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

[Masafumi Miura (Jira)]

Masafumi Miura created ISPN-11679:
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             Summary: 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: 10.1.6.Final, 9.4.19.Final
            Reporter: Masafumi Miura


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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