[jira] [Created] (CASSANDRA-7282) Faster Memtable map

["Benedict (JIRA)" <ji...@apache.org>]

Benedict created CASSANDRA-7282:
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             Summary: Faster Memtable map
                 Key: CASSANDRA-7282
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7282
             Project: Cassandra
          Issue Type: Improvement
          Components: Core
            Reporter: Benedict
             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 skip list / hash map. The skip list would impose the order on the collection, but a hash index would live alongside as part of the same data structure, simply mapping into the skip list and permitting O(1) lookups. It should be possible to define the hash map to also permit O(1) inserts. Our decorated keys are in fact perfectly designed for this scheme.

At the same time, we can potentially improve the data locality in the skip list by baking the initial 64 token bits directly into the structure, and storing multiple values per skip list entry to improve cache performance, bringing down memory and constant factor cpu overheads.



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