neon-sunset / fast-cache / 5372606542

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Reduce contention in parallel eviction by counting opportunistically/inaccurately which is acceptable...
Just substract the difference and report if it's positive, as in, the count after eviction is lower, it can be higher due to overlap with a lot of insertions, in which case we can simply report 0.
Ideally, we would use a spaced counter with per-core hashed offsets to prevent false sharing but that would be hard to do reliably due to enumerator.AsParallel() behavior. Don't forget that this is a naive implementation imposed by inherent limitations of ConcurrentDictionary API. It still gives significant (+2x) performance improvement on multi-core systems but is otherwise less efficient due to SPMC behavior of the underlying parallel logic.

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