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

On Thu, Feb 13, 2020 at 9:46 AM 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.

IMHO, in this case, there is no point in using the "group clear" type
of mechanism mainly for two reasons 1) It will unnecessarily make
PGPROC structure heavy.
2) For our case, we don't need any specific pieces of information from
other waiters, we just need the count.

Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com