perf: increase min buckets on very small types #524
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This was referenced May 8, 2024
Amanieu
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Jun 17, 2024
|
Also see #47 for previous discussion of this problem. |
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 | | 32 | 28 | 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 4 -> 8 buckets leading to only 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 may be a useful property to utilize over-sized allocations (see rust-lang#523).
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morrisonlevi
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Jun 24, 2024
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| let cap = cap.max(match (Group::WIDTH, table_layout.size) { | ||
| (16, 0..=1) => 14, | ||
| (16, 2..=3) => 7, | ||
| (8, 0..=1) => 7, | ||
| _ => 3, | ||
| }); |
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It could also be written this way. Not sure what is better:
Suggested change
| let cap = cap.max(match (Group::WIDTH, table_layout.size) { | |
| (16, 0..=1) => 14, | |
| (16, 2..=3) => 7, | |
| (8, 0..=1) => 7, | |
| _ => 3, | |
| }); | |
| let cap = cap.max(match table_layout.size { | |
| 0..=1 if Group::WIDTH == 16 => 14, | |
| 2..=3 if Group::WIDTH == 16 => 7, | |
| 0..=1 if Group::WIDTH == 8 => 7, | |
| _ => 3, | |
| }); |
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What I had in mind was to move the max check inside the if cap < 8 block entirely (and changing the bound to 15). Then just have hard-coded selection depending on the requested capacity and the group width/item size.
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Consider
HashSet<u8>on x86_64 with SSE with various bucket sizes and how many bytes the allocation ends up being: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 theGroup::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>:Notice doubling from 4 to 8 buckets only lead to 4 more bytes (20 -> 24) instead of roughly doubling.
Generalized,
buckets * table_layout.sizeneeds to be at least as big astable_layout.ctrl_align. For the cases I listed above, we'd get these new minimum bucket sizes: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 #523).