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

On Wed, Jan 17, 2018 at 6:20 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I had forgotten about the previous discussion. The sketch in my
> previous email supposed that we would use dynamic barriers since the
> whole point, after all, is to handle the fact that we don't know how
> many participants will really show up. Thomas's idea seems to be that
> the leader will initialize the barrier based on the anticipated number
> of participants and then tell it to forget about the participants that
> don't materialize. Of course, that would require that the leader
> somehow figure out how many participants didn't show up so that it can
> deduct then from the counter in the barrier. And how is it going to
> do that?

I don't know; Thomas?

> It's true that the leader will know the value of nworkers_launched,
> but as the comment in LaunchParallelWorkers() says: "The caller must
> be able to tolerate ending up with fewer workers than expected, so
> there is no need to throw an error here if registration fails. It
> wouldn't help much anyway, because registering the worker in no way
> guarantees that it will start up and initialize successfully." So it
> seems to me that a much better plan than having the leader try to
> figure out how many workers failed to launch would be to just keep a
> count of how many workers did in fact launch.

> So my position (at least until Thomas or Andres shows up and tells me
> why I'm wrong) is that you can use the Barrier API just as it is
> without any yak-shaving, just by following the sketch I set out
> before. The additional API I proposed in that sketch isn't really
> required, although it might be more efficient. But it doesn't really
> matter: if that comes along later, it will be trivial to adjust the
> code to take advantage of it.

Okay. I'll work on adopting dynamic barriers in the way you described.
I just wanted to make sure that we're all on the same page about what
that looks like.

--
Peter Geoghegan