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From Rick Hillegas <rick.hille...@oracle.com>
Subject Re: [VOTE] release
Date Wed, 21 Sep 2011 16:37:22 GMT
On 9/20/11 1:32 PM, Kathey Marsden wrote:
> On 9/20/2011 12:20 PM, Rick Hillegas wrote:
>> at sounds like a good idea.
>>> Sadly, I think it is something that users are not likely to hit 
>>> until they are in a heavily loaded production environment.  Is there 
>>> a way to acheive the prior behavior where there is not a risk of the 
>>> error occurring?
>> Here are some options:
>> 1) It's easy to achieve the prior behavior by backing out the fix for 
>> DERBY-4437. That would give up the concurrency boost provided by that 
>> work. I believe the user experience of the prior behavior is that 
>> access to the identity column goes down to single-threaded usage even 
>> at low contention levels. This could be done for 10.8.2.
> I was thinking more in terms of whether there is some safe value for a 
> user setting derby.language.sequence.preallocator,   if an application 
> is not prepared for the new error,  or some way for Derby to 
> dynamically slow things down to single threaded  if there are   more 
> than 20 simultaneous inserts. (Is that the limitation is if the value 
> is set to 20?)
Hi Kathey,

How much contention is too much depends on a lot of variables, including 
transaction workload and how the VM schedules threads. I don't think 
there is any straightforward mapping of threadcount to optimal value for 
this property. In general, higher is better. This property sets how many 
sequence/identity values are preallocated every time we grab a new 
chunk. It is the max size of the gap in the sequence/identity which will 
appear if the engine is not shutdown in an orderly fashion.

If you set derby.locks.waitTimeout to a negative number, then a session 
will never timeout trying to get a sequence/identity value. That will 
eliminate the "too much contention" error. If Derby is configured to 
have no lock timeout, then it is possible that some sessions may hang a 
long time before obtaining a sequence/identity column.


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