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From "Yi Liu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8792) Improve BlockManager#postponedMisreplicatedBlocks and BlockManager#excessReplicateMap
Date Thu, 06 Aug 2015 01:18:07 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8792?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14659302#comment-14659302

Yi Liu commented on HDFS-8792:

Thanks [~cmccabe] for the review.

This JavaDoc comment is stale. It seems like it should be "the current modification epoch"
or similar.
Will update it in the new patch right now.

Can you put this in a separate JIRA? It's less obvious to me that this is a win. Java HashMaps
don't ever shrink, whereas TreeMap uses less memory when elements are removed.
Sure, I will put it in a separate JIRA.  
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, the empty HashMap entry is just a {{null}} reference
(4 bytes),  so they are nearly 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.

> Improve BlockManager#postponedMisreplicatedBlocks and BlockManager#excessReplicateMap
> -------------------------------------------------------------------------------------
>                 Key: HDFS-8792
>                 URL: https://issues.apache.org/jira/browse/HDFS-8792
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Yi Liu
>            Assignee: Yi Liu
>         Attachments: HDFS-8792.001.patch, HDFS-8792.002.patch
> {{LightWeightHashSet}} requires fewer memory than java hashset. 
> Furthermore, for {{excessReplicateMap}}, we can use {{HashMap}} instead of {{TreeMap}}
instead, since no need to sort. 

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