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

On Thu, 13 Feb 2020 at 09:46, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Feb 11, 2020 at 9:13 PM 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.
> > * When we want to extend some relation R, choose one of those locks
> > (say, R's relfilenode number mod N) and lock it.
> >
>
> I am imagining something on the lines of BufferIOLWLockArray (here it
> will be RelExtLWLockArray). The size (N) could MaxBackends or some
> percentage of it (depending on testing) and indexing into an array
> could be as suggested (R's relfilenode number mod N). We need to
> initialize this during shared memory initialization. Then, to extend
> the relation with multiple blocks at-a-time (as we do in
> RelationAddExtraBlocks), we can either use the already proven
> technique of group clear xid mechanism (see ProcArrayGroupClearXid) or
> have an additional state in the RelExtLWLockArray which will keep the
> count of waiters (as done in latest patch of Sawada-san [1]). We
> might want to experiment with both approaches and see which yields
> better results.

Thanks all for the suggestions. I have started working on the
implementation based on the suggestion. I will post a patch for this
in few days.

--
Thanks and Regards
Mahendra Singh Thalor
EnterpriseDB: http://www.enterprisedb.com