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

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

     [ https://issues.apache.org/jira/browse/HADOOP-17408?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Work on HADOOP-17408 started by Ahmed Hussein.
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> 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
>            Priority: Major
>
> 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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