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From "Eric Newton (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-3982) Add LRU mechanism to address memory concerns and possibly pre-fetch Tablet Locations.
Date Mon, 31 Aug 2015 23:53:46 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-3982?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14724419#comment-14724419
] 

Eric Newton commented on ACCUMULO-3982:
---------------------------------------

It took about 0.5G. I also did experiments with a ConcurrentSkipListMap. The TreeMap was slightly
faster than a SkipList. The SkipList used slightly less RAM (by a few percent).

The ConcurrentSkipListMap doesn't have the rest of the stuff needed for all the Guava cache
goodness, but it would be able to support different concurrent update strategies.

If we have both data structures, we can do our first look-ups on Cache, which will normally
find the tablet, and that's a hash lookup, which is significantly faster.  So that would be
a nice benefit.

I was surprised at how fast the deletes went.


> Add LRU mechanism to address memory concerns and possibly pre-fetch Tablet Locations.
> -------------------------------------------------------------------------------------
>
>                 Key: ACCUMULO-3982
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-3982
>             Project: Accumulo
>          Issue Type: New Feature
>          Components: client, tserver
>    Affects Versions: 1.6.3, 1.7.0, 1.8.0
>            Reporter: marco polo
>            Assignee: Eric Newton
>             Fix For: 1.8.0
>
>
> On larger systems, especially those of which may split frequently, I've seen churn in
the TabletLocator caches. Could we possibly pre-fetch entries for a table so that we don't
constantly access the metadata table for tablet locations. 
> Additionally, instead of clearing the cache in the case of a miss or failure, could we
simply update those entries in the cache? If we do this we may minimize cache misses overall
and the need to scan the meta data table. If need be, we could have a threshold before clearing
the cache entirely if people see the need for that.
> It would also be helpful to add an LRU cache ( or maybe even priority based LRU cache
). In ACCUMULO-3549, we added a cache clear to TabletServer to avoid overwhelming memory.
An LRU cache might address this concern and make the clear cache call unnecessary.



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