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

On Wed, Jan 24, 2018 at 12:20 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Tue, Jan 23, 2018 at 10:36 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> As Amit says, what remains is the case where fork() fails or the
>> worker dies before it reaches the line in ParallelWorkerMain that
>> reads shm_mq_set_sender(mq, MyProc). In those cases, no error will be
>> signaled until you call WaitForParallelWorkersToFinish(). If you wait
>> prior to that point for a number of workers equal to
>> nworkers_launched, you will wait forever in those cases.
>
> Another option might be to actually call
> WaitForParallelWorkersToFinish() in place of a condition variable or
> barrier, as Amit suggested at one point.
>

Yes, the only thing that is slightly worrying about using
WaitForParallelWorkersToFinish is that backend leader needs to wait
for workers to finish rather than just finishing sort related work. I
think there shouldn't be much difference between when the sort is done
and the workers actually finish the remaining resource cleanup.
However, OTOH, if we are not okay with this solution and want to go
with some kind of usage of barriers to solve this problem, then we can
evaluate that as well, but I feel it is better if we can use the
method which is used in other parallelism code to solve this problem
(which is to use WaitForParallelWorkersToFinish).

>> I am going to repeat my previous suggest that we use a Barrier here.
>> Given the discussion subsequent to my original proposal, this can be a
>> lot simpler than what I suggested originally. Each worker does
>> BarrierAttach() before beginning to read tuples (exiting if the phase
>> returned is non-zero) and BarrierArriveAndDetach() when it's done
>> sorting. The leader does BarrierAttach() before launching workers and
>> BarrierArriveAndWait() when it's done sorting.
>>

How does leader detect if one of the workers does BarrierAttach and
then fails (either exits or error out) before doing
BarrierArriveAndDetach?

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