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https://issues.apache.org/jira/browse/CASSANDRA-14318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16405580#comment-16405580
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Jeff Jirsa commented on CASSANDRA-14318:
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If you havent yet started the second run of benchmarks, you may also want to
move the debug log statements from ReadCallback.java (one deals with digest
mismatch exceptions, forgot what the other is) to trace as well.
> Debug logging can create massive performance issues
> ---------------------------------------------------
>
> Key: CASSANDRA-14318
> URL: https://issues.apache.org/jira/browse/CASSANDRA-14318
> Project: Cassandra
> Issue Type: Bug
> Reporter: Alexander Dejanovski
> Assignee: Alexander Dejanovski
> Priority: Major
> Labels: lhf, performance
> Fix For: 2.2.x, 3.0.x, 3.11.x, 4.x
>
> Attachments: debuglogging.png, flame22 nodebug sjk svg.png,
> flame22-nodebug-sjk.svg, flame22-sjk.svg, flame_graph_snapshot.png
>
>
> Debug logging can involve in many cases (especially very low latency ones) a
> very important overhead on the read path in 2.2 as we've seen when upgrading
> clusters from 2.0 to 2.2.
> The performance impact was especially noticeable on the client side metrics,
> where p99 could go up to 10 times higher, while ClientRequest metrics
> recorded by Cassandra didn't show any overhead.
> Below shows latencies recorded on the client side with debug logging on
> first, and then without it :
> !debuglogging.png!
> We generated a flame graph before turning off debug logging that shows the
> read call stack is dominated by debug logging :
> !flame_graph_snapshot.png!
> I've attached the original flame graph for exploration.
> Once disabled, the new flame graph shows that the read call stack gets
> extremely thin, which is further confirmed by client recorded metrics :
> !flame22 nodebug sjk svg.png!
> The query pager code has been reworked since 3.0 and it looks like
> log.debug() calls are gone there, but for 2.2 users and to prevent such
> issues to appear with default settings, I really think debug logging should
> be disabled by default.
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