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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-9937) Improvement of the group information refer frequency
Date Mon, 09 Sep 2013 17:23:52 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-9937?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13762056#comment-13762056

Colin Patrick McCabe commented on HADOOP-9937:

We put a lock around getgrname_r by default, since a large percentage of systems out there
have an implementation that actually is not re-entrant (despite the _r).  So you may not get
as large a speedup as you think by removing the lock in {{UserGroupInformation#getGroupNames}}
(unless you also set {{hadoop.workaround.non.threadsafe.getpwuid}} to {{false}}).

You could create a custom lock just for group information (rather than synchronizing on the
big UGI lock), but I'm not sure how much that would help.  Do you have a more detailed proposal
for how to change the synchronization here?
> Improvement of the group information refer frequency
> ----------------------------------------------------
>                 Key: HADOOP-9937
>                 URL: https://issues.apache.org/jira/browse/HADOOP-9937
>             Project: Hadoop Common
>          Issue Type: Improvement
>          Components: security
>    Affects Versions: 3.0.0
>            Reporter: Shinichi Yamashita
>            Priority: Minor
>         Attachments: namenode-threaddump.txt
> The node (e.g. NameNode, ResourceManager) uses UGI.getGroupNames() now to get the information
of the user's group who accessed it.
> In UGI.getGroupsNames(), synchronized is declared, but UGI instance by various methods
each time and the node get different lock in getGroupName().
> For example, when cache time limit in userToGroupsMap of Groups expired and the node
accepts many requests at the same time, each refers for group information in id command or
> To improve the refer frequency of the group information, it should change about synchronization.

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