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

On Sat, Jan 20, 2018 at 8:38 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> It would have been better if there were some comments besides that
> field, but I think it has been covered at another place in the code.
> See comments in LaunchParallelWorkers().
>
> /*
> * Start workers.
> *
> * 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.
> */

Why is this okay for Gather nodes, though? nodeGather.c looks at
pcxt->nworkers_launched during initialization, and appears to at least
trust it to indicate that more than zero actually-launched workers
will also show up when "nworkers_launched > 0". This trust seems critical
when parallel_leader_participation is off, because "node->nreaders ==
0" overrides the parallel_leader_participation GUC's setting (note
that node->nreaders comes directly from pcxt->nworkers_launched). If
zero workers show up, and parallel_leader_participation is off, but
pcxt->nworkers_launched/node->nreaders is non-zero, won't the Gather
never make forward progress?

Parallel CREATE INDEX does go a bit further. It assumes that
nworkers_launched *exactly* indicates the number of workers that
successfully underwent parallel initialization, and therefore can be
expected to show up.

Is there actually a meaningful difference between the way
nworkers_launched is depended upon in each case, though?

-- 
Peter Geoghegan