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From "Ankur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3856) Asynchronous IO Handling in Hadoop and HDFS
Date Mon, 11 Aug 2008 14:29:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12621460#action_12621460
] 

Ankur commented on HADOOP-3856:
-------------------------------

I read a bit about Grizzly framework and tried the attached patch. Grizzly definitely looks
simpler and flexible compared to Mina. It lets you implement custom protocol handlers with
ease and in a way that's easy to read and understand. 

The thing that I especially liked is its simple architecture and the fact that it does not
hide the underlying NIO objects thus providing more control and being flexible.

It appears to be a lightweight NIO framework and their reported performance comparisons with
other NIO frameworks look encouraging.

The only flip-side I see is lack of documentation but shouldn't pose too much of a problem
considering the fact that code seem to have decent javadoc comments.

Licensing of our choice would be CDDL which is compatible with Apache License, I think. So
even that's a non-issue.

Looks like a good candidate to me get started :-).

If folks agree then the next step I guess would be to cook up a patch replacing read/write
in datanode with Async I/O using Grizzly.

> 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
>            Assignee: 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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