| From: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
|---|---|
| To: | Josh berkus <josh(at)agliodbs(dot)com> |
| Cc: | Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Peter Geoghegan <pg(at)heroku(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Rename max_parallel_degree? |
| Date: | 2016-05-31 18:29:12 |
| Message-ID: | CAKFQuwYjeEmM1yi7C=7hOa72AP9trCUPSKML_eHuNAoYnxg6Sw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, May 31, 2016 at 2:22 PM, Josh berkus <josh(at)agliodbs(dot)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.
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