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

["Todd Lipcon (JIRA)" <ji...@apache.org>]

Todd Lipcon created HDFS-4879:
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             Summary: 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: 2.0.4-alpha, 3.0.0
            Reporter: Todd Lipcon
            Assignee: Todd Lipcon


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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