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From "Hadoop QA (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4070) DFSClient ignores bufferSize argument & always performs small writes
Date Wed, 24 Oct 2012 18:10:13 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4070?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13483440#comment-13483440
] 

Hadoop QA commented on HDFS-4070:
---------------------------------

{color:red}-1 overall{color}.  Here are the results of testing the latest attachment 
  http://issues.apache.org/jira/secure/attachment/12549678/HDFS-4090-dfs%2Bpacketsize.patch
  against trunk revision .

    {color:green}+1 @author{color}.  The patch does not contain any @author tags.

    {color:red}-1 tests included{color}.  The patch doesn't appear to include any new or modified
tests.
                        Please justify why no new tests are needed for this patch.
                        Also please list what manual steps were performed to verify this patch.

    {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:red}-1 core tests{color}.  The patch failed these unit tests in hadoop-hdfs-project/hadoop-hdfs:

                  org.apache.hadoop.hdfs.server.blockmanagement.TestUnderReplicatedBlocks

    {color:green}+1 contrib tests{color}.  The patch passed contrib unit tests.

Test results: https://builds.apache.org/job/PreCommit-HDFS-Build/3395//testReport/
Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/3395//console

This message is automatically generated.
                
> DFSClient ignores bufferSize argument & always performs small writes
> --------------------------------------------------------------------
>
>                 Key: HDFS-4070
>                 URL: https://issues.apache.org/jira/browse/HDFS-4070
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: hdfs client
>    Affects Versions: 1.0.3, 2.0.3-alpha
>         Environment: RHEL 5.5 x86_64 (ec2)
>            Reporter: Gopal V
>            Priority: Minor
>              Labels: optimization
>         Attachments: gistfe319436b880026cbad4-aad495d50e0d6b538831327752b984e0fdcc74db.tar.gz,
HDFS-4090-dfs+packetsize.patch
>
>
> The following code illustrates the issue at hand 
> {code}
>  protected void map(LongWritable offset, Text value, Context context) 
>     	        throws IOException, InterruptedException {
> 			OutputStream out = fs.create(new Path("/tmp/benchmark/",value.toString()), true, 1024*1024);

> 			int i;
> 			for(i = 0; i < 1024*1024; i++) {
> 				out.write(buffer, 0, 1024);
> 			}
> 			out.close();
> 			context.write(value, new IntWritable(i));
>     	}
> {code}
> This code is run as a single map-only task with an input file on disk and map-output
to disk.
> {{# su - hdfs -c 'hadoop jar /tmp/dfs-test-1.0-SNAPSHOT-job.jar  file:///tmp/list file:///grid/0/hadoop/hdfs/tmp/benchmark'}}
> In the data node disk access patterns, the following consistent pattern was observed
irrespective of bufferSize provided.
> {code}
> 21119 read(58,  <unfinished ...>
> 21119 <... read resumed> "\0\1\0\0\0\0\0\0\0034\212\0\0\0\0\0\0\0+\220\0\0\0\376\0\262\252ux\262\252u"...,
65557) = 65557
> 21119 lseek(107, 0, SEEK_CUR <unfinished ...>
> 21119 <... lseek resumed> )             = 53774848
> 21119 write(107, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"...,
65024 <unfinished ...>
> 21119 <... write resumed> )             = 65024
> 21119 write(108, "\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux"...,
508 <unfinished ...>
> 21119 <... write resumed> )             = 508
> {code}
> Here fd 58 is the incoming socket, 107 is the blk file and 108 is the .meta file.
> The DFS packet size ignores the bufferSize argument and suffers from suboptimal syscall
& disk performance because of the default 64kb value, as is obvious from the interrupted
read/write operations.
> Changing the packet size to a more optimal 1056405 bytes results in a decent spike in
performance, by cutting down on disk & network iops.
> h3. Average time (milliseconds) for a 10 GB write as 10 files in a single map task
> ||timestamp||65536||1056252||
> |1350469614|88530|78662|
> |1350469827|88610|81680|
> |1350470042|92632|78277|
> |1350470261|89726|79225|
> |1350470476|92272|78265|
> |1350470696|89646|81352|
> |1350470913|92311|77281|
> |1350471132|89632|77601|
> |1350471345|89302|81530|
> |1350471564|91844|80413|
> That is by average an increase from ~115 MB/s to ~130 MB/s, by modifying the global packet
size setting.
> This suggests that there is value in adapting the user provided buffer sizes to hadoop
packet sizing, per stream.

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