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Date: Sat, 8 Dec 2007 00:53:43 -0800 (PST)
From: "Mark Thomas (JIRA)" <jira@apache.org>
To: issues@commons.apache.org
Subject: [jira] Commented: (POOL-86) GenericKeyedObjectPool retaining too
 many idle objects
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    [ https://issues.apache.org/jira/browse/POOL-86?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12549673 ] 

Mark Thomas commented on POOL-86:
---------------------------------

I've looked through the code and the patch looks good to me.

> GenericKeyedObjectPool retaining too many idle objects
> ------------------------------------------------------
>
>                 Key: POOL-86
>                 URL: https://issues.apache.org/jira/browse/POOL-86
>             Project: Commons Pool
>          Issue Type: Bug
>    Affects Versions: 1.3
>            Reporter: Mike Martin
>            Assignee: Phil Steitz
>             Fix For: 2.0
>
>         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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