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From "Mike Martin (JIRA)" <j...@apache.org>
Subject [jira] Created: (POOL-86) GenericKeyedObjectPool retaining too many idle objects
Date Tue, 10 Oct 2006 23:03:19 GMT
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


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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