Re: [Proxool-developer] concurrency bug in Prototyper class?

[Matt S. <msa...@gm...>]

In this specific case, I believe making 'connectionCount' a volatile
variable satisfies both goals: correctness and performance.  It even
establishes the 'out-of-thin-air' guarantee you don't have when reading a
non-synchronized, non-volatile 64-bit value.

I think sacrificing correctness for performance is OK in places where stale
(although correct) values won't hurt you.  I don't believe it works with the
variable holding keys to the connection pool.

My application is massively multithreaded.  There are a large number of
concurrent threads acquiring and releasing connections. The fact that these
threads will always see the 'connectionCount' increment done in
Prototyper.buildConnection( ) but might not see any decrements done in
Prototyper.connectionRemoved( ) seems serious to me.


On Fri, Jun 19, 2009 at 3:59 PM, Bill Horsman <bi...@lo...>wrote:

> I see your point. We've jumped through a few hoops in Proxool to get the
> balance right between concurrency and excessive blocking. As you'd expect,
> our first priority is to ensure the code doesn't break in concurrent
> situations, but it's closely followed by our second priority: that it must
> be very fast and not do unnecessary locking. I'm not saying you're right or
> wrong - just that it would be hasty to grab a lock to read the
> connectionCount unless we explore the implications.
>
> 2009/6/19 Matt Salafia <msa...@gm...>
>
>> Possibly, but I don't know - it's indeterminate.  Without the proper
>> memory visibility semantics (e.g. synchronization, immutability, volatile
>> et. al.) I think all bets are off.
>>
>>
>>
>> On Fri, Jun 19, 2009 at 1:32 PM, Bill Horsman <bi...@lo...>wrote:
>>
>>> Is that a problem that corrects itself? In other words, is it a temporary
>>> problem where a connection is refused when it didn't have to be but that the
>>> connection count does end up being correct? - Bill
>>>
>>> 2009/6/19 Matt Salafia <msa...@gm...>
>>>
>>>>  It seems Class Prototyper should either acquire the lock on private
>>>> member 'lock' whenever it reads or modifies 'connectionCount' or make
>>>> 'connectionCount' a volatile variable.
>>>>
>>>> Prototyper.buildConnection( ) acquires the lock to read and modify
>>>> 'connectionCount' but other methods do not: Prototyper.sweep( ),
>>>> Prototyper.connectionRemoved( ) and Prototyper.getConnectionCount().
>>>>
>>>> I started poking around after my application started seeing
>>>> ProxoolExceptions indicating the maximum connection count had been reached
>>>> and discovered what appears to be a memory visibility issue.  If calls to
>>>> Prototyper.connectionRemoved( ) and Prototyper.buildConnection( ) happen in
>>>> different threads there could be unwarranted ProxoolExceptions for reaching
>>>> the maximum connection count - a 'connectionPool' decrement might not be
>>>> visible to the threads calling Prototyper.buildConnection( ).
>>>>
>>>>
>>>>
>>>>
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>
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