BELUGA BEHR created HDFS-14102:
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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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