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https://issues.apache.org/jira/browse/CASSANDRA-15049?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16806417#comment-16806417
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Sumanth Pasupuleti commented on CASSANDRA-15049:
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FYI, I have submitted a patch on CASSANDRA-15013.
> Requests blocked at NTR stage should be rejected
> ------------------------------------------------
>
> Key: CASSANDRA-15049
> URL: https://issues.apache.org/jira/browse/CASSANDRA-15049
> Project: Cassandra
> Issue Type: Bug
> Reporter: Michaël Figuière
> Priority: Normal
>
> CASSANDRA-11363 has emphasized that if the NTR stage's thread pool and queue
> are full, the Netty Event Loops may block waiting on the NTR queue. The
> solution that was brought in CASSANDRA-11363 was to increase the default
> queue size from 128 to 1024. This significantly reduced the number of blocked
> requests observed but hasn't removed the problem entirely. Whenever a Netty
> Event Loop is blocked, the responsiveness of Cassandra is significantly
> impacted so it seems inappropriate to rely solely on increasing this queue
> size until everything looks fine... at the time the tuning was done.
> In fact, this situation looks exactly like the definition of the
> {{Overloaded}} error of the CQL Protocol:
> {code:java}
> 0x1001 Overloaded: the request cannot be processed because the
> coordinator node is overloaded{code}
> Therefore, whenever a request can't make it to the NTR stage, it should be
> rejected with an {{Overloaded}} error to the client. This can be done at low
> cost as we're already in the Netty Event Loop owning the channel to that
> client.
> It would then be the client responsibility to retry with another coordinator,
> which is likely to lead to a better P99 latency than blocking on an already
> too long queue.
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