Re: Rename max_parallel_degree?

David G. Johnston <david.g.johnston@gmail.com>

From: "David G. Johnston" <david.g.johnston@gmail.com>

To: Josh berkus <josh@agliodbs.com>

Cc: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Peter Geoghegan <pg@heroku.com>, Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Noah Misch <noah@leadboat.com>, David Rowley <david.rowley@2ndquadrant.com>, Amit Kapila <amit.kapila16@gmail.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Magnus Hagander <magnus@hagander.net>, Bruce Momjian <bruce@momjian.us>, PostgreSQL-development <pgsql-hackers@postgresql.org>

Date: 2016-05-31T18:29:12Z

Lists: pgsql-hackers

On Tue, May 31, 2016 at 2:22 PM, Josh berkus <josh@agliodbs.com> wrote:

> On 05/31/2016 11:17 AM, Peter Eisentraut wrote:
> > On 5/31/16 2:02 PM, Josh berkus wrote:
> >> I get where you're coming from, but I think Haas's query plan output is
> >> going to show us the confusion we're going to get.  So we need to either
> >> change the parameter, the explain output, or brace ourselves for endless
> >> repeated questions.
> >
> > Changing the explain output doesn't sound so bad to me.
> >
> > The users' problem is that the parameter setting ought to match the
> > EXPLAIN output.
> >
> > The developers' problem is that the EXPLAIN output actually corresponds
> > to leader + (N-1) workers internally.
> >
> > I think we can hope that developers are going to be less confused about
> > that than users.
>
> Makes sense.
>
> One more consistency question: what's the effect of running out of
> max_parallel_workers?
>
> That is, say max_parallel_workers is set to 10, and 8 are already
> allocated.  If I ask for max_parallel_X = 4, how many cores to I use?
>
> Presumably the leader isn't counted towards max_parallel_workers?
>

You'd have three O/S processes - the one dedicated to your session and
you'd pick up two additional processes from the worker pool to assist.

How the O/S assigns those to cores is outside PostgreSQL's jurisdiction.

David J.