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From Phil Steitz <phil.ste...@gmail.com>
Subject Re: svn commit: r1164051 - /commons/proper/pool/trunk/src/java/org/apache/commons/pool2/impl/GenericKeyedObjectPool.java
Date Fri, 02 Sep 2011 00:23:35 GMT
On 9/1/11 4:59 PM, Mark Thomas wrote:
> On 02/09/2011 00:40, Phil Steitz wrote:
>> On 9/1/11 5:41 AM, markt@apache.org wrote:
>>> Author: markt
>>> Date: Thu Sep  1 12:41:54 2011
>>> New Revision: 1164051
>>>
>>> URL: http://svn.apache.org/viewvc?rev=1164051&view=rev
>>> Log:
>>> Remove sync that wasn't helping.
>>> Make thread safe
>> I agree that the sync wasn't helping; but I am not sure exactly what
>> "thread safe" means for this method.
> For me, thread safe means that a local copy is taken for any value used
> more than once that needs to be consistent between uses. I'm not sure we
> can do much more without tying ourselves in synchronisation knots.
>
>> I have always worried about
>> this method.  I have usually been able to convince myself that it's
>> incorrectness is harmless (i.e., there are guards to prevent pool
>> invariants being violated due to incorrect / inconsistent return
>> value), but I am not sure that is right.
> My analysis was that if you reduce the limits (maxTotal, maxIdle etc.)
> while this method is executing then there is a risk that it could exceed
> those limits. However, there is a fair chance that the pool will already
> be exceeding the limits when the limits are changed. Therefore, there is
> actually very little difference between these two scenarios. In both
> cases the pool will conform to the limits as soon as circumstances allow
> and once operating within the limits, will not exceed them.

I have never worried much about races against setters for maxTotal,
MaxIdle, though it is good to fix this as you did.  What I have
always worried about is that once we removed the sync on this and
the pool statistics getters, what it computes becomes suspect.  For
example, if things enter / exit the idle object pool between the
time objectDeficit and growLimit are computed, the return value will
be incorrect.  As I said, IIRC previous traces through the code, the
use of the return value has guards to prevent too much badness from
this; but since the correctness of the computation done inside this
method depends on the idle instance pool keeping the same size over
the duration of its execution, it is neither really threadsafe nor
correct.
>
>> It might be good to
>> temporarily make this method protected and run some concurrency
>> tests directly against it, so we can understand and document what
>> happens when instances come in and out of the pool while it is
>> executing.
> I'm not against that but I don't have the time to do it right now and am
> unlikely to any time soon.

I will do some tracing and more analysis of the code to verify that
the problems that I think may exist in the correctness (or maybe
meaningfulness) of the return value are harmless.

Phil
>
> Mark
>
>
>> Phil
>>> Modified:
>>>     commons/proper/pool/trunk/src/java/org/apache/commons/pool2/impl/GenericKeyedObjectPool.java
>>>
>>> Modified: commons/proper/pool/trunk/src/java/org/apache/commons/pool2/impl/GenericKeyedObjectPool.java
>>> URL: http://svn.apache.org/viewvc/commons/proper/pool/trunk/src/java/org/apache/commons/pool2/impl/GenericKeyedObjectPool.java?rev=1164051&r1=1164050&r2=1164051&view=diff
>>> ==============================================================================
>>> --- commons/proper/pool/trunk/src/java/org/apache/commons/pool2/impl/GenericKeyedObjectPool.java
(original)
>>> +++ commons/proper/pool/trunk/src/java/org/apache/commons/pool2/impl/GenericKeyedObjectPool.java
Thu Sep  1 12:41:54 2011
>>> @@ -1577,25 +1577,30 @@ public class GenericKeyedObjectPool<K,T>
>>>       * @param pool the ObjectPool to calculate the deficit for
>>>       * @return The number of objects to be created
>>>       */
>>> -    private synchronized int calculateDeficit(ObjectDeque<T> objectDeque)
{
>>> +    private int calculateDeficit(ObjectDeque<T> objectDeque) {
>>>          
>>>          if (objectDeque == null) {
>>>              return getMinIdlePerKey();
>>>          }
>>> +
>>> +        // Used more than once so keep a local copy so the value is consistent
>>> +        int maxTotal = getMaxTotal();
>>> +        int maxTotalPerKey = getMaxTotalPerKey();
>>> +
>>>          int objectDefecit = 0;
>>> -        
>>> +
>>>          // Calculate no of objects needed to be created, in order to have
>>>          // the number of pooled objects < maxTotalPerKey();
>>>          objectDefecit = getMinIdlePerKey() - objectDeque.getIdleObjects().size();
>>> -        if (getMaxTotalPerKey() > 0) {
>>> +        if (maxTotalPerKey > 0) {
>>>              int growLimit = Math.max(0,
>>> -                    getMaxTotalPerKey() - objectDeque.getIdleObjects().size());
>>> +                    maxTotalPerKey - objectDeque.getIdleObjects().size());
>>>              objectDefecit = Math.min(objectDefecit, growLimit);
>>>          }
>>>  
>>>          // Take the maxTotal limit into account
>>> -        if (getMaxTotal() > 0) {
>>> -            int growLimit = Math.max(0, getMaxTotal() - getNumActive() - getNumIdle());
>>> +        if (maxTotal > 0) {
>>> +            int growLimit = Math.max(0, maxTotal - getNumActive() - getNumIdle());
>>>              objectDefecit = Math.min(objectDefecit, growLimit);
>>>          }
>>>  
>>>
>>>
>>>
>>
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