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https://issues.apache.org/jira/browse/HDFS-12922?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Misha Dmitriev updated HDFS-12922:
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Attachment: screenshot-1.png
> Arrays of length 1 cause 9.2% memory overhead
> ---------------------------------------------
>
> Key: HDFS-12922
> URL: https://issues.apache.org/jira/browse/HDFS-12922
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Misha Dmitriev
> Assignee: Misha Dmitriev
> Attachments: screenshot-1.png
>
>
> I recently obtained a big (over 60GiB) heap dump from a customer and analyzed
> it using jxray (www.jxray.com). One source of memory waste that the tool
> detected is arrays of length 1 that come from {{BlockInfo[]
> org.apache.hadoop.hdfs.server.namenode.INodeFile.blocks}} and
> {{INode$Feature[]
> org.apache.hadoop.hdfs.server.namenode.INodeFile.features}}. Only a small
> fraction of these arrays (less than 10%) have a length greater than 1.
> Collectively these arrays waste 5.5GiB, or 9.2% of the heap. See the attached
> screenshot for more details.
> The reason why an array of length 1 is problematic is that every array in the
> JVM has a header, that takes between 16 and 20 bytes depending on the JVM
> configuration. For a big enough array this 16-20 byte overhead is not a
> concern, but if the array has only one element (that takes 4-8 bytes
> depending on the JVM configuration), the overhead becomes bigger than the
> array's "workload".
> In such a situation it makes sense to replace the array data field {{Foo[]
> ar}} with an {{Object obj}}, that would contain either a direct reference to
> the array's single workload element, or a reference to the array if there is
> more than one element. This change will require further code changes and type
> casts. For example, code like {{return ar[i];}} becomes {{return (obj
> instanceof Foo) ? (Foo) obj : ((Foo[]) obj)[i];}} and so on. This doesn't
> look very pretty, but as far as I see, the code that deals with e.g.
> INodeFile.blocks already contains various null checks, etc. So we will not
> make the code much less readable.
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