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https://issues.apache.org/jira/browse/HDFS-4461?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13568920#comment-13568920
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Colin Patrick McCabe commented on HDFS-4461:
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bq. I doubt that the directory scanner is the cause of OOM error. It is
probably happening due to some other issue. How many blocks per storage
directory do you have, when OOME happened?
we analyzed a DN heap dump from a production cluster with eclipse memory
analyzer and found that the memory was full of ScanInfo objects. The memory
histogram showed that {{java.lang.String}} was the third-largest consumer of
memory in the system. Unfortunately I can't share the heap dump.
bq. I have hard time understanding the picture. How many bytes are we saving
per ScanInfo?
In the particular case shown in memory-analysis.png, we save 86 characters in
each string. The volume prefix that we avoid storing is
{{/home/cmccabe/hadoop4/hadoop-hdfs-project/hadoop-hdfs/build//test/data/dfs/data/data1/}}.
Java uses 2 bytes per character (UCS-2 encoding), and we store both metaPath
and blockPath, so multiply that by 4 to get 344. Then add the overhead of
using two objects File that contain the path string instead of just the string
itself-- probably around an extra 16 bytes per object, for 376 bytes in total
saved per {{ScanInfo}}.
You might think that
{{/home/cmccabe/hadoop4/hadoop-hdfs-project/hadoop-hdfs/build//test/data/dfs/data/data1/}}
is an unrealistically long volume path, but here is an example of a real
volume path in use on a production cluster:
{{/mnt/hdfs/hdfs01/10769eef-a23a-4300-b45b-749221786109/dfs/dn}}.
Putting the disk UUID into the volume is an obvious thing to do if you're a
system administrator.
> DirectoryScanner: volume path prefix takes up memory for every block that is
> scanned
> -------------------------------------------------------------------------------------
>
> Key: HDFS-4461
> URL: https://issues.apache.org/jira/browse/HDFS-4461
> Project: Hadoop HDFS
> Issue Type: Improvement
> Affects Versions: 2.0.3-alpha
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Priority: Minor
> Attachments: HDFS-4461.002.patch, HDFS-4461.003.patch,
> memory-analysis.png
>
>
> In the {{DirectoryScanner}}, we create a class {{ScanInfo}} for every block.
> This object contains two File objects-- one for the metadata file, and one
> for the block file. Since those File objects contain full paths, users who
> pick a lengthly path for their volume roots will end up using an extra
> N_blocks * path_prefix bytes per block scanned. We also don't really need to
> store File objects-- storing strings and then creating File objects as needed
> would be cheaper. This has been causing out-of-memory conditions for users
> who pick such long volume paths.
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