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From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-4070) DFSClient ignores bufferSize argument & always performs small writes
Date Wed, 17 Oct 2012 11:08:03 GMT
Gopal V created HDFS-4070:
-----------------------------

             Summary: 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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