[infinispan-issues] [JBoss JIRA] (ISPN-11679) Bad uneven segment distributions can happen after the rebalancing with SyncConsistentHashFactory

[Masafumi Miura (Jira)]

     [ https://issues.redhat.com/browse/ISPN-11679?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Masafumi Miura updated ISPN-11679:
----------------------------------
    Steps to Reproduce: 
- Add a unit test "testRebalanceDistribution" in SyncConsistentHashFactoryKeyDistributionTest.

https://github.com/infinispan/infinispan/compare/master...msfm:master_SyncCHFKeyDistTest

- Execute unit test

{code}
$ mvn -pl core test -Dtest=distribution.ch.SyncConsistentHashFactoryKeyDistributionTest#testRebalanceDistribution
{code}

- Check the test result, especially "Primary: max", "Primary: min" and "Segments per node - max/min ratio". You will see "Primary: min" that indicates around half (around 0.500-0.600) and "Segments per node - max/min ratio" indicates around two times (around 1.7-2.0).



> Bad uneven segment distributions can happen after the rebalancing with SyncConsistentHashFactory
> ------------------------------------------------------------------------------------------------
>
>                 Key: ISPN-11679
>                 URL: https://issues.redhat.com/browse/ISPN-11679
>             Project: Infinispan
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 9.4.19.Final, 10.1.6.Final
>            Reporter: Masafumi Miura
>            Priority: Major
>
> Uneven segment distribution can happen after rebalancing with SyncConsistentHashFactory. 
> Even if a larger number of segments than "20 * number of nodes" is configured, some nodes have only around 50-60% number of segments than other nodes after the rebalance in the worst case.
> I understand the documentation (like the following) states about the possibility of slight uneven distribution as a potential drawback of SyncConsistentHashFactory.
> [Infinispan 10 configuration guide|https://infinispan.org/docs/stable/titles/configuring/configuring.html#key_ownership] says:
> {quote}
> This implementation does have some negative points in that the load distribution is slightly uneven. It also moves more segments than strictly necessary on a join or leave.
> {quote}
> [JDG 7 Developer Guide|https://access.redhat.com/documentation/en-us/red_hat_data_grid/7.2/html/developer_guide/high_availability_using_server_hinting#consistenthashfactories-1] says:
> {quote}
> It has a potential drawback in that it can move a greater number of segments than necessary during re-balancing. This can be mitigated by using a larger number of segments.
> Another potential drawback is that the segments are not distributed as evenly as possible, and actually using a very large number of segments can make the distribution of segments worse.
> {quote}
> However, only half segments than others is not a slight uneven.



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