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Hadoop QA commented on HDFS-347: -------------------------------- {color:green}+1 overall{color}. Here are the results of testing the latest attachment http://issues.apache.org/jira/secure/attachment/12569632/2012.02.15.consolidated3.patch against trunk revision . {color:green}+1 @author{color}. The patch does not contain any @author tags. {color:green}+1 tests included{color}. The patch appears to include 23 new or modified test files. {color:green}+1 javac{color}. The applied patch does not increase the total number of javac compiler warnings. {color:green}+1 javadoc{color}. The javadoc tool did not generate any warning messages. {color:green}+1 eclipse:eclipse{color}. The patch built with eclipse:eclipse. {color:green}+1 findbugs{color}. The patch does not introduce any new Findbugs (version 1.3.9) warnings. {color:green}+1 release audit{color}. The applied patch does not increase the total number of release audit warnings. {color:green}+1 core tests{color}. The patch passed unit tests in . {color:green}+1 contrib tests{color}. The patch passed contrib unit tests. Test results: https://builds.apache.org/job/PreCommit-HDFS-Build/3975//testReport/ Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/3975//console This message is automatically generated. > 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 > Components: datanode, hdfs-client, performance > Reporter: George Porter > Assignee: Colin Patrick McCabe > Attachments: 2012.02.15.consolidated3.patch, 2013.01.28.design.pdf, > 2013.01.31.consolidated2.patch, 2013.01.31.consolidated.patch, all.tsv, > BlockReaderLocal1.txt, full.patch, HADOOP-4801.1.patch, HADOOP-4801.2.patch, > HADOOP-4801.3.patch, HDFS-347-016_cleaned.patch, HDFS-347.016.patch, > HDFS-347.017.clean.patch, HDFS-347.017.patch, HDFS-347.018.clean.patch, > HDFS-347.018.patch2, HDFS-347.019.patch, HDFS-347.020.patch, > HDFS-347.021.patch, HDFS-347.022.patch, HDFS-347.024.patch, > HDFS-347.025.patch, HDFS-347.026.patch, HDFS-347.027.patch, > HDFS-347.029.patch, HDFS-347.030.patch, HDFS-347.033.patch, > HDFS-347.035.patch, HDFS-347-branch-20-append.txt, hdfs-347-merge.txt, > hdfs-347-merge.txt, hdfs-347-merge.txt, 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. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira