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https://issues.apache.org/jira/browse/CASSANDRA-7032?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14232857#comment-14232857
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Benedict edited comment on CASSANDRA-7032 at 12/3/14 10:38 AM:
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Well, NetworkTopologyStrategy already enforces some degree of balance across
racks, and absolutely guarantees balance across DCs as far as replication
ownership is concerned. It _would_ be nice to migrate this behaviour to the
token selection so that we could reason about ownership a bit more clearly (NTS
might enforce our general ownership constraints, but having a predictably cheap
generation strategy for end points would be great, as the amount of state
necessary to route queries could shrink dramatically. if we could rely on a
sequence of adjacent tokens ensuring these properties, for instance), but a
simpler goal of simply ensuring that for any given arbitrary slice of the
global token range, all nodes have a share of the range that is within epsilon
of perfect, should be more than sufficient.
TL;DR; our goal should probably be: "for any given arbitrary slice of the
global token range, all nodes have a share of the range that is within epsilon*
of perfect"
\* with epsilon probably inversely proportional to the size of the slice
was (Author: benedict):
Well, NetworkTopologyStrategy already enforces some degree of balance across
racks, and absolutely guarantees balance across DCs as far as replication
ownership is concerned. It _would_ be nice to migrate this behaviour to the
token selection so that we could reason about ownership a bit more clearly (NTS
might enforce our general ownership constraints, but having a predictably cheap
generation strategy for end points would be great, as the amount of state
necessary to route queries could shrink dramatically. if we could rely on a
sequence of adjacent tokens ensuring these properties, for instance), but a
simpler goal of simply ensuring that for any given arbitrary slice of the
global token range, all nodes have a share of the range that is within epsilon
of perfect, should be more than sufficient.
TL;DR; our goal should probably be: "for any given arbitrary slice of the
global token range, all nodes have a share of the range that is within epsilon*
of perfect"
* with epsilon probably inversely proportional to the size of the slice
> 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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