Re: strange parallel query behavior after OOM crashes

On Fri, Mar 31, 2017 at 2:05 AM, Kuntal Ghosh
<kuntalghosh.2007@gmail.com> wrote:
>
> 1. Put an Assert(0) in ParallelQueryMain(), start server and execute
> any parallel query.
>  In LaunchParallelWorkers, you can see
>        nworkers = n nworkers_launched = n (n>0)
> But, all the workers will crash because of the assert statement.
> 2. the server restarts automatically, initialize
> BackgroundWorkerData->parallel_register_count and
> BackgroundWorkerData->parallel_terminate_count in the shared memory.
> After that, it calls ForgetBackgroundWorker and it increments
> parallel_terminate_count. In LaunchParallelWorkers, we have the
> following condition:
> if ((BackgroundWorkerData->parallel_register_count -
>                      BackgroundWorkerData->parallel_terminate_count) >=
>         max_parallel_workers)
> DO NOT launch any parallel worker.
> Hence, nworkers = n nworkers_launched = 0.
parallel_register_count and parallel_terminate_count, both are
unsigned integer. So, whenever the difference is negative, it'll be a
well-defined unsigned integer and certainly much larger than
max_parallel_workers. Hence, no workers will be launched. I've
attached a patch to fix this.



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