Re: Back-patch use of unnamed POSIX semaphores for Linux?

On Tue, Dec 6, 2016 at 9:53 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Just saw another report of what's probably systemd killing off Postgres'
> SysV semaphores, as we've discussed previously at, eg,
> https://www.postgresql.org/message-id/flat/57828C31.5060409%40gmail.com
> Since the systemd people are generally impervious to suggestions that
> they might be mistaken, I do not expect this problem to go away.
>
> I think we should give serious consideration to back-patching commit
> ecb0d20a9, which changed the default semaphore type to unnamed-POSIX
> on Linux. We've seen no problems in the buildfarm in the two months
> that that's been in HEAD. If we don't do this, we can expect to
> continue seeing complaints of this sort until pre-v10 PG releases
> fall out of use ... and I don't want to wait that long.
>
> Commit ecb0d20a9 also changed the default for FreeBSD. I'm not convinced
> we should back-patch that part, because (a) unnamed-POSIX semas have
> only been there since FreeBSD 9.0, which isn't that long ago, and (b)
> the argument for switching is "it'll perform better" not "your server
> will fail randomly without this change".
>
> Comments?

Urk. That sounds like a scary thing to back-patch. The fact that the
buildfarm has reported no problems is good as far as it goes, but user
environments can be expected to be considerably more diverse than the
buildfarm. I wouldn't mind giving users the option to select unnamed
POSIX semas, but I don't think there's any guarantee that that's 100%
certain to work every place where the current implementation works -
and if not, then people will upgrade to the latest minor release and
everything will completely stop working. Granted, that might not
happen, because maybe unnamed POSIX semas are one of those really
awesome operating system primitives that never has problems on any
system anywhere ever. But I think it's pretty hard to be certain of
that.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company