[jira] [Assigned] (CASSANDRA-15752) Range read concurrency factor didn't consider range merger

["ZhaoYang (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-15752?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

ZhaoYang reassigned CASSANDRA-15752:
------------------------------------

    Assignee: ZhaoYang

> Range read concurrency factor didn't consider range merger
> ----------------------------------------------------------
>
>                 Key: CASSANDRA-15752
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-15752
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Legacy/Coordination
>            Reporter: ZhaoYang
>            Assignee: ZhaoYang
>            Priority: Normal
>
> During range read, coordinator computes concurrency factor which is the number of vnode ranges to contact in parallel for the next batch.
> But in {{RangeCommandIterator}}, vnode ranges are merged by {{RangeMerger}} if vnode ranges share enough replicas to satisfy consistency level. eg. vnode range [a,b) has replica n1,n2,n3 and vnode range [b,c) has replica n2,n3,n4, so they can be merged as range [a,c) with replica n2, n3 for Quorum.
> Currently it counts number of merged ranges towards concurrency factor. Coordinator may fetch more ranges than needed.
> ----
> Another issue is that when executing range read on table with very small amount of data, concurrency factor can be bumped to {{size of total vnode ranges}}, eg. 10k, depending on the num of vnodes and cluster size. As a result, coordinator will send large number of concurrent range requests, potentially slowing down the cluster.. We should cap the max concurrency factor..



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