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

Hi,

On 2018-11-21 09:00:58 +1300, Thomas Munro wrote:
> On Wed, Nov 21, 2018 at 4:37 AM REIX, Tony <tony(dot)reix(at)atos(dot)net> wrote:
> > YES ! Reading this file, your suggestion should work ! Thx !
> >
> > I've rebuilt and run the basic tests. We'll relaunch our tests asap.
>
> I would be surprised if that makes a difference:
> anonymous-mmap-then-fork and SysV shm are just two different ways to
> exchange mappings between processes, but I'd expect the virtual memory
> object itself to be basically the same, in terms of constraints that
> might affect page size at least.

I don't think that's true on many systems, FWIW. On linux there's
certainly different behaviour, and e.g. the way to get hugepages for
anon-mmap and SysV shmem aren't the same. [1] strongly suggests that
that's not the case on FreeBSD either (with sysv shmem being
better). I'd attached a patch to implement a GUC to allow users to
choose the shmem implementation back then [2].

[1] http://archives.postgresql.org/message-id/2AE143D2-87D3-4AD1-AC78-CE2258230C05%40FreeBSD.org
[2] http://archives.postgresql.org/message-id/20140422121921.GD4449%40awork2.anarazel.de

Greetings,

Andres Freund