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Use SmallVec in Bitfield (#3025)

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## Issue Addressed

Alternative to #2935

## Proposed Changes

Replace the `Vec<u8>` inside `Bitfield` with a `SmallVec<[u8; 32>`. This eliminates heap allocations for attestation bitfields until we reach 500K validators, at which point we can consider increasing `SMALLVEC_LEN` to 40 or 48.

While running Lighthouse under `heaptrack` I found that SSZ encoding and decoding of bitfields corresponded to 22% of all allocations by count. I've confirmed that with this change applied those allocations disappear entirely.

## Additional Info

We can win another 8 bytes of space by using `smallvec`'s [`union` feature](https://docs.rs/smallvec/1.8.0/smallvec/#union), although I might leave that for a future PR because I don't know how experimental that feature is and whether it uses some spicy `unsafe` blocks.
This commit is contained in:
Michael Sproul
2022-02-17 23:55:04 +00:00
parent 0a6a8ea3b0
commit da4ca024f1
6 changed files with 174 additions and 110 deletions
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consensus/types/Cargo.toml
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@@ -45,6 +45,7 @@ parking_lot = "0.11.1"
itertools = "0.10.0"
superstruct = "0.4.0"
serde_json = "1.0.74"
smallvec = "1.8.0"
[dev-dependencies]
criterion = "0.3.3"