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Date: Sat, 13 Dec 2014 15:26:16 +0000 (UTC)
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Subject: [jira] [Commented] (HADOOP-11238) Update the NameNode's Group Cache
 in the background when possible
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    [ https://issues.apache.org/jira/browse/HADOOP-11238?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14245375#comment-14245375 ] 

Hudson commented on HADOOP-11238:
---------------------------------

FAILURE: Integrated in Hadoop-Mapreduce-trunk #1991 (See [https://builds.apache.org/job/Hadoop-Mapreduce-trunk/1991/])
HADOOP-11238. Update the NameNode's Group Cache in the background when possible (Chris Li via Colin P. McCabe) (cmccabe: rev e5a692519956aefb3a540ed0137b63cf598ac10d)
* hadoop-common-project/hadoop-common/CHANGES.txt
* hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/security/TestGroupsCaching.java
* hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/security/Groups.java


> 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
>         Attachments: HADOOP-11238.003.patch, HADOOP-11238.003.patch, HADOOP-11238.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 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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