[jira] [Commented] (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:comment-tabpanel&focusedCommentId=16131453#comment-16131453 ] 

Jeff Jirsa commented on CASSANDRA-13756:
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Shouldn't need a version for trunk, but [~jasobrown] if you can check me there to be sure that'd be nice (I think in the faster rewrite for trunk, we now [build|https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/utils/streamhist/StreamingTombstoneHistogramBuilder.java#L182-L186] a snapshot that is no longer modified on read).
 
|| branch || utest || dtest ||
| [3.0|https://github.com/jeffjirsa/cassandra/tree/cassandra-3.0-13756] | [3.0 circle|https://circleci.com/gh/jeffjirsa/cassandra/tree/cassandra-3.0-13756] | [3.0 dtest|https://builds.apache.org/view/A-D/view/Cassandra/job/Cassandra-devbranch-dtest/189/] |
| [3.11|https://github.com/jeffjirsa/cassandra/tree/cassandra-3.11-13756] | [3.11 circle|https://circleci.com/gh/jeffjirsa/cassandra/tree/cassandra-3.11-13756] | [3.11 dtest|https://builds.apache.org/view/A-D/view/Cassandra/job/Cassandra-devbranch-dtest/190/] |


> 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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