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From "Tony Wu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-12482) Race condition in JMX cache update
Date Fri, 30 Oct 2015 17:52:27 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-12482?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14982967#comment-14982967
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Tony Wu commented on HADOOP-12482:
----------------------------------

Oops. I will correct it and post a new patch. Thanks a lot for looking at the patch!


> Race condition in JMX cache update
> ----------------------------------
>
>                 Key: HADOOP-12482
>                 URL: https://issues.apache.org/jira/browse/HADOOP-12482
>             Project: Hadoop Common
>          Issue Type: Bug
>    Affects Versions: 2.7.1
>            Reporter: Tony Wu
>            Assignee: Tony Wu
>         Attachments: HADOOP-12482.001.patch, HADOOP-12482.002.patch
>
>
> updateJmxCache() was updated in HADOOP-11301. However the patch introduced a race condition.
In updateJmxCache() function in MetricsSourceAdapter.java:
> {code:java}
>   private void updateJmxCache() {
>     boolean getAllMetrics = false;
>     synchronized (this) {
>       if (Time.now() - jmxCacheTS >= jmxCacheTTL) {
>         // temporarilly advance the expiry while updating the cache
>         jmxCacheTS = Time.now() + jmxCacheTTL;
>         if (lastRecs == null) {
>           getAllMetrics = true;
>         }
>       } else {
>         return;
>       }
>       if (getAllMetrics) {
>         MetricsCollectorImpl builder = new MetricsCollectorImpl();
>         getMetrics(builder, true);
>       }
>       updateAttrCache();
>       if (getAllMetrics) {
>         updateInfoCache();
>       }
>       jmxCacheTS = Time.now();
>       lastRecs = null; // in case regular interval update is not running
>     }
>   }
> {code}
> Notice that getAllMetrics is set to true when:
> # jmxCacheTTL has passed
> # lastRecs == null
> lastRecs is set to null in the same function, but gets reassigned by getMetrics().
> However getMetrics() can be called from a different thread:
> # MetricsSystemImpl.onTimerEvent()
> # MetricsSystemImpl.publishMetricsNow()
> Consider the following sequence:
> # updateJmxCache() is called by getMBeanInfo() from a thread getting cached info. 
> ** lastRecs is set to null.
> # metrics sources is updated with new value/field.
> # getMetrics() is called by publishMetricsNow() or onTimerEvent() from a different thread
getting the latest metrics. 
> ** lastRecs is updated (!= null).
> # jmxCacheTTL passed.
> # updateJmxCache() is called again via getMBeanInfo().
> ** However because lastRecs is already updated (!= null), getAllMetrics will not be set
to true. So updateInfoCache() is not called and getMBeanInfo() returns the old cached info.
> We ran into this issue on a cluster where a new metric did not get published until much
later.
> The case can be made worse by a periodic call to getMetrics() (driven by an external
program or script). In such case getMBeanInfo() may never be able to retrieve the new record.
> The desired behavior should be that updateJmxCache() will guarantee to call updateInfoCache()
once after jmxCacheTTL, if lastRecs has been set to null by updateJmxCache() itself.



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