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From "Yi Liu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8862) BlockManager#excessReplicateMap should use a HashMap
Date Tue, 18 Aug 2015 01:33:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8862?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14700577#comment-14700577

Yi Liu commented on HDFS-8862:

One more discussion, do you think it's worth to extend java HashMap and implement the {{shrink}}?
Since it's better to have the shrinked HashMap in some places. From my point of review, I
think it's worth and for quick glance, seems few code is needed.

> BlockManager#excessReplicateMap should use a HashMap
> ----------------------------------------------------
>                 Key: HDFS-8862
>                 URL: https://issues.apache.org/jira/browse/HDFS-8862
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: namenode
>            Reporter: Yi Liu
>            Assignee: Yi Liu
>             Fix For: 2.8.0
>         Attachments: HDFS-8862.001.patch
> Per [~cmccabe]'s comments in HDFS-8792, this JIRA is to discuss improving {{BlockManager#excessReplicateMap}}.
> That's right HashMap don't ever shrink when elements are removed,  but TreeMap entry
needs to store more (memory) references (left,  right, parent) than HashMap entry (only one
reference next),  even when there is element removing and cause some entry empty, the empty
HashMap entry is just a {{null}} reference (4 bytes),  so they are close at this point.  On
the other hand, the key of {{excessReplicateMap}} is datanode uuid, so the entries number
is almost fixed, so HashMap memory is good than TreeMap memory in this case.   I think the
most important is the search/insert/remove performance, HashMap is absolutely better than
TreeMap.  Because we don't need to sort,  we should use HashMap instead of TreeMap

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