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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