[jira] [Created] (HDFS-8862) Improve BlockManager#excessReplicateMap

["Yi Liu (JIRA)" <ji...@apache.org>]

Yi Liu created HDFS-8862:
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             Summary: 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


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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