Re: strange parallel query behavior after OOM crashes

On Wed, Apr 5, 2017 at 7:45 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Apr 5, 2017 at 10:09 AM, Kuntal Ghosh
> <kuntalghosh.2007@gmail.com> wrote:
>>> Did you intend to attach that patch to this email?
>>>
>> Actually, I'm confused how we should ensure (register_count >
>> terminate_count) invariant. I think there can be a system crash what
>> Tomas has suggested up in the thread.
>>
>> Assert(parallel_register_count - parallel_terminate_count <=
>> max_parallel_workers);
>> Backend 1 > SET max_parallel_worker = 8;
>> Backend 1 > Execute a long running parallel query q1 with number of
>> parallel worker spawned is say, 4.
>
> At this point, parallel_register_count should be equal to
> parallel_terminate_count.  4 workers were started, and 4 have
> terminated.
>
Actually, I'm referring to the case when q1 is still running. In that
case, parallel_register_count = 4 and parallel_terminate_count = 0.

>> Backend 2> SET max_parallel_worker = 3;
Now, parallel_register_count - parallel_terminate_count = 4 >
max_parallel_worker.

>> Backend 2 > Try to execute any parallel query q2 with number of
>> parallel worker spawned > 0.
>
Hence, the assert will fail here.


-- 
Thanks & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com