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From "Benedict (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7282) Faster Memtable map
Date Sat, 13 Sep 2014 19:41:34 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7282?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14132906#comment-14132906
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Benedict commented on CASSANDRA-7282:
-------------------------------------

As already stated, I disagree. Always, when making these decisions, the important factors
are: 1) what workloads / portions of workloads are affected; and 2) do we consider these common
enough (or to become common enough) to warrant inclusion?

Since we are very constrained on our hardware and workload generation, picking a "realistic
workload" that we can perform at best helps us rule out regressions if it is not compatible
with exhibiting the change. The important question is simply: do we consider it likely it
will impact other workloads we are not capable of benchmarking, given the known information
we have from isolating its effect in a manner we _can_.

> 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
>
>         Attachments: 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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