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

On 2014-10-13 07:49:44 -0700, Sean Chittenden wrote:
> >> 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 practial 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.
> 
> While you're not wrong in that use of mmap(2) here is potentially a
> bad idea, much of that is mitigated through the correct use of flags
> to mmap(2) (i.e. prevent mmap(2) pages from hooking in to the syncer).
> In the same breath, it would also be nice if the following were
> committed:

Unless I'm mistaken the pages will still be written back to disk (and
not just swap, the actual backing file) if there's memory pressure, no?

> > --- src/template/freebsd.orig   2014-05-26 23:54:53.854165855 +0300
> > +++ src/template/freebsd        2014-05-26 23:55:12.307880900 +0300
> > @@ -3,3 +3,4 @@
> >  case $host_cpu in
> >    alpha*)   CFLAGS="-O";;  # alpha has problems with -O2
> >  esac
> > +USE_NAMED_POSIX_SEMAPHORES=1

If so, that should be a separate change. But why?

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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