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https://issues.apache.org/jira/browse/CASSANDRA-7282?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Benedict updated CASSANDRA-7282:
--------------------------------
Description:
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.
was:
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.
> 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
> 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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