Re: Use fadvise in wal replay

On Wed, Jun 22, 2022 at 2:07 PM Andrey Borodin <x4mmm(at)yandex-team(dot)ru> wrote:

>
>
> > On 21 Jun 2022, at 20:52, Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
> >
> > > On 21 Jun 2022, at 16:59, Jakub Wartak <jakub(dot)wartak(at)tomtom(dot)com>
> wrote:
> > Oh, wow, your benchmarks show really impressive improvement.
> >
> > FWIW I was trying to speedup long sequential file reads in Postgres
> using fadvise hints. I've found no detectable improvements.
> > Then I've written 1Mb - 1Gb sequential read test with both fadvise
> POSIX_FADV_WILLNEED and POSIX_FADV_SEQUENTIAL in Linux.
> Did you drop caches?
>
Yes. I saw nothing changes speed of long file (50Mb+) read.

> > The only improvement I've found was
> >
> > 1. when the size of read was around several Mb and fadvise len also
> around several Mb.
> > 2. when before fdavice and the first read there was a delay (which was
> supposedly used by OS for reading into prefetch buffer)
> That's the case of startup process: you read a xlog page, then redo
> records from this page.
>
Then I'd guess that your speedup is due to speeding up the first several
Mb's in many files opened (and delay for kernel prefetch is due to some
other reason). That may differ from the case I've tried to measure speedup
and this could be the cause of speedup in your case.

--
Best regards,
Pavel Borisov

Postgres Professional: http://postgrespro.com <http://www.postgrespro.com>