RE: Shared Memory: How to use SYSV rather than MMAP ?

From: "REIX, Tony" <tony(dot)reix(at)atos(dot)net>
To: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, "EMPEREUR-MOT, SYLVIE" <sylvie(dot)empereur-mot(at)atos(dot)net>
Subject: RE: Shared Memory: How to use SYSV rather than MMAP ?
Date: 2018-12-18 15:17:28
Message-ID: DB7PR02MB4663EEED8A2B058773C90C5B86BD0@DB7PR02MB4663.eurprd02.prod.outlook.com
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Hi Thomas,

Here is the patch we are using now on AIX for enabling SysV shm for AIX, which improves greatly the performance on AIX.

It is compile time.

It seems to me that you'd like this to become a shared_memory_type GUC. Correct? However, I do not know how to do.

Even as-is, this patch would greatly improve the performance of PostgreSQL v11.1 in the field on AIX machines. So, we'd like this change to be available for AIX asap.

What are the next steps to get this patch accepted? or What are your suggestions for improving it?

Thanks/Regards

Cordialement,

Tony Reix

tony(dot)reix(at)atos(dot)net

ATOS / Bull SAS
ATOS Expert
IBM Coop Architect & Technical Leader
Office : +33 (0) 4 76 29 72 67
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________________________________
De : REIX, Tony
Envoyé : lundi 26 novembre 2018 18:00:15
À : Thomas Munro
Cc : Andres Freund; Robert Haas; Pg Hackers; EMPEREUR-MOT, SYLVIE; BERGAMINI, DAMIEN
Objet : RE: Shared Memory: How to use SYSV rather than MMAP ?

Hi Thomas,

You said:

I think we should respect the huge_pages GUC, as we do on Linux and
Windows (since there are downsides to using large pages, maybe not
everyone would want that). It could even be useful to allow different
page sizes to be requested by GUC (I see that DB2 has an option to use
16GB pages -- yikes). It also seems like a good idea to have a
shared_memory_type GUC as Andres proposed (see his link), instead of
using a compile time option. I guess it was made a compile time
option because nobody could imagine wanting to go back to SysV shm!
(I'm still kinda surprised that MAP_ANONYMOUS memory can't be coaxed
into large pages by environment variables or loader controls, since
apparently other things like data segments etc apparently can, though
I can't find any text that says that's the case and I have no AIX
system).

I guess that you are talking about CPP & C variables:
#ifndef MAP_HUGETLB
HUGE_PAGES_ON
HUGE_PAGES_TRY)
in addition to : huge_pages = .... in postgresql.conf file.

For now, these variables for Huge Pages are used only for MMAP.
About SysV shared memory, as far as I know, shmget() options for AIX and Linux are different.
Moreover, AIX also provides Large Pages (16MB).

About Andres proposal, I've read his link. However, the patch he proposed:
0001-Add-shared_memory_type-GUC.patch<https://www.postgresql.org/message-id/attachment/33090/0001-Add-shared_memory_type-GUC.patch>
is no more available (Attachment not found).

I confirm that I got the SysV Shared Memory by means of a "compile time option".

About "still kinda surprised that MAP_ANONYMOUS memory can't be coaxed
into large pages by environment variables or loader controls" I confirm that,
on AIX, only 4K pages are available for mmap().

I do agree that options in the postgresql.conf file would be the best solution,
since the code for SysV shared memory and MMAP shared memory seems always present.

Regards,

Tony

Attachment Content-Type Size
postgresql-11.1-AIX-SysV-SharedMemory-LargePagesV2.patch text/x-patch 1.9 KB

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