Re: MVCC catalog access

On Sun, May 26, 2013 at 9:10 PM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> Perhaps we see little difference in performance because PGPROC has been
> separated into PGPROC and PGXACT, reducing lock contention with getting
> snapshot data?
>
> By the way, I grabbed a 32-core machine and did some more performance tests
> with some open connections with XIDs assigned using pg_cxn v2 given by
> Robert in his previous mail to make sure that the snapshots get pretty
> large.

Thanks for checking this on another machine. It's interesting that
you were able to measure a hit for relcache rebuild, whereas I was
not, but it doesn't look horrible.

IMHO, we should press forward with this approach. Considering that
these are pretty extreme test cases, I'm inclined to view the
performance loss as acceptable. We've never really viewed DDL as
something that needs to be micro-optimized, and there is ample
testimony to that fact in the existing code and in the treatment of
prior patches in this area. This is not to say that we want to go
around willy-nilly making it slower, but I think there will be very
few users for which the number of microseconds it takes to create or
drop an SQL object is performance-critical, especially when you
consider that (1) the effect will be quite a bit less when the objects
are tables, since in that case the snapshot cost will tend to be
drowned out by the filesystem cost and (2) people who don't habitually
keep hundreds and hundreds of connections open - which hopefully most
people don't - won't see the effect anyway. Against that, this
removes the single largest barrier to allowing more concurrent DDL, a
feature that I suspect will make a whole lot of people *very* happy.

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