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From "Benedict (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-7282) Faster Memtable map
Date Sun, 11 Jan 2015 21:18:36 GMT


Benedict commented on CASSANDRA-7282:

Just a note for the future (I haven't yet addressed your comments): It may be we want to harden
the data structure against determined attackers that produce a lot of colliding tokens with
different partition keys. 

It's not a terribly complex modification to make this a skip-list, instead of a linked-list
(since it is add-only, maintaining a skip-list is just applying the current linked-list logic
N times, and the majority of complexity here is for the hash index). The hash index would
remain an _optimisation_ for expected constant time access into a logarithmic structure, instead
of a linear one, giving us best- and expected- case constant time, worst case logarithmic.
The skip-list index levels could perhaps be dropped as the hash index extends to overlap them,
although it may be cheaper to encode them all in the nodes themselves. This should probably
be a follow up ticket, but I wanted to log the thought as it occurred to me.

> Faster Memtable map
> -------------------
>                 Key: CASSANDRA-7282
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Benedict
>              Labels: performance
>             Fix For: 3.0
>         Attachments: jasobrown-sample-run.txt, profile.yaml, reads.svg, run1.svg, writes.svg
> 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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