Re: [HACKERS] parallel.c oblivion of worker-startup failures

On Tue, Jan 23, 2018 at 9:02 PM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> Yes, this is what I am trying to explain on parallel create index
>> thread. I think there we need to either use
>> WaitForParallelWorkersToFinish or WaitForParallelWorkersToAttach (a
>> new API as proposed in that thread) if we don't want to use barriers.
>> I see a minor disadvantage in using WaitForParallelWorkersToFinish
>> which I will say on that thread.
>
> Ah, I see. So if you wait for them to attach you can detect
> unexpected dead workers (via shm_mq_receive), at the cost of having
> the leader wasting time waiting around for forked processes to say
> hello when it could instead be sorting tuples.

The leader can go ahead and sort before calling something like a new
WaitForParallelWorkersToAttach() function (or even
WaitForParallelWorkersToFinish()). If we did add a
WaitForParallelWorkersToAttach() function, then the performance hit
would probably not be noticeable in practice. The
parallel_leader_participates case would still work without special
care (that's the main hazard that makes using a barrier seem
unappealing).

--
Peter Geoghegan