Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Tue, Mar 10, 2020 at 6:48 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Sat, Mar 7, 2020 at 10:23 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > I continue to think that we'd be better off getting all of this
> > out of the heavyweight lock manager. There is no reason why we
> > should need deadlock detection, or multiple holds of the same
> > lock, or pretty much anything that LWLocks don't give you.
>
> Well, that was my initial inclination too, but Andres didn't like it.
> I don't know whether it's better to take his advice or yours.
>
> The one facility that we need here which the heavyweight lock facility
> does provide and the lightweight lock facility does not is the ability
> to take locks on an effectively unlimited number of distinct objects.
> That is, we can't have a separate LWLock for every relation, because
> there ~2^32 relation OIDs per database, and ~2^32 database OIDs, and a
> patch that tried to allocate a tranche of 2^64 LWLocks would probably
> get shot down.
>

I think if we have to follow any LWLock based design, then we also
need to think about a case where if it is already acquired by the
backend (say in X mode), then it should be granted if the same backend
tries to acquire it in same mode (or mode that is compatible with the
mode in which it is already acquired). As per my analysis above [1],
we do this at multiple places for relation extension lock.

[1] - https://www.postgresql.org/message-id/CAA4eK1%2BE8Vu%3D9PYZBZvMrga0Ynz_m6jmT3G_vJv-3L1PWv9Krg%40mail.gmail.com

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com