[jira] [Commented] (HDFS-4879) Add "blocked ArrayList" collection to avoid CMS full GCs

["Colin Patrick McCabe (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-4879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13675424#comment-13675424 ] 

Colin Patrick McCabe commented on HDFS-4879:
--------------------------------------------

This is a good improvement; it should be really helpful in keeping those GCs down.

Since you're not actually implementing {{java.util.List}}, why not name it something like {{BlockedArrayContainer}}?

As long as we're merely implementing {{Iterable}}, do we have to implement {{get}} at all?  it seems to be fairly inefficient, so it might encourage the wrong kind of programmer behavior.
                
> 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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