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From "BELUGA BEHR (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-14102) verifyBlockPlacement
Date Tue, 27 Nov 2018 16:07:00 GMT
BELUGA BEHR created HDFS-14102:
----------------------------------

             Summary: verifyBlockPlacement
                 Key: HDFS-14102
                 URL: https://issues.apache.org/jira/browse/HDFS-14102
             Project: Hadoop HDFS
          Issue Type: Improvement
            Reporter: BELUGA BEHR


 
{code:java|title=BlockPlacementPolicyDefault.java}
    // 1. Check that all locations are different.
    // 2. Count locations on different racks.
    Set<String> racks = new TreeSet<>();
    for (DatanodeInfo dn : locs)
      racks.add(dn.getNetworkLocation());
...
    racks.size(){code}
 
 Here, the code is counting the number of distinct Network Locations. However, it is using
a TreeSet which has overhead to maintain item order and uses a linked structure internally.
This overhead is unneeded since all that is required here is a count.
{quote}A NavigableSet implementation based on a TreeMap. The elements are ordered using their
natural ordering, or by a Comparator provided at set creation time, depending on which constructor
is used.
 This implementation provides guaranteed log(n) time cost for the basic operations (add, remove
and contains).

[https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html]
{quote}
 
 Use Java streams for readability and because it uses a {{HashSet}} under the covers to perform
the distinct action. {{HashSet}} uses an array internally and has constant time performance
for the {{add}} method.

[https://github.com/apache/hadoop/blob/27978bcb66a9130cbf26d37ec454c0b7fcdc2530/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockPlacementPolicyDefault.java#L1042]



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