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From "Arpit Agarwal (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HADOOP-13263) Reload cached groups in background after expiry
Date Wed, 27 Jul 2016 23:53:20 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-13263?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Arpit Agarwal updated HADOOP-13263:
-----------------------------------
    Release Note: hadoop.security.groups.cache.background.reload can be set to true to enable
background reload of expired groups cache entries. This setting can improve the performance
of services that use Groups.java (e.g. the NameNode) when group lookups are slow. The setting
is disabled by default.  (was: hadoop.security.groups.cache.background.reload can be set to
true to enable background reload of expired groups cache entries. This setting can improve
the performance of processes that use Groups.java (e.g. the NameNode) when group lookups are
slow. The setting is disabled by default.)

> Reload cached groups in background after expiry
> -----------------------------------------------
>
>                 Key: HADOOP-13263
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13263
>             Project: Hadoop Common
>          Issue Type: Improvement
>            Reporter: Stephen O'Donnell
>            Assignee: Stephen O'Donnell
>             Fix For: 2.8.0
>
>         Attachments: HADOOP-13263.001.patch, HADOOP-13263.002.patch, HADOOP-13263.003.patch,
HADOOP-13263.004.patch, HADOOP-13263.005.patch, HADOOP-13263.006.patch, HADOOP-13263.007.patch
>
>
> In HADOOP-11238 the Guava cache was introduced to allow refreshes on the Namenode group
cache to run in the background, avoiding many slow group lookups. Even with this change, I
have seen quite a few clusters with issues due to slow group lookups. The problem is most
prevalent in HA clusters, where a slow group lookup on the hdfs user can fail to return for
over 45 seconds causing the Failover Controller to kill it.
> The way the current Guava cache implementation works is approximately:
> 1) On initial load, the first thread to request groups for a given user blocks until
it returns. Any subsequent threads requesting that user block until that first thread populates
the cache.
> 2) When the key expires, the first thread to hit the cache after expiry blocks. While
it is blocked, other threads will return the old value.
> I feel it is this blocking thread that still gives the Namenode issues on slow group
lookups. If the call from the FC is the one that blocks and lookups are slow, if can cause
the NN to be killed.
> Guava has the ability to refresh expired keys completely in the background, where the
first thread that hits an expired key schedules a background cache reload, but still returns
the old value. Then the cache is eventually updated. This patch introduces this background
reload feature. There are two new parameters:
> 1) hadoop.security.groups.cache.background.reload - default false to keep the current
behaviour. Set to true to enable a small thread pool and background refresh for expired keys
> 2) hadoop.security.groups.cache.background.reload.threads - only relevant if the above
is set to true. Controls how many threads are in the background refresh pool. Default is 1,
which is likely to be enough.



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