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From "Benoy Antony (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-11238) Group cache expiry causes namenode slowdown
Date Wed, 12 Nov 2014 22:57:34 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-11238?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14208842#comment-14208842
] 

Benoy Antony commented on HADOOP-11238:
---------------------------------------

Nice way to refresh the cache asynchronously to avoid penalizing few requests.

In the new implementation, it looks like a user's group will be refreshed periodically forever
even if the user accessed Hadoop only once. Is there a way to prevent that ? 

Line #63 is more than 100 lines. 
In line#63 could you keep the type of the variable as _Map_ instead of _ConcurrentHashMap_
?

> 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
>         Attachments: HADOOP-11238.patch
>
>
> Our namenode pauses for 12-60 seconds several times every hour. 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. Each entry has a cache expiry.
> 2. When a cache entry expires, multiple threads can see this expiration and then we have
a thundering herd effect where all these threads hit the wire and overwhelm 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
> 3. Gate the cache refresh so that only one thread is responsible for refreshing the cache



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