d