Re: Should we cacheline align PGXACT?

On 2016-08-22 11:25:55 +0800, Craig Ringer wrote:
> On 22 August 2016 at 10:40, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> > On 2016-08-19 09:46:12 -0400, Tom Lane wrote:
> > > Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru> writes:
> > > > originally this idea was proposed by Andres Freund while experimenting
> > with
> > > > lockfree Pin/UnpinBuffer [1].
> > > > The patch is attached as well as results of pgbench -S on 72-cores
> > > > machine. As before it shows huge benefit in this case.
> > >
> > > That's one mighty ugly patch.
> >
> > My version of it was only intended to nail down some variability on the
> > pgpro machine, it wasn't intended for submission.
> >
> >
> > > Can't you do it without needing to introduce the additional layer of
> > > struct nesting?
> >
> > If we required support for anonymous unions, such things would be a lot
> > easier to do. That aside, the only alternative seems tob e hard-coding
> > padding space - which probably isn't all that un-fragile either.
> > <http://www.postgresql.org/mailpref/pgsql-hackers>
> >
>
> Somewhat naïve question from someone with much less clue about low level
> cache behaviour trying to follow along: given that we determine such
> padding at compile time, how do we ensure that the cacheline size we're
> targeting is right at runtime?

There's basically only very few common cacheline sizes. Pretty much only
64 byte and 128 bytes are common these days. By usually padding to the
larger of those two, we waste a bit of memory, but not actually cache
space on platforms with smaller lines, because the padding is never
accessed.

> Or is it safe to assume that using 16 bytes so we don't cross cache
> line boundaries is always helpful, whether we have 4 PGXACT entries
> (64 byte line) or some other number per cacheline?

That's generally a good thing to do, yes. It's probably not going to
give the full benefits here though.

Regards

Andres