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