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https://issues.apache.org/jira/browse/HADOOP-11238?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Chris Li updated HADOOP-11238:
------------------------------
Description:
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
was:
Our namenode pauses for 12-60 seconds several times 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. 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
> 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
> Assignee: Chris Li
> Priority: Minor
>
> 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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