Re: SYSV shared memory vs mmap performance

Hi,

Robert Haas wrote:
> On Fri, Jan 25, 2013 at 8:40 AM, Bruce Momjian <bruce@momjian.us>
> wrote:
>> Just a reminder we might have *BSD performance issues with our use
>> of Posix shared memory in Postgres 9.3.  I am attaching the PDF the
>> user posted.
>
> This is a good point.  The question which I believe I asked before
> and haven't gotten an answer to is whether there's some way to get
> the benefit of shm_use_phys with an anonymous mapping.

There is. Postgres 9.3+mmap performance on DragonFly is now much better 
than these old benchmark results show.

After the initial disappointing result, I went on a benchmarking/tuning 
binge with our Dear Leader Matt Dillon. Taking advantage of some 
previous cpu topology work from Mihai Carabas, he heavily improved most 
performance shortcomings we found in the DragonFly kernel.

There were a few mail about this changes on the DragonFly mailing-lists 
and Justin Sherill wrote some interesting articles on his blog.

Some links with more details about improvements and final results:
http://www.shiningsilence.com/dbsdlog/2012/09/19/10403.html
http://www.shiningsilence.com/dbsdlog/2012/10/11/10544.html
http://www.dragonflybsd.org/performance/

> It seems to me to be slightly insane to impose draconian shared
> memory limits out of the box and then complain when people switch to
> some other type of shared memory to get around them.  I realize that
> Dragonfly may not be doing that (because I think they may have
> raised the default shared-memory limits), but I believe some of the
> more mainstream BSDs are.

The original SYSV limits looked like something straight from the 1980s; 
we're now autotuning them on DragonFly.
FreeBSD and NetBSD still needed manual tuning last time I had a look.

-- 
Francois Tigeot