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From "Lukas Majercak (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-12288) Fix DataNode's xceiver count calculation
Date Fri, 11 Aug 2017 16:42:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-12288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16123589#comment-16123589
] 

Lukas Majercak commented on HDFS-12288:
---------------------------------------

Thanks for the review [~shahrs87].

# Do you mind explaining why would we start creating twice the number of threads compared
to today with this change? I don't see a reason why.
# Not sure what else can we do here, metrics are initialized as long as they are not null
afaik.

> Fix DataNode's xceiver count calculation
> ----------------------------------------
>
>                 Key: HDFS-12288
>                 URL: https://issues.apache.org/jira/browse/HDFS-12288
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode, hdfs
>            Reporter: Lukas Majercak
>            Assignee: Lukas Majercak
>         Attachments: HDFS-12288.001.patch
>
>
> The problem with the ThreadGroup.activeCount() method is that the method is only a very
rough estimate, and in reality returns the total number of threads in the thread group as
opposed to the threads actually running.
> In some DNs, we saw this to return 50~ for a long time, even though the actual number
of DataXceiver threads was next to none.
> This is a big issue as we use the xceiverCount to make decisions on the NN for choosing
replication source DN or returning DNs to clients for R/W.
> The plan is to reuse the DataNodeMetrics.dataNodeActiveXceiversCount value which only
accounts for actual number of DataXcevier threads currently running and thus represents the
load on the DN much better.



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