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https://issues.apache.org/jira/browse/HDFS-8862?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14700560#comment-14700560
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Yi Liu commented on HDFS-8862:
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Thanks [~cmccabe] for the review. Will commit the patch later.
{{LightWeightLinkedSet<BlockInfo>}} can shrink..
> Improve BlockManager#excessReplicateMap
> ---------------------------------------
>
> 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
> 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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