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From "Jason Brown (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13756) StreamingHistogram is not thread safe
Date Tue, 29 Aug 2017 19:19:01 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13756?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16145944#comment-16145944
] 

Jason Brown commented on CASSANDRA-13756:
-----------------------------------------

I'm +1 on the snapshot version of the code. You may want to wait a day to see if [~krummas]
has any additional feedback.

> StreamingHistogram is not thread safe
> -------------------------------------
>
>                 Key: CASSANDRA-13756
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13756
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: xiangzhou xia
>            Assignee: Jeff Jirsa
>             Fix For: 3.0.x, 3.11.x
>
>
> When we test C*3 in shadow cluster, we notice after a period of time, several data node
suddenly run into 100% cpu and stop process query anymore.
> After investigation, we found that threads are stuck on the sum() in streaminghistogram
class. Those are jmx threads that working on expose getTombStoneRatio metrics (since jmx is
kicked off every 3 seconds, there is a chance that multiple jmx thread is access streaminghistogram
at the same time).  
> After further investigation, we find that the optimization in CASSANDRA-13038 led to
a spool flush every time when we call sum(). Since TreeMap is not thread safe, threads will
be stuck when multiple threads visit sum() at the same time.
> There are two approaches to solve this issue. 
> The first one is to add a lock to the flush in sum() which will introduce some extra
overhead to streaminghistogram.
> The second one is to avoid streaminghistogram to be access by multiple threads. For our
specific case, is to remove the metrics we added.  



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