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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9520) PeerCache evicts too frequently causing connection restablishments
Date Tue, 15 Dec 2015 20:38:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9520?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15058753#comment-15058753

Colin Patrick McCabe commented on HDFS-9520:

Right.  {{dfs.client.socketcache.capacity}} is the capacity of the socket cache, not the number
of distinct datanodes it holds.

The right size for {{dfs.client.socketcache.capacity}} depends on a few things.  The more
HDFS input streams you have open at once, the more sockets you will use at once.  The more
datanodes you have in your cluster, the larger you may want the cache to be, so that you get
a better hit rate.

We could certainly raise the default value for this.  Probably it should be at least 32 or

> PeerCache evicts too frequently causing connection restablishments
> ------------------------------------------------------------------
>                 Key: HDFS-9520
>                 URL: https://issues.apache.org/jira/browse/HDFS-9520
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: Rajesh Balamohan
>         Attachments: HDFS-9520.png
> Env: 20 node setup
> dfs.client.socketcache.capacity = 16
> Issue:
> ======
> Monitored PeerCache and it was evicting lots of connections during close. Set "dfs.client.socketcache.capacity=20"
and tested again. Evictions still happened. Screenshot of profiler is attached in the JIRA.
> Workaround:
> ===========
> Temp fix was to set "dfs.client.socketcache.capacity=1000" to prevent eviction. 
> Added more debug logs revealed that multimap.size() was 40 instead of 20. LinkedListMultimap
returns the total values instead of key size causing lots of evictions.
> {code}
>    if (capacity == multimap.size()) {
>       evictOldest();
>     }
> {code}
> Should this be (capacity == multimap.keySet().size())  or is it expected that the "dfs.client.socketcache.capacity"
be set to very high value?
> \cc [~gopalv], [~sseth]

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