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https://issues.apache.org/jira/browse/HDFS-4070?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13478094#comment-13478094
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Andy Isaacson commented on HDFS-4070:
-------------------------------------
I'm a little surprised that increasing buffer size from 64k to 1M would make a
15% difference in throughput. Could you post a complete example benchmark that
demonstrates the effect? Your {{map(\)}} makes sense, but I don't want to
flesh it out with code that "makes sense" to me but might be different from how
you're testing.
What kernel precisely are you observing this on? RHEL5 has merged a fair
number of IO performance-bug-fixes over the years.
Does the issue reproduce on a more modern kernel, such as RHEL6? (I would be
happy to test this case for you given a working example.)
> 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
>
> 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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