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https://issues.apache.org/jira/browse/HDFS-14292?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16777303#comment-16777303
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Kihwal Lee commented on HDFS-14292:
-----------------------------------
There was also an effort to make data transfer use http (by [~wheat9]?). The
rationale was that we could use existing potentially async engines and also
take advantage of more standard encryption (TLS). It didn't go far, but I
thought it needs to be revisited.
> Introduce Java ExecutorService to DataXceiverServer
> ---------------------------------------------------
>
> Key: HDFS-14292
> URL: https://issues.apache.org/jira/browse/HDFS-14292
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 3.2.0
> Reporter: BELUGA BEHR
> Assignee: BELUGA BEHR
> Priority: Major
> Attachments: HDFS-14292.1.patch, HDFS-14292.2.patch,
> HDFS-14292.3.patch, HDFS-14292.4.patch, HDFS-14292.5.patch,
> HDFS-14292.6.patch, HDFS-14292.6.patch, HDFS-14292.7.patch
>
>
> I wanted to investigate {{dfs.datanode.max.transfer.threads}} from
> {{hdfs-site.xml}}. It is described as "Specifies the maximum number of
> threads to use for transferring data in and out of the DN." The default
> value is 4096. I found it interesting because 4096 threads sounds like a lot
> to me. I'm not sure how a system with 8-16 cores would react to this large a
> thread count. Intuitively, I would say that the overhead of context
> switching would be immense.
> During mt investigation, I discovered the
> [following|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/DataXceiverServer.java#L203-L216]
> setup in the {{DataXceiverServer}} class:
> # A peer connects to a DataNode
> # A new thread is spun up to service this connection
> # The thread runs to completion
> # The tread dies
> It would perhaps be better if we used a thread pool to better manage the
> lifecycle of the service threads and to allow the DataNode to re-use existing
> threads, saving on the need to create and spin-up threads on demand.
> In this JIRA, I have added a couple of things:
> # Added a thread pool to {{DataXceiverServer}} class that, on demand, will
> create up to {{dfs.datanode.max.transfer.threads}}. A thread that has
> completed its prior duties will stay idle for up to 60 seconds
> (configurable), it will be retired if no new work has arrived.
> # Added new methods to the {{Peer}} Interface to allow for better logging and
> less code within each Thread ({{DataXceiver}}).
> # Updated the Thread code ({{DataXceiver}}) regarding its interactions with
> {{blockReceiver}} instance variable
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