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From Mark Thomas <>
Subject Re: [pool] equal instances
Date Sat, 11 Jun 2011 11:31:02 GMT
On 11/06/2011 12:26, Honton, Charles wrote:
> Please look at java.util.IdentityHashMap again.  I think it does
> everything you're looking for except concurrent access.  Given other
> requirements, I'm not sure a ConcurrentHashMap will buy much performance
> anyway.

I suggest you take a look in the archives. Synchronisation has a major
negative impact on performance. IdentityHashMap is completely the wrong
solution for this use case.


> chas
> On 6/11/11 5:54 AM, "Mark Thomas" <> wrote:
>> On 11/06/2011 08:44, Phil Steitz wrote:
>>> I have looked at this some more and I think we have two choices,
>>> depending on how much we want to be able to do in terms of
>>> monitoring and instance management.  The simplest solution is to
>>> enable only holding references to instances checked out to clients,
>>> but no tracking of last active times, etc. by the pool itself
>>> (leaving this to the pooled objects themselves, as
>>> AbandonedObjectPool does now).  That could be done with less
>>> ambitious _allObjects and PoolableObject classes.  Actually,
>>> _allObjects would go away, replaced by a List of _circulatingObjects
>>> (like AbandonedObjectPool trace) and PoolableObject would really
>>> only maintain state for idle instances or instances undergoing
>>> lifecycle transitions.
>> The problem with a List and objects where A.equals(B) is that we'll have
>> to iterate through the list testing objects for equality when an object
>> is returned in order to remove the right object. That will be slow. (If
>> we want to prevent the same object being returned multiple times).
>>> If we want to be able to track things like last active time (as the
>>> trunk code now enables), we need to be able to identify returning
>>> objects uniquely.  That means either we impose the factory
>>> discernibility requirement or use system identity hashcodes.
>>> I would like to keep the monitoring / management options open, so my
>>> vote is for the second option.  I wonder, though, if it makes sense
>>> to keep the base implementation simple (and a little faster) and
>>> restrictive and provide the equals-tolerant functionality in a
>>> subclass or via a config option.
>> I too think that the second option is the way to go using system
>> identity hashcodes. I'm not sure that the first option will be faster
>> since iterating through the List of circulating objects is likely to be
>> slower than generating a system identity hashcode and doing a lookup in
>> a Map. If testing pool2/dbcp2 shows that the system identity hashcode
>> makes it significantly slower than Tomcat's jdbc-pool then we can look
>> at this again.
>> Mark
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