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

On Fri, Feb 14, 2020 at 8:12 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:
> > I think MaxBackends will generally limit the number of different
> > relations that can simultaneously extend, but maybe tables with many
> > partitions might change the situation. You are right that some tests
> > might suggest a good number, let Mahendra write a patch and then we
> > can test it. Do you have any better idea?
>
> In the first place, there certainly isn't more than one extension
> happening at a time per backend, else the entire premise of this
> thread is wrong. Handwaving about partitions won't change that.
>

Having more number of partitions theoretically increases the chances
of false-sharing with the same number of concurrent sessions. For ex.
two sessions operating on two relations vs. two sessions working on
two relations with 100 partitions each would increase the chances of
false-sharing. Sawada-San and Mahendra have done many tests on
different systems and some monitoring with the previous patch that
with a decent number of fixed slots (1024), the false-sharing was very
less and even if it was there the effect was close to nothing. So, in
short, this is not the point to worry about, but to ensure that we
don't create any significant regressions in this area.

> In the second place, it's ludicrous to expect that the underlying
> platform/filesystem can support an infinite number of concurrent
> file-extension operations. At some level (e.g. where disk blocks
> are handed out, or where a record of the operation is written to
> a filesystem journal) it's quite likely that things are bottlenecked
> down to *one* such operation at a time per filesystem. So I'm not
> that concerned about occasional false-sharing limiting our ability
> to issue concurrent requests. There are probably worse restrictions
> at lower levels.
>

Agreed and what we have observed during the tests is what you have
said in this paragraph.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com