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https://issues.apache.org/jira/browse/HDFS-12288?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16123822#comment-16123822
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Rushabh S Shah commented on HDFS-12288:
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[~hkoneru]: yes, that's correct.
bq. DataXceiverServer
There is only 1 instance of this per datanode. So I don't care much about this.
But we need to accommodate {{BlockRecoveryWorker}} and {{PacketResponder}} 
threads in our calculation.
bq. I would say we need to have 3 metrics to capture all the thread counts:
Instead of having 3 different metrics, my suggestion would be having just one 
metric "NumActiveThreads".

> 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, HDFS-12288.002.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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