Details
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Bug
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Status: Open
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Minor
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Resolution: Unresolved
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1.0.3, 2.0.3-alpha
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None
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RHEL 5.5 x86_64 (ec2)
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Allow write packet sizes to be configurable for DFS input streams
Description
The following code illustrates the issue at hand
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)); }
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.
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
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.
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.
Attachments
Attachments
Issue Links
- is part of
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HDFS-4225 Improve HDFS write performance
- Open