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From "Branimir Lambov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7032) Improve vnode allocation
Date Wed, 14 Jan 2015 20:52:36 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7032?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14277652#comment-14277652
] 

Branimir Lambov commented on CASSANDRA-7032:
--------------------------------------------

No, I have not tried asymmetric configs yet, but I don't foresee significant problems adapting
the method to them. Assuming configuration is done by specifying a number of vnodes in the
node it should be just a matter of setting the optimal size based on that number; of course
it remains to be seen if that won't cause some weird behaviour.

The speed problem is now mostly solved, I'll post a new version soon and try that out. 

> Improve vnode allocation
> ------------------------
>
>                 Key: CASSANDRA-7032
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7032
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Benedict
>            Assignee: Branimir Lambov
>              Labels: performance, vnodes
>             Fix For: 3.0
>
>         Attachments: TestVNodeAllocation.java, TestVNodeAllocation.java, TestVNodeAllocation.java,
TestVNodeAllocation.java
>
>
> It's been known for a little while that random vnode allocation causes hotspots of ownership.
It should be possible to improve dramatically on this with deterministic allocation. I have
quickly thrown together a simple greedy algorithm that allocates vnodes efficiently, and will
repair hotspots in a randomly allocated cluster gradually as more nodes are added, and also
ensures that token ranges are fairly evenly spread between nodes (somewhat tunably so). The
allocation still permits slight discrepancies in ownership, but it is bound by the inverse
of the size of the cluster (as opposed to random allocation, which strangely gets worse as
the cluster size increases). I'm sure there is a decent dynamic programming solution to this
that would be even better.
> If on joining the ring a new node were to CAS a shared table where a canonical allocation
of token ranges lives after running this (or a similar) algorithm, we could then get guaranteed
bounds on the ownership distribution in a cluster. This will also help for CASSANDRA-6696.



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