On Wed, Jan 17, 2018 at 6:20 PM, Robert Haas <robertmh...@gmail.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
