rust-lang
perf: increase min buckets on very small types
Consider HashSet<u8> on x86_64 with SSE with various bucket sizes and how many bytes the allocation ends up being:
| buckets | capacity | allocated bytes |
|---|---|---|
| 4 | 3 | 36 |
| 8 | 7 | 40 |
| 16 | 14 | 48 |
| 28 | 32 | 80 |
In general, doubling the number of buckets should roughly double the number of bytes used. However, for small bucket sizes for these small TableLayouts (4 -> 8, 8 -> 16), it doesn't happen. This is an edge case which happens because of padding of the control bytes and adding the Group::WIDTH. Taking the buckets from 4 to 16 (4x) only takes the allocated bytes from 36 to 48 (~1.3x).
This platform isn't the only one with edges. Here's aarch64 on an M1 for the same HashSet<u8>:
| buckets | capacity | allocated bytes |
|---|---|---|
| 4 | 3 | 20 |
| 8 | 7 | 24 |
| 16 | 14 | 40 |
Notice doubling from 4 to 8 buckets only lead to 4 more bytes (20 -> 24) instead of roughly doubling.
Generalized, buckets * table_layout.size needs to be at least as big as table_layout.ctrl_align. For the cases I listed above, we'd get these new minimum bucket sizes:
- x86_64 with SSE: 16
- aarch64: 8
This is a niche optimization. However, it also removes possible undefined behavior edge case in resize operations. In addition, it would be a useful property when utilizing over-sized allocations (see rust-lang/hashbrown#523).
