Re: Should we cacheline align PGXACT?

pgsql-hackers-owner(at)postgresql(dot)org wrote on 04/03/2017 01:58:03 PM:

> From: Andres Freund <andres(at)anarazel(dot)de>
> To: Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>
> Cc: David Steele <david(at)pgmasters(dot)net>, Ashutosh Sharma
> <ashu(dot)coek88(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Alvaro
> Herrera <alvherre(at)2ndquadrant(dot)com>, Robert Haas
> <robertmhaas(at)gmail(dot)com>, Bernd Helmle <mailings(at)oopsware(dot)de>, Tomas
> Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-
> hackers(at)postgresql(dot)org>
> Date: 04/03/2017 01:59 PM
> Subject: Re: [HACKERS] Should we cacheline align PGXACT?
> Sent by: pgsql-hackers-owner(at)postgresql(dot)org
>
> 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 :/
>
My approach has been to, generally, pad "everything" as well. In my
testing on power, I padded PGXACT to 16 bytes. To my surprise, with the
padding in isolation, the performance (on hammerdb) was slightly degraded.

Jim Van Fleet
>
> I'm inclined to push this to the next CF, it seems we need a lot more
> benchmarking here.
>
> Greetings,
>
> Andres Freund
>
>
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