Horton

Re: Estimating HugePages Requirements?

Magnus Hagander <magnus@hagander.net>

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From: Magnus Hagander <magnus@hagander.net>
To: Michael Paquier <michael@paquier.xyz>
Cc: "Bossart, Nathan" <bossartn@amazon.com>, Andres Freund <andres@anarazel.de>, Mark Dilger <mark.dilger@enterprisedb.com>, Don Seiler <don@seiler.us>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2021-08-27T14:40:27Z
Lists: pgsql-hackers 
On Fri, Aug 27, 2021 at 8:46 AM Michael Paquier <michael@paquier.xyz> wrote:
>
> On Wed, Aug 11, 2021 at 11:23:52PM +0000, Bossart, Nathan wrote:
> > I think BootstrapModeMain() makes the most sense.  It fits in nicely
> > with the --check logic that's already there.  With v3, the following
> > command can be used to retrieve the amount of shared memory required.
> >
> >         postgres --output-shmem -D dir
> >
> > While testing this new option, I noticed that you can achieve similar
> > results today with the following command, although this one will
> > actually try to create the shared memory, too.
>
> That may not be the best option.

I would say that can be a disastrous option.

First of all it would probably not work if you already have something
running -- especially when using huge pages. And if it does work, in
that or other scenarios, it can potentially have significant impact on
a running cluster to suddenly allocate many GB of more memory...


> > IMO the new option is still handy, but I can see the argument that it
> > might not be necessary.
>
> A separate option looks handy.  Wouldn't it be better to document it
> in postgres-ref.sgml then?

I'd say a lot more than just handy. I don't think the workaround is
really all that useful.

(haven't looked at the actual patch yet, just commenting on the principle)

-- 
 Magnus Hagander
 Me: https://www.hagander.net/
 Work: https://www.redpill-linpro.com/

Commits

  1. Silence extra logging when using "postgres -C" on runtime-computed GUCs

  2. doc: Improve postgres command for shared_memory_size_in_huge_pages

  3. Introduce GUC shared_memory_size_in_huge_pages

  4. Support "postgres -C" with runtime-computed GUCs

  5. Make shared_memory_size a preset option

  6. Introduce GUC shared_memory_size

  7. Move the shared memory size calculation to its own function

  8. Add new GUC, max_worker_processes, limiting number of bgworkers.