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From "Mike Martin (JIRA)" <j...@apache.org>
Subject [jira] Commented: (POOL-86) GenericKeyedObjectPool retaining too many idle objects
Date Tue, 31 Oct 2006 16:54:49 GMT
    [ http://issues.apache.org/jira/browse/POOL-86?page=comments#action_12445951 ] 
            
Mike Martin commented on POOL-86:
---------------------------------

Sandy wrote:
> I think the changes you suggest to the evict method will fail to make forward
> progress if getNumTests() is less than the number of idle objects for the
> current key. This is why the _evictLastIndex was needed.

_recentlyEvictedKeys can serve the same purpose.  This change would do that:

--- GenericKeyedObjectPool.java.prev    Tue Oct 31 10:24:44 2006
+++ GenericKeyedObjectPool.java Tue Oct 31 10:25:06 2006
@@ -1207,6 +1207,7 @@
                         return;
                     }
                     key = keyIter.next();
+                    _recentlyEvictedKeys.add(key);
                     LinkedList list = (LinkedList)_poolMap.get(key);
                     objIter = list.listIterator(list.size());
                 }
@@ -1259,7 +1260,6 @@
                     }
                 } else {
                     // else done evicting keyed pool
-                    _recentlyEvictedKeys.add(key);
                     key = null;
                 }
             }

> 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.

I think what I want, and what most DB connection pools need, is the existing
idle eviction facility as advertised.

I've never encountered the thread-safety problems you mention.  Is there an
outstanding bug regarding that?  If so, it's not occurring in my environment
which as I said involves hundreds of keys (DB users), thousands of connections,
and in fact is used by hundreds of threads.

> 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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