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From "Alexander Dejanovski (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-14318) Debug logging can create massive performance issues
Date Mon, 19 Mar 2018 16:11:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-14318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16405051#comment-16405051
] 

Alexander Dejanovski commented on CASSANDRA-14318:
--------------------------------------------------

Fair point. I'll write the patch and run the benchmarks.

> 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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