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From "Sandy McArthur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (POOL-86) GenericKeyedObjectPool retaining too many idle objects
Date Tue, 31 Oct 2006 07:01:18 GMT
    [ http://issues.apache.org/jira/browse/POOL-86?page=comments#action_12445803 ] 
            
Sandy McArthur commented on POOL-86:
------------------------------------

Back to the first part of your original submission: What I understand you really want is a
way to prune the pool size down after a peak load spike.

Would a decorator that either occasionally discards returned objects or uses some heuristics
to determine when to discard returned objects meet your needs? This could be implemented so
that it doesn't need an eviction thread which means we can guarantee thread-safety of the
pool implementation but it would require some pool activity to do it's work.

> GenericKeyedObjectPool retaining too many idle objects
> ------------------------------------------------------
>
>                 Key: POOL-86
>                 URL: http://issues.apache.org/jira/browse/POOL-86
>             Project: Commons Pool
>          Issue Type: Bug
>    Affects Versions: 1.3
>            Reporter: Mike Martin
>         Assigned To: Sandy McArthur
>         Attachments: pool-86.patch, pool-86.withtest.patch
>
>
> There are two somewhat related problems in GenericKeyedObjectPool that cause
> many more idle objects to be retained than should be, for much longer than they
> should be.
> Firstly, borrowObject() is returning the LRU object rather than the MRU object.
> That minimizes rather than maximizes object reuse and tends to refresh all the
> idle objects, preventing them from becoming evictable.
> The idle LinkedList is being maintained with:
>     borrowObject:   list.removeFirst()
>     returnObject:   list.addLast()
> These should either both be ...First() or both ...Last() so the list maintains
> a newer-to-older, or vice-versa, ordering.  The code in evict() works from the
> end of the list which indicates newer-to-older might have been originally
> intended.
> Secondly, evict() itself has a couple of problems, both of which only show up
> when many keys are in play:
> 1.  Once it processes a key it doesn't advance to the next key.
> 2.  _evictLastIndex is not working as documented ("Position in the _pool where
>     the _evictor last stopped").  Instead it's the position where the last scan
>     started, and becomes the position at which it attempts to start scanning
>     *in the next pool*.  That just causes objects eligible for eviction to
>     sometimes be skipped entirely.
> Here's a patch fixing both problems:
> GenericKeyedObjectPool.java
> 990c990
> <             pool.addLast(new ObjectTimestampPair(obj));
> ---
> >             pool.addFirst(new ObjectTimestampPair(obj));
> 1094,1102c1094,1095
> <                 }
> <
> <                 // if we don't have a keyed object pool iterator
> <                 if (objIter == null) {
> <                     final LinkedList list = (LinkedList)_poolMap.get(key);
> <                     if (_evictLastIndex < 0 || _evictLastIndex > list.size())
{
> <                         _evictLastIndex = list.size();
> <                     }
> <                     objIter = list.listIterator(_evictLastIndex);
> ---
> >                     LinkedList list = (LinkedList)_poolMap.get(key);
> >                     objIter = list.listIterator(list.size());
> 1154,1155c1147
> <                     _evictLastIndex = -1;
> <                     objIter = null;
> ---
> >                     key = null;
> 1547,1551d1538
> <
> <     /**
> <      * Position in the _pool where the _evictor last stopped.
> <      */
> <     private int _evictLastIndex = -1;
> I have a local unit test for this but it depends on some other code I can't
> donate.  It works like this:
> 1.  Fill the pool with _maxTotal objects using many different keys.
> 2.  Select X as a small number, e.g. 2.
> 3.  Compute:
>         maxEvictionRunsNeeded = (maxTotal - X) / numTestsPerEvictionRun + 2;
>         maxEvictionTime = minEvictableIdleTimeMillis + maxEvictionRunsNeeded * timeBetweenEvictionRunsMillis;
> 4.  Enter a loop:
>     a.  Borrow X objects.
>     b.  Exit if _totalIdle = 0
>     c.  Return the X objects.
> Fail if loop doesn't exit within maxEvictionTime.
> Mike

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