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https://issues.apache.org/jira/browse/HDFS-8855?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14704102#comment-14704102
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Xiaobing Zhou commented on HDFS-8855:
-------------------------------------
Thanks [~wheat9] and [~bobhansen] for review. Tracing down to ProtobufRpcEngine
that calls Client.getConnection which fetches connection from
cache(com.google.common.cache.Cache) by using ConnectionId as key. ConnectionId
is different for every webhdfs request even if the url and user are same.
That's why the NN connection is constantly created by DN in this case. Need to
refactor ConnectionId(hashCode or equals) somehow to meet comparison invariant
assumed by cache(com.google.common.cache.Cache) to make it work properly.
> 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.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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