Re: strange parallel query behavior after OOM crashes

On Wed, Apr 5, 2017 at 7:31 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Apr 5, 2017 at 6:36 AM, Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
>> Yes. But, as Robert suggested up in the thread, we should not use
>> (parallel_register_count = 0) as an alternative to define a bgworker
>> crash. Hence, I've added an argument named 'wasCrashed' in
>> ForgetBackgroundWorker to indicate a bgworker crash.
>
> 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.
Backend 2> SET max_parallel_worker = 3;
Backend 2 > Try to execute any parallel query q2 with number of
parallel worker spawned > 0.

The above assert statement will bring down the server unnecessarily
while executing q2. If the assert statement was not there, it could
have gone ahead without launching any workers.

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