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From "Vinod Kumar Vavilapalli (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HADOOP-11238) Update the NameNode's Group Cache in the background when possible
Date Sat, 18 Jul 2015 00:52:04 GMT

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

Vinod Kumar Vavilapalli updated HADOOP-11238:
    Labels: 2.6.1-candidate  (was: )

> Update the NameNode's Group Cache in the background when possible
> -----------------------------------------------------------------
>                 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
>              Labels: 2.6.1-candidate
>             Fix For: 2.7.0
>         Attachments: HADOOP-11238.003.patch, HADOOP-11238.003.patch, HADOOP-11238.patch,
> This patch addresses an issue where the namenode pauses during group resolution by only
allowing a single group resolution query on expiry. There are two scenarios:
> 1. When there is not yet a value in the cache, all threads which make a request will
block while a single thread fetches the value.
> 2. When there is already a value in the cache and it is expired, the new value will be
fetched in the background while the old value is used by other threads
> This is handled by guava's cache.
> Negative caching is a feature built into the groups cache, and since guava's caches don't
support different expiration times, we have a separate negative cache which masks the guava
cache: if an element exists in the negative cache and isn't expired, we return it.
> In total the logic for fetching a group is:
> 1. If username exists in static cache, return the value (this was already present)
> 2. If username exists in negative cache and negative cache is not expired, raise an exception
as usual
> 3. Otherwise Defer to guava cache (see two scenarios above)
> Original Issue Below:
> ----------------------------
> 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
> 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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