Re: OpenBSD versus semaphores

On Tue, Jan 8, 2019 at 12:14 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> I've been toying with OpenBSD lately, and soon noticed a seriously
> annoying problem for running Postgres on it: by default, its limits
> for SysV semaphores are only SEMMNS=60, SEMMNI=10. Not only does that
> greatly constrain the number of connections for a single installation,
> it means that our TAP tests fail because you can't start two postmasters
> concurrently (cf [1]).
>
> Raising the annoyance factor considerably, AFAICT the only way to
> increase these settings is to build your own custom kernel.
>

This is not accurate, you can change this values via sysctl(1), extracted
from OpenBSD postgresql port:

Tuning for busy servers

=======================
The default sizes in the GENERIC kernel for SysV semaphores are only
just large enough for a database with the default configuration
(max_connections 40) if no other running processes use semaphores.
In other cases you will need to increase the limits. Adding the
following in /etc/sysctl.conf will be reasonable for many systems:

kern.seminfo.semmni=60
kern.seminfo.semmns=1024

To serve a large number of connections (>250), you may need higher
values for the above.

http://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/ports/databases/postgresql/pkg/README-server?rev=1.25&content-type=text/plain

> So I looked around for an alternative, and found out that modern
> OpenBSD releases support named POSIX semaphores (though not unnamed
> ones, at least not shared unnamed ones). What's more, it appears that
> in this implementation, named semaphores don't eat open file descriptors
> as they do on macOS, removing our major objection to using them.
>
> I don't have any OpenBSD installation on hardware that I'd take very
> seriously for performance testing, but some light testing with
> "pgbench -S" suggests that a build with PREFERRED_SEMAPHORES=NAMED_POSIX
> has just about the same performance as a build with SysV semaphores.
>
> This all leads to the thought that maybe we should be selecting
> PREFERRED_SEMAPHORES=NAMED_POSIX on OpenBSD. At the very least,
> our docs ought to recommend it as a credible alternative for
> people who don't want to get into building custom kernels.
>
> I've checked that this works back to OpenBSD 6.0, and scanning
> their man pages suggests that the feature appeared in 5.5.
> 5.5 isn't that old (2014) so possibly people are still running
> older versions, but we could easily put in version-specific
> default logic similar to what's in src/template/darwin.
>
> Thoughts?
>
> regards, tom lane
>
> [1]
> https://www.postgresql.org/message-id/e6ecf989-9d5a-9dc5-12de-96985b6e5a5f%40mksoft.nu
>
>