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From "Allen Wittenauer (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-347) DFS read performance suboptimal when client co-located on nodes with data
Date Thu, 10 Feb 2011 16:48:57 GMT

    [ https://issues.apache.org/jira/browse/HDFS-347?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12993109#comment-12993109
] 

Allen Wittenauer commented on HDFS-347:
---------------------------------------

Short Story:

I need to look at the patch but keep in mind this *should* work on pretty every modern UNIX
and UNIX-like system. Heck, even less-than-mainstream-these-days OSes like Tru64 support these
interfaces.  So we should make sure that this optimization isn't escaped off to only work
on Linux machines.

Long Story:

If I remember my history correctly, UNIX domain sockets were originally SVID and "bumped up"
to be a POSIX standard later on.  So any System V-compliant OS should work--thus covering
HP-UX and Solaris. The BSDs and Linux kernels are covered when one compiles with the STREAMS
interfaces enabled, IIRC.  (In other words, someone using Gentoo or a hand-built FreeBSD install
might need to recompile their kernel). Apple does appear to compile in STREAMS for at least
10.6.  I think AIX falls under SystemV based (even though I worked on it for years, I'm still
not sure of its origin. ;) )

I'm not sure if cygwin or Windows natively support STREAMS.  So that should be the only OS
this change may not work on.

> DFS read performance suboptimal when client co-located on nodes with data
> -------------------------------------------------------------------------
>
>                 Key: HDFS-347
>                 URL: https://issues.apache.org/jira/browse/HDFS-347
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: George Porter
>            Assignee: Todd Lipcon
>         Attachments: BlockReaderLocal1.txt, HADOOP-4801.1.patch, HADOOP-4801.2.patch,
HADOOP-4801.3.patch, HDFS-347-branch-20-append.txt, all.tsv, hdfs-347.png, hdfs-347.txt, local-reads-doc
>
>
> One of the major strategies Hadoop uses to get scalable data processing is to move the
code to the data.  However, putting the DFS client on the same physical node as the data blocks
it acts on doesn't improve read performance as much as expected.
> After looking at Hadoop and O/S traces (via HADOOP-4049), I think the problem is due
to the HDFS streaming protocol causing many more read I/O operations (iops) than necessary.
 Consider the case of a DFSClient fetching a 64 MB disk block from the DataNode process (running
in a separate JVM) running on the same machine.  The DataNode will satisfy the single disk
block request by sending data back to the HDFS client in 64-KB chunks.  In BlockSender.java,
this is done in the sendChunk() method, relying on Java's transferTo() method.  Depending
on the host O/S and JVM implementation, transferTo() is implemented as either a sendfilev()
syscall or a pair of mmap() and write().  In either case, each chunk is read from the disk
by issuing a separate I/O operation for each chunk.  The result is that the single request
for a 64-MB block ends up hitting the disk as over a thousand smaller requests for 64-KB each.
> Since the DFSClient runs in a different JVM and process than the DataNode, shuttling
data from the disk to the DFSClient also results in context switches each time network packets
get sent (in this case, the 64-kb chunk turns into a large number of 1500 byte packet send
operations).  Thus we see a large number of context switches for each block send operation.
> I'd like to get some feedback on the best way to address this, but I think providing
a mechanism for a DFSClient to directly open data blocks that happen to be on the same machine.
 It could do this by examining the set of LocatedBlocks returned by the NameNode, marking
those that should be resident on the local host.  Since the DataNode and DFSClient (probably)
share the same hadoop configuration, the DFSClient should be able to find the files holding
the block data, and it could directly open them and send data back to the client.  This would
avoid the context switches imposed by the network layer, and would allow for much larger read
buffers than 64KB, which should reduce the number of iops imposed by each read block operation.

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