Re: Should we cacheline align PGXACT?

On 2017-03-25 19:35:35 +0300, Alexander Korotkov wrote:
> On Wed, Mar 22, 2017 at 12:23 AM, David Steele <david(at)pgmasters(dot)net> wrote:
>
> > Hi Alexander
> >
> > On 3/10/17 8:08 AM, Alexander Korotkov wrote:
> >
> > Results look good for me. Idea of committing both of patches looks
> >> attractive.
> >> We have pretty much acceleration for read-only case and small
> >> acceleration for read-write case.
> >> I'll run benchmark on 72-cores machine as well.
> >>
> >
> > Have you had a chance to run those tests yet?
> >
>
> I discovered an interesting issue.
> I found that ccce90b3 (which was reverted) gives almost same effect as
> PGXACT alignment on read-only test on 72-cores machine.

That's possibly because it changes alignment?

> That shouldn't be related to the functionality of ccce90b3 itself, because
> read-only test don't do anything with clog. And that appears to be true.
> Padding of PGPROC gives same positive effect as ccce90b3. Padding patch
> (pgproc-pad.patch) is attached. It's curious that padding changes size of
> PGPROC from 816 bytes to 848 bytes. So, size of PGPROC remains 16-byte
> aligned. So, probably effect is related to distance between PGPROC
> members...
>
> See comparison of 16-bytes alignment of PGXACT + reduce PGXACT access vs.
> padding of PGPROC.

My earlier testing had showed that padding everything is the best
approach :/

I'm inclined to push this to the next CF, it seems we need a lot more
benchmarking here.

Greetings,

Andres Freund