Re: [PATCH] PostgreSQL 9.4 mmap(2) performance regression on FreeBSD...

On 2014-10-11 20:33:57 -0400, Bruce Momjian wrote:
> On Tue, Aug 12, 2014 at 07:08:06PM -0400, Robert Haas wrote:
> > On Tue, Aug 12, 2014 at 12:59 PM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > > On 2014-08-12 09:42:30 -0700, Sean Chittenden wrote:
> > >> One of the patches that I've been sitting on and am derelict in punting
> > >> upstream is the attached mmap(2) flags patch for the BSDs. Is there any
> > >> chance this can be squeezed in to the PostreSQL 9.4 release?
> > >>
> > >> The patch is trivial in size and is used to add one flag to mmap(2) calls in
> > >> dsm_impl.c. Alan Cox (FreeBSD alc, not Linux) and I went back and forth
> > >> regarding PostgreSQL's use of mmap(2) and determined that the following is
> > >> correct and will prevent a likely performance regression in PostgreSQL 9.4.
> > >> In PostgreSQL 9.3, all mmap(2) calls were called with the flags MAP_ANON |
> > >> MAP_SHARED, whereas in PostgreSQL 9.4 this is not the case.
> > >
> > > The performancewise important call to mmap will still use that set of
> > > flags, no? That's the one backing shared_buffers.
> > >
> > > The mmap backend for *dynamic* shared memory (aka dsm) is *NOT* supposed
> > > to be used on common platforms. Both posix and sysv shared memory will
> > > be used before falling back to the mmap() backend.
> >
> > Hmm, yeah. This might still be a good thing to do (because what do we
> > lose?) but it shouldn't really be an issue in practice.
>
> Is there a reason this was not applied?

IIRC, as pointed out above, it's primarily based on a misunderstanding
about when mmap is used for in dsm. I.e. that it's essentially just a
fallback/toy implementation and that posix or sysv should rather be
used.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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