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  1. Hadoop Common
  2. HADOOP-1565

DFSScalability: reduce memory usage of namenode

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major
    • Resolution: Fixed
    • Affects Version/s: None
    • Fix Version/s: 0.15.0
    • Component/s: None
    • Labels:
      None

      Description

      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;

        Attachments

        1. memoryReduction3.patch
          5 kB
          Dhruba Borthakur

          Activity

            People

            • Assignee:
              dhruba Dhruba Borthakur
              Reporter:
              dhruba Dhruba Borthakur
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              Dates

              • Created:
                Updated:
                Resolved: