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https://issues.apache.org/jira/browse/HDFS-4879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13746946#comment-13746946
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Aaron T. Myers commented on HDFS-4879:
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I agree with Colin's comment, but would also add that I believe the following
code comment is no longer correct as of the latest rev, due to the lazy
initialization:
{code}
+ * The chunks which make up the full list. This array always
+ * has at least one element, even in an empty list, to make
+ * some of the code simpler.
{code}
+1 from me once both of the above are addressed.
> Add "blocked ArrayList" collection to avoid CMS full GCs
> --------------------------------------------------------
>
> Key: HDFS-4879
> URL: https://issues.apache.org/jira/browse/HDFS-4879
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Affects Versions: 3.0.0, 2.0.4-alpha
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
> Attachments: hdfs-4879.txt, hdfs-4879.txt, hdfs-4879.txt
>
>
> We recently saw an issue where a large deletion was issued which caused 25M
> blocks to be collected during {{deleteInternal}}. Currently, the list of
> collected blocks is an ArrayList, meaning that we had to allocate a
> contiguous 25M-entry array (~400MB). After a NN has been running for a long
> amount of time, the old generation may become fragmented such that it's hard
> to find a 400MB contiguous chunk of heap.
> In general, we should try to design the NN such that the only large objects
> are long-lived and created at startup time. We can improve this particular
> case (and perhaps some others) by introducing a new List implementation which
> is made of a linked list of arrays, each of which is size-limited (eg to 1MB).
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