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https://issues.apache.org/jira/browse/HADOOP-1565?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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dhruba borthakur updated HADOOP-1565:
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Attachment: memoryReduction2.patch
Merged patch with latest trunk.
> DFSScalability: reduce memory usage of namenode
> -----------------------------------------------
>
> Key: HADOOP-1565
> URL: https://issues.apache.org/jira/browse/HADOOP-1565
> Project: Hadoop
> Issue Type: Bug
> Reporter: dhruba borthakur
> Assignee: dhruba borthakur
> Attachments: memoryReduction2.patch
>
>
> Experiments have demonstrated that a single file/block needs about 300 to 500
> bytes of main memory on a 64-bit Namenode. This puts some limitations on the
> size of the file system that a single namenode can support. Most of this
> overhead occurs because a block and/or filename is inserted into multiple
> TreeMaps and/or HashSets.
> Here are a few ideas that can be measured to see if an appreciable reduction
> of memory usage occurs:
> 1. Change FSDirectory.children from a TreeMap to an array. Do binary search
> in this array while looking up children. This saves a TreeMap object for
> every intermediate node in the directory tree.
> 2. Change INode from an inner class. This saves on one "parent object"
> reference for each INODE instance. 4 bytes per inode.
> 3. Keep all DatanodeDescriptors in an array. BlocksMap.nodes[] is currently a
> 64-bit reference to the DatanodeDescriptor object. Instead, it can be a
> 'short'. This will probably save about 16 bytes per block.
> 4. Change DatanodeDescriptor.blocks from a SortedTreeMap to a HashMap? Block
> report processing CPU cost can increase.
> For the records: TreeMap has the following fields:
> Object key;
> Object value;
> Entry left = null;
> Entry right = null;
> Entry parent;
> boolean color = BLACK;
> and HashMap object:
> final Object key;
> Object value;
> final int hash;
> Entry next;
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