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From "Chris Li (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-11238) Group cache expiry causes namenode slowdown
Date Mon, 27 Oct 2014 21:36:34 GMT
Chris Li created HADOOP-11238:

             Summary: 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
            Priority: Minor

Our namenode pauses for 12-60 seconds 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

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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