Benedict commented on CASSANDRA-7282:

Just pushed a minor update removing an extraneous comment and making the resize 
threshold triggering uniform + clearer.

Agree it would be good to get some performance numbers on this, but I'm not 
sure which magical facilities you're referring to? New stress isn't likely to 
stress this bit out any more interestingly than old stress, and we don't yet 
have a working magical performance service I don't think...

> Faster Memtable map
> -------------------
>                 Key: CASSANDRA-7282
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7282
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>              Labels: performance
>             Fix For: 3.0
> Currently we maintain a ConcurrentSkipLastMap of DecoratedKey -> Partition in 
> our memtables. Maintaining this is an O(lg(n)) operation; since the vast 
> majority of users use a hash partitioner, it occurs to me we could maintain a 
> hybrid ordered list / hash map. The list would impose the normal order on the 
> collection, but a hash index would live alongside as part of the same data 
> structure, simply mapping into the list and permitting O(1) lookups and 
> inserts.
> I've chosen to implement this initial version as a linked-list node per item, 
> but we can optimise this in future by storing fatter nodes that permit a 
> cache-line's worth of hashes to be checked at once,  further reducing the 
> constant factor costs for lookups.

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