Chris Li created HADOOP-11238:
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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 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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