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https://issues.apache.org/jira/browse/HADOOP-3856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12670029#action_12670029
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Raghu Angadi commented on HADOOP-3856:
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Thanks for working on the patch.
> However this is only benificial if, once an xceiver is engaged, the
> transaction usually completes quickly
This is usually not the case. Many of these threads stay active in the order of
10s of seconds to minutes. These threads are involved in disk i/o as well. As
long as the patch makes sure that a thread is never blocked on network I/O and
blocks only on disk i/o, that is essentially what is needed.
Currently DN threads block more often on network I/O which is the core of the
problem.
> Asynchronous IO Handling in Hadoop and HDFS
> -------------------------------------------
>
> Key: HADOOP-3856
> URL: https://issues.apache.org/jira/browse/HADOOP-3856
> Project: Hadoop Core
> Issue Type: New Feature
> Components: dfs, io
> Reporter: Raghu Angadi
> Attachments: GrizzlyEchoServer.patch, MinaEchoServer.patch
>
>
> I think Hadoop needs utilities or framework to make it simpler to deal with
> generic asynchronous IO in Hadoop.
> Example use case :
> Its been a long standing problem that DataNode takes too many threads for
> data transfers. Each write operation takes up 2 threads at each of the
> datanodes and each read operation takes one irrespective of how much activity
> is on the sockets. The kinds of load that HDFS serves has been expanding
> quite fast and HDFS should handle these varied loads better. If there is a
> framework for non-blocking IO, read and write pipeline state machines could
> be implemented with async events on a fixed number of threads.
> A generic utility is better since it could be used in other places like
> DFSClient. DFSClient currently creates 2 extra threads for each file it has
> open for writing.
> Initially I started writing a primitive "selector", then tried to see if such
> facility already exists. [Apache MINA|http://mina.apache.org] seemed to do
> exactly this. My impression after looking the the interface and examples is
> that it does not give kind control we might prefer or need. First use case I
> was thinking of implementing using MINA was to replace "response handlers" in
> DataNode. The response handlers are simpler since they don't involve disk
> I/O. I [asked on MINA user
> list|http://www.nabble.com/Async-events-with-existing-NIO-sockets.-td18640767.html],
> but looks like it can not be done, I think mainly because the sockets are
> already created.
> Essentially what I have in mind is similar to MINA, except that read and
> write of the sockets is done by the event handlers. The lowest layer
> essentially invokes selectors, invokes event handlers on single or on
> multiple threads. Each event handler is is expected to do some non-blocking
> work. We would of course have utility handler implementations that do read,
> write, accept etc, that are useful for simple processing.
> Sam Pullara mentioned that [xSockets|http://xsocket.sourceforge.net/] is more
> flexible. It is under GPL.
> Are there other such implementations we should look at?
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