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https://issues.apache.org/jira/browse/HDFS-8912?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Yi Liu resolved HDFS-8912.
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Resolution: Invalid
I forgot one thing: many variables/methods of java {{HashMap}} we need to touch
in {{ShrinkableHashMap}} are {{package-private}}, so we can't simply extend
{{HashMap}}. Implementing a new {{ShrinkableHashMap}} is a bit heavy, close it
as "Invalid".
> Implement ShrinkableHashMap extends java HashMap and use properly
> -----------------------------------------------------------------
>
> Key: HDFS-8912
> URL: https://issues.apache.org/jira/browse/HDFS-8912
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Reporter: Yi Liu
> Assignee: Yi Liu
>
> Currently {{LightWeightHashSet}} and {{LightWeightLinkedSet}} are used in
> hdfs, there are two advantages compared to java HashSet: one is the entry
> requires fewer memory, another is it's shrinkable. In real cluster, hdfs is
> a long running service, and {{set}} may become large at some time and may
> become small after that, so shrinking the {{set}} when size hits the shrink
> threshold is necessary, it can improve the NN memory.
> Same situation for {{map}}, some HashMap used in BlockManager (e.g., the
> hashmap in CorruptReplicasMap), it's better to be shrinkable.
> I think it's worth to implement ShrinkableHashMap extends the java HashMap,
> for quick glance, seems few code is needed.
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