[CASSANDRA-7282] Faster Memtable map

• Type: Improvement
• Status: Open
• Priority: Normal
• Resolution: Unresolved
• Fix Version/s: 5.x
• Component/s: Legacy/Local Write-Read Paths
• Labels: performance
• Assignee: Michael Burman
• Reporter: Benedict Elliott Smith

Authors

Michael Burman

Reviewers

Benedict Elliott Smith

• Created: 21/May/14 18:38
• Updated: 07/Mar/23 10:53

Description

Currently we maintain a ConcurrentSkipLastMap of DecoratedKey -> Partition in our memtables. Maintaining this is an O(lg) 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.