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From Michael Bouschen <mbo.t...@spree.de>
Subject Re: TCK tests : QueryCancel
Date Sun, 14 Mar 2010 18:50:52 GMT
Hi Andy, hi Craig,

I'm surprised about the query execution time of 60 ms to 2 ms. Andy, 
your hardware seems to be much faster than mine :-).

I agree we should change the test such that the query takes longer to 
execute. I propose to create more test instances, then the join (as part 
of the query) becomes more expensive.

Attached you find a patch with the following changes of the test class:
- it creates 5000 PCPoint and PCPoint2 instances (instead of 1000)
- uses a barrier to synchronize the threads
- compiles the query before staring the thread

Regards Michael

> Hi Andy,
> On Mar 13, 2010, at 3:29 AM, Andy Jefferson wrote:
>> Hi Michael,
>>> I added a Thread.sleep(1000) to the main thread in order to give the
>>> other thread a chance to start the query before the main thread  
>>> cancels
>>> it. Could you please give it a try?
>> Giving something a second to get started seems excessive, I know  
>> Derby is slow
>> but ... ;-)
>> I tried with a value more like 80 or 100 (millis) and managed **on  
>> occasions**
>> to get a JDOQueryInterruptedException (and pass the test). Other  
>> times the
>> cancel still got in first. I also added a query.compile() before  
>> starting the
>> threads hence less for the query execute to do before starting, but  
>> still
>> intermittent. The intermittency will also be affected by the machine  
>> being
>> used, hence the number is arbitrary. The query execution time (in the
>> datastore) is anywhere between 60ms and 2ms on my machine.
> This test will always be somewhat sensitive to timing, but we should  
> be able to minimize the timing effects.
> The query itself should be long enough to allow the cancel to have an  
> effect. The query execution time of 60 ms to 2 ms. [sic] doesn't sound  
> long enough. Maybe we need to make the query more complex. But still,  
> as machines get faster the query execution times will get smaller.
> The other thing we discussed earlier was the possibility of needing a  
> barrier so we could guarantee that the two threads were synchronized  
> at some point. The main thread calling start doesn't quite guarantee  
> synchronization, while a barrier of size two should do it. Still,  
> assuming that the query is compiled before the barrier, and the main  
> thread yields to the query thread (sleep should do this just fine) we  
> should be able to get the query to start and the cancel to interrupt it.
>> Depends what the cancel is allowing cancel over; to be consistent  
>> with the
>> timeout I assume it is cancel the datastore operation (as opposed to  
>> the whole
>> query execute process ... compile, generate PreparedStatement,  
>> populate
>> parameter values, execute in the datastore, return results), and so
>> DataNucleus only starts the execution in a separate thread and  
>> maintains the
>> handle on that thread for cancellation purposes. If this execution  
>> thread is
>> either not yet created, or is now finished then the test will fail,  
>> always.
> Another good high level discussion. Since we have the datastore read  
> timeout, the purpose of the cancel is to stop query execution before  
> the timeout expires. Since we assume that the main thread and the  
> query thread have ways of communicating as long as the query thread is  
> not stuck in a datastore wait, we are only concerned about getting the  
> query thread unstuck.
> So if the query has already returned results, the purpose for cancel  
> doesn't exist. There still may be a timing issue in real life where  
> the main thread sees that the query thread is stuck, but by the time  
> it gets around to canceling the query, the query thread is no longer  
> stuck. So it doesn't really matter whether the cancel worked or not.  
> That's why I agree that a reasonable behavior is having the cancel be  
> a no-op in case it can't find anything to do.
> But we still want a test case. Do we need the test case to perform  
> some timing tests to see how long queries take and adjust the wait  
> time accordingly? Do we want to try different values and require that  
> at least one value succeed? Start with a value of 1 ms and raise by  
> increments until the cancel works?
>> Shouldn't the test also take into account if the query does actually  
>> return
>> the results before the cancel can be called ?
> This could be part of the heuristics as noted above. Much better if we  
> can figure out a good barrier/sleep strategy that at least works for  
> now.
> Craig
>> Regards
>> -- 
>> Andy
>> DataNucleus (http://www.datanucleus.org)
> Craig L Russell
> Architect, Oracle
> http://db.apache.org/jdo
> 408 276-5638 mailto:Craig.Russell@oracle.com
> P.S. A good JDO? O, Gasp!

*Michael Bouschen*

akquinet tech@spree GmbH
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Fax:   +49 30 217 520-12
Email: michael.bouschen@akquinet.de
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