ZhaoYang created CASSANDRA-15752:
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Summary: 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
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.
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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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