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https://issues.apache.org/jira/browse/HDFS-1114?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12861725#action_12861725
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Tsz Wo (Nicholas), SZE commented on HDFS-1114:
----------------------------------------------
In Java 1.6, java.util.HashMap has an array of HashMap.Entry
{code}
//HashMap
transient Entry[] table;
{code}
where HashMap.Entry has the following members
{code}
//HashMap.Entry
final K key;
V value;
Entry<K,V> next;
final int hash;
{code}
In the BlocksMap, we have the following invariants
- K == V == BlockInfo
- key == value, i.e. they point to the same BlockInfo object.
Therefore, we may reduce the memory footprint by
# eliminating HashMap.Entry class
# keeping only one object reference
# possibly eliminating next and hash
Will provide more design details.
> Reducing NameNode memory usage by an alternate hash table
> ---------------------------------------------------------
>
> Key: HDFS-1114
> URL: https://issues.apache.org/jira/browse/HDFS-1114
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node
> Reporter: Tsz Wo (Nicholas), SZE
> Assignee: Tsz Wo (Nicholas), SZE
>
> NameNode uses a java.util.HashMap to store BlockInfo objects. When there are
> many blocks in HDFS, this map uses a lot of memory in the NameNode. We may
> optimize the memory usage by a light weight hash table implementation.
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