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

On 2014-10-13 11:18:26 -0400, Bruce Momjian wrote:
> On Mon, Oct 13, 2014 at 04:19:39PM +0200, Andres Freund wrote:
> > On 2014-10-13 10:15:29 -0400, Robert Haas wrote:
> > > On Sun, Oct 12, 2014 at 5:36 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > > > 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.
> > >
> > > Perhaps, but I still see no reason not to apply it. It may not help
> > > many people, but it won't hurt anything, either. So why not?
> >
> > More complicated, less tested code. For no practical benefit, it'll still
> > be slower than posix shm if there's any memmory pressure. But if you
> > want to apply it, go ahead, I won't cry louder than this email.
> >
> > I still think the mmap dsm implementation is a bad idea. We shouldn't
> > put additional effort into it. If anything we should remove it.
>
> If we have it, we should improve it, or remove it. We might want to use
> this code for something else in the future, so it should be improved
> where feasible.

Meh. We don't put in effort into code that doesn't matter just because
it might get used elsewhere some day. By that argument we'd need to
performance optimize a lot of code. And actually, using that code
somewhere else is more of a counter indication than a pro
argument. MAP_NOSYNC isn't a general purpose flag.

Greetings,

Andres Freund

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