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From "Brandon Williams (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7032) Improve vnode allocation
Date Thu, 18 Sep 2014 16:58:35 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7032?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14139167#comment-14139167
] 

Brandon Williams commented on CASSANDRA-7032:
---------------------------------------------

It seems that our main problem when choosing a distribution for vnodes is not knowing what
the existing distribution in the cluster is.  Now that we have shadow gossip (and already
use it during bootstrap) we could know this before choosing our own tokens and be able to
achieve a perfect distribution.  Of course for a new cluster where you aren't bootstrapping
it's still worth algorithmic improvements.

> 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
>
>
> 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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