(tokio-util JoinMap) Remove raw-entry feature in favour of HashTable API. #7252
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Darksonn
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Apr 24, 2025
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| let hash_builder = self.hashes_by_task.hasher(); | ||
| self.tasks_by_key | ||
| .shrink_to_fit(|(k, _)| hash_one(hash_builder, k)); |
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I'm confused. Why is hashes_by_task needed inside tasks_by_key here?
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We only need the hasher inside hashes_by_task. It saves us from having to store a separate instance of the hasher.
An alternative solution could be to have a separate hasher: S, and then make the HashMap<Id, u64> use a cheaper hash function like a simple fibonacci hash (so no state is needed). That might be overkill though.
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For reference: conradludgate@5086eeb
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Co-authored-by: Alice Ryhl <[email protected]>
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Motivation
While rebasing #7075, I noticed the change from #7219 which updates hashbrown, requiring the addition of the
raw-entryfeature. That inspired me to take a look at the actual JoinMap impl and I thought it could be improved by usingHashTableinstead.Solution
Replacing the
tasks_by_keymap with aHashTable, we unlock a few benefits:tasks_by_key, since we can fetch the ID from theAbortHandle.S: BuildHasher, as we are in-control over the hash-function, we can fetch the existing hasher fromhashes_by_task.However, removing the ID from
tasks_by_keywill slow down those lookups (only injoin_next()), as they now need a vtable lookup, rather than comparing against the id already in the current cache line.I don't feel strongly about this PR, I'm just quite fond of the HashTable API and thought I'd try it here.