0b4858726c
# Objective There are two problems this aims to solve. First, `Entity::index` is currently a `u32`. That means there are `u32::MAX + 1` possible entities. Not only is that awkward, but it also make `Entity` allocation more difficult. I discovered this while working on remote entity reservation, but even on main, `Entities` doesn't handle the `u32::MAX + 1` entity very well. It can not be batch reserved because that iterator uses exclusive ranges, which has a maximum upper bound of `u32::MAX - 1`. In other words, having `u32::MAX` as a valid index can be thought of as a bug right now. We either need to make that invalid (this PR), which makes Entity allocation cleaner and makes remote reservation easier (because the length only needs to be u32 instead of u64, which, in atomics is a big deal), or we need to take another pass at `Entities` to make it handle the `u32::MAX` index properly. Second, `TableRow`, `ArchetypeRow` and `EntityIndex` (a type alias for u32) all have `u32` as the underlying type. That means using these as the index type in a `SparseSet` uses 64 bits for the sparse list because it stores `Option<IndexType>`. By using `NonMaxU32` here, we cut the memory of that list in half. To my knowledge, `EntityIndex` is the only thing that would really benefit from this niche. `TableRow` and `ArchetypeRow` I think are not stored in an `Option` in bulk. But if they ever are, this would help. Additionally this ensures `TableRow::INVALID` and `ArchetypeRow::INVALID` never conflict with an actual row, which in a nice bonus. As a related note, if we do components as entities where `ComponentId` becomes `Entity`, the the `SparseSet<ComponentId>` will see a similar memory improvement too. ## Solution Create a new type `EntityRow` that wraps `NonMaxU32`, similar to `TableRow` and `ArchetypeRow`. Change `Entity::index` to this type. ## Downsides `NonMax` is implemented as a `NonZero` with a binary inversion. That means accessing and storing the value takes one more instruction. I don't think that's a big deal, but it's worth mentioning. As a consequence, `to_bits` uses `transmute` to skip the inversion which keeps it a nop. But that also means that ordering has now flipped. In other words, higher indices are considered less than lower indices. I don't think that's a problem, but it's also worth mentioning. ## Alternatives We could keep the index as a u32 type and just document that `u32::MAX` is invalid, modifying `Entities` to ensure it never gets handed out. (But that's not enforced by the type system.) We could still take advantage of the niche here in `ComponentSparseSet`. We'd just need some unsafe manual conversions, which is probably fine, but opens up the possibility for correctness problems later. We could change `Entities` to fully support the `u32::MAX` index. (But that makes `Entities` more complex and potentially slightly slower.) ## Testing - CI - A few tests were changed because they depend on different ordering and `to_bits` values. ## Future Work - It might be worth removing the niche on `Entity::generation` since there is now a different niche. - We could move `Entity::generation` into it's own type too for clarity. - We should change `ComponentSparseSet` to take advantage of the new niche. (This PR doesn't change that yet.) - Consider removing or updating `Identifier`. This is only used for `Entity`, so it might be worth combining since `Entity` is now more unique. --------- Co-authored-by: atlv <email@atlasdostal.com> Co-authored-by: Zachary Harrold <zac@harrold.com.au> |
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| README.md | ||
Bevy Benchmarks
This is a crate with a collection of benchmarks for Bevy.
Running benchmarks
Benchmarks can be run through Cargo:
# Run all benchmarks. (This will take a while!)
cargo bench -p benches
# Just compile the benchmarks, do not run them.
cargo bench -p benches --no-run
# Run the benchmarks for a specific crate. (See `Cargo.toml` for a complete list of crates
# tracked.)
cargo bench -p benches --bench ecs
# Filter which benchmarks are run based on the name. This will only run benchmarks whose name
# contains "name_fragment".
cargo bench -p benches -- name_fragment
# List all available benchmarks.
cargo bench -p benches -- --list
# Save a baseline to be compared against later.
cargo bench -p benches -- --save-baseline before
# Compare the current benchmarks against a baseline to find performance gains and regressions.
cargo bench -p benches -- --baseline before
Criterion
Bevy's benchmarks use Criterion. If you want to learn more about using Criterion for comparing performance against a baseline or generating detailed reports, you can read the Criterion.rs documentation.