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From "Suresh Srinivas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4461) DirectoryScanner: volume path prefix takes up memory for every block that is scanned
Date Fri, 01 Feb 2013 18:16:14 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4461?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13568930#comment-13568930
] 

Suresh Srinivas commented on HDFS-4461:
---------------------------------------

bq. 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.
A server generally has a lot of String objects. There are also file objects in ReplicasMap,
string paths tracked in many other places as well.

This patch indeed saves few bytes. However I do not think this is either the cause of the
OOME or is likely to solve that issue. ScanInfo is a short lived object, unlike other data
structures that are long lived.

Can you answer the following question, I previously asked:
bq. How many blocks per storage directory do you have, when OOME happened?

Or at least the number of ScanInfo objects you saw.
                
> 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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