[jira] [Created] (HADOOP-17408) Optimize NetworkTopology while sorting of block locations

["Ahmed Hussein (Jira)" <ji...@apache.org>]

Ahmed Hussein created HADOOP-17408:
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             Summary: Optimize NetworkTopology while sorting of block locations
                 Key: HADOOP-17408
                 URL: https://issues.apache.org/jira/browse/HADOOP-17408
             Project: Hadoop Common
          Issue Type: Improvement
          Components: common, net
            Reporter: Ahmed Hussein
            Assignee: Ahmed Hussein


In {{NetworkTopology}}, I noticed that there are some hanging fruits to improve the performance.

Inside {{sortByDistance}}, collections.shuffle is performed on the list before calling {{secondarySort}}.

{code:java}
Collections.shuffle(list, r);
if (secondarySort != null) {
  secondarySort.accept(list);
}
{code}

However, in different call sites, {{collections.shuffle}} is passed as the secondarySort to {{sortByDistance}}. This means that the shuffle is executed twice on each list.
Also, logic wise, it is useless to shuffle before applying a tie breaker which might make the shuffle work obsolete.

In addition, [~daryn] reported that:
* topology is unnecessarily locking/unlocking to calculate the distance for every node
* shuffling uses a seeded Random, instead of ThreadLocalRandom, which is heavily synchronized




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