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From "Chris Li (JIRA)" <j...@apache.org>
Subject [jira] [Assigned] (HADOOP-11238) Group cache expiry causes namenode slowdown
Date Mon, 27 Oct 2014 21:53:34 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-11238?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Chris Li reassigned HADOOP-11238:
---------------------------------

    Assignee: Chris Li

> Group cache expiry causes namenode slowdown
> -------------------------------------------
>
>                 Key: HADOOP-11238
>                 URL: https://issues.apache.org/jira/browse/HADOOP-11238
>             Project: Hadoop Common
>          Issue Type: Bug
>    Affects Versions: 2.5.1
>            Reporter: Chris Li
>            Assignee: Chris Li
>            Priority: Minor
>
> Our namenode pauses for 12-60 seconds several times every hour or so. During these pauses,
no new requests can come in.
> Around the time of pauses, we have log messages such as:
> 2014-10-22 13:24:22,688 WARN org.apache.hadoop.security.Groups: Potential performance
problem: getGroups(user=xxxxx) took 34507 milliseconds.
> The current theory is:
> 1. Groups has a cache that is refreshed periodically. 
> 2. When the cache is cleared, we have a thundering herd effect which overwhelms our LDAP
servers (we are using ShellBasedUnixGroupsMapping with sssd, how this happens has yet to be
established)
> 3. group resolution queries begin to take longer, I've observed it taking 1.2 seconds
instead of the usual 0.01-0.03 seconds when measuring in the shell `time groups myself`
> 4. If there is mutual exclusion somewhere along this path, a 1 second pause could lead
to a 60 second pause as all the threads compete for the resource. The exact cause hasn't been
established
> Potential solutions include:
> 1. Increasing group cache time, which will make the issue less frequent
> 2. Rolling evictions of the cache so we prevent the large spike in LDAP queries



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