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From dhutchis <...@git.apache.org>
Subject [GitHub] accumulo pull request: ACCUMULO-4229 BatchWriter Locator cache out...
Date Fri, 29 Apr 2016 01:48:13 GMT
Github user dhutchis commented on the pull request:

    https://github.com/apache/accumulo/pull/96#issuecomment-215609729
  
    The desired behavior is: remove a TabletLocator from the locators map one hour *after*
no other strong references to it exist. Reset the timer when a new thread accesses the TabletLocator.
    
    Sadly I don't see an easy way to implement this behavior.  
    
    1. If we use a timer with Guava, then a TabletLocator may be removed from the map if a
thread uses the TabletLocator for at least an hour, which could easily happen if the thread
locally caches the locator.
    2. If we use soft references, then TabletLocators may be removed from the map (in an LRU
manner) at any time there are no other references to them if the gc is pressured. This could
affect performance in edge cases.
    3. Using both a timer and soft references has both problems.
    
    The approach in this PR forces threads to re-fetch a TabletLocator from the map if the
locators in the map are cleared.  This happens once per hour and is done automatically by
SyncingTabletLocator.  This solves the problem on its own and may be good enough. 
    
    Perhaps the best solution is to combine it with the Guava timer (point 1 above): replacing
the locators map with a Guava cache, letting entries expire one hour after they are last accessed.
We may then remove the `synchronized` statements on TabletLocator.
    
    Think its worth going for, or shall we stick with this? 


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