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https://issues.apache.org/jira/browse/HDFS-8855?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14720070#comment-14720070
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Bob Hansen commented on HDFS-8855:
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Xiaobing - is there a race condition in initializing the ugiCache? If two
threads make simultaneous requests, one of them will succeed in the CAS for
ugiCacheInit, and the other will proceed ahead. If the latter thread
immediately tries to reference the ugiCache while the first is still
initializing it, we will get an NPE or a partially-constructed object.
See
http://www.journaldev.com/1377/java-singleton-design-pattern-best-practices-with-examples
for a nice little discussion of idiomatic singletons in Java; if we're
supporting JRE <= 1.5, the Pugh construction is clean and works well for
concurrent access.
> Webhdfs client leaks active NameNode connections
> ------------------------------------------------
>
> Key: HDFS-8855
> URL: https://issues.apache.org/jira/browse/HDFS-8855
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: webhdfs
> Environment: HDP 2.2
> Reporter: Bob Hansen
> Assignee: Xiaobing Zhou
> Attachments: HDFS-8855.1.patch, HDFS-8855.2.patch,
> HDFS_8855.prototype.patch
>
>
> The attached script simulates a process opening ~50 files via webhdfs and
> performing random reads. Note that there are at most 50 concurrent reads,
> and all webhdfs sessions are kept open. Each read is ~64k at a random
> position.
> The script periodically (once per second) shells into the NameNode and
> produces a summary of the socket states. For my test cluster with 5 nodes,
> it took ~30 seconds for the NameNode to have ~25000 active connections and
> fails.
> It appears that each request to the webhdfs client is opening a new
> connection to the NameNode and keeping it open after the request is complete.
> If the process continues to run, eventually (~30-60 seconds), all of the
> open connections are closed and the NameNode recovers.
> This smells like SoftReference reaping. Are we using SoftReferences in the
> webhdfs client to cache NameNode connections but never re-using them?
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