[jira] [Assigned] (CASSANDRA-13756) StreamingHistogram is not thread safe

["Jeff Jirsa (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-13756?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Jeff Jirsa reassigned CASSANDRA-13756:
--------------------------------------

    Assignee: Jeff Jirsa

> 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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