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

On Sun, Jan 21, 2018 at 1:39 AM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> On Fri, Jan 19, 2018 at 9:29 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Actually, though it doesn't really look like it from the way things
> are structured within nbtsort.c, I don't need to wait for workers to
> start up (call the WaitForParallelWorkerToAttach() function you
> sketched) before doing any real work within the leader. The leader can
> participate as a worker, and only do this check afterwards. That will
> work because the leader Tuplesortstate has yet to do any real work.
> Nothing stops me from adding a new function to tuplesort, for the
> leader, that lets the leader say: "New plan -- you should now expect
> this many participants" (leader takes this reliable number from
> eventual call to WaitForParallelWorkerToAttach()).
>
> I admit that I had no idea that there is this issue with
> nworkers_launched until very recently. But then, that field has
> absolutely no comments.
>

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.
*/

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