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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4879) Add "blocked ArrayList" collection to avoid CMS full GCs
Date Thu, 06 Jun 2013 14:23:22 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13677056#comment-13677056

Daryn Sharp commented on HDFS-4879:

I think this is a great change, but agree that ChunkedArrayList should ideally be a full-fledged
list.  We may find this list implementation to be useful in other places, which is a benefit
over using an actual linked list.

* Default ctor should invoke the ctor with capacity/size to avoid code duplication.
* Consider avoiding need to compute size by tracking it via add/remove?  This would simplify
isEmpty() to size == 0.
* Consider removing multiple calls to addChunk() to seed the main list by folding the logic
into add?  It could add a new chunk if the list is either empty, or the existing full chunk
* Why does each additional chunk's capacity quadruple?  If necessary, it would be more understandable
to multiple by 4.
> 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
> 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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