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

On Wed, Feb 12, 2020 at 7:36 AM Masahiko Sawada
<masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>
> On Wed, 12 Feb 2020 at 00:43, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >
> > I took a brief look through this patch. I agree with the fundamental
> > idea that we shouldn't need to use the heavyweight lock manager for
> > relation extension, since deadlock is not a concern and no backend
> > should ever need to hold more than one such lock at once. But it feels
> > to me like this particular solution is rather seriously overengineered.
> > I would like to suggest that we do something similar to Robert Haas'
> > excellent hack (daa7527af) for the !HAVE_SPINLOCK case in lmgr/spin.c,
> > that is,
> >
> > * Create some predetermined number N of LWLocks for relation extension.
>
> My original proposal used LWLocks and hash tables for relation
> extension but there was a discussion that using LWLocks is not good
> because it's not interruptible[1]. Because of this reason and that we
> don't need to have two lock level (shared, exclusive) for relation
> extension lock we ended up with implementing dedicated lock manager
> for extension lock. I think we will have that problem if we use LWLocks.
>

Hmm, but we use LWLocks for (a) WALWrite/Flush (see the usage of
WALWriteLock), (b) writing the shared buffer contents (see
io_in_progress lock and its usage in FlushBuffer) and might be for few
other similar stuff. Many times those take more time than extending a
block in relation especially when we combine the WAL write for
multiple commits. So, if this is a problem for relation extension
lock, then the same thing holds true there also. Now, there are cases
like when we extend the relation with multiple blocks, finding victim
buffer under this lock, etc. where this can be also equally or more
costly, but I think we can improve some of those cases (some of this
is even pointed by Andres in his email) if we agree on a fundamental
idea of using LWLocks as proposed by Tom. I am not telling that we
implement Tom's idea without weighing its pros and cons, but it has an
appeal due to its simplicity.

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