Don't lock eviction parent set if it's likely empty - #1047
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| /// An estimate of the number of elements in [`Self::children`], used | ||
| /// to avoid locking and querying consistently empty elements. | ||
| n_children: AtomicUsize, |
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May want to consider wrapping n_children and children up in a struct with an interface that makes it impossible for them to get out of sync. Seems like it'd be pretty easy to update children in some future PR and forget to update n_children.
This prevents us doing meaningless work when calculating eviction priority of a UFT/TCP entry while evaluating LFT candidates.
The use of `n_children` makes it a little quicker for us to early exit from eviction if we come across an entry which is genuinely expired.
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By keeping a counter outside of the locked
childrenlist, we can avoid the cost of acquiring a readlock when an entry has nothing which depends on it. This is most relevant for UFT and TCP flow table entries, which should never have any children, but calculating the eviction score for an LFT entry benefits indirectly since they calleviction_priorityfor each child.We also use this now to identify flows which have no children, and thus can be evicted because they are ready for expiry but are not cleaned up by the periodic task yet. When this occurs we can exit the eviction candidate scan early.