Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

On Tue, Jan 23, 2018 at 1:45 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Mon, Jan 22, 2018 at 3:52 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> The difference is that nodeGather.c doesn't have any logic like the
>> one you have in _bt_leader_heapscan where the patch waits for each
>> worker to increment nparticipantsdone. For Gather node, we do such a
>> thing (wait for all workers to finish) by calling
>> WaitForParallelWorkersToFinish which will have the capability after
>> Robert's patch to detect if any worker is exited abnormally (fork
>> failure or failed before attaching to the error queue).
>
> FWIW, I don't think that that's really much of a difference.
>
> ExecParallelFinish() calls WaitForParallelWorkersToFinish(), which is
> similar to how _bt_end_parallel() calls
> WaitForParallelWorkersToFinish() in the patch. The
> _bt_leader_heapscan() condition variable wait for workers that you
> refer to is quite a bit like how gather_readnext() behaves. It
> generally checks to make sure that all tuple queues are done.
> gather_readnext() can wait for developments using WaitLatch(), to make
> sure every tuple queue is visited, with all output reliably consumed.
>

The difference lies in the fact that in gather_readnext, we use tuple
queue mechanism which has the capability to detect that the workers
are stopped/exited whereas _bt_leader_heapscan doesn't have any such
capability, so I think it will loop forever.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com