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From "ZhaoYang (Jira)" <>
Subject [jira] [Updated] (CASSANDRA-15752) Range read concurrency factor didn't consider range merger
Date Fri, 12 Jun 2020 07:25:00 GMT


ZhaoYang updated CASSANDRA-15752:
    Test and Documentation Plan: 
Added unit tests.

Circle Ci: []

Added unit tests.

Circle Ci: []

> Range read concurrency factor didn't consider range merger
> ----------------------------------------------------------
>                 Key: CASSANDRA-15752
>                 URL:
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Legacy/Coordination
>            Reporter: ZhaoYang
>            Assignee: ZhaoYang
>            Priority: Normal
>             Fix For: 3.0.x, 3.11.x, 4.0-beta
> 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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