| From: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
|---|---|
| To: | Tomas Vondra <tomas(at)vondra(dot)me> |
| Cc: | Peter Geoghegan <pg(at)bowt(dot)ie>, Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Georgios <gkokolatos(at)protonmail(dot)com>, Konstantin Knizhnik <knizhnik(at)garret(dot)ru>, Dilip Kumar <dilipbalaut(at)gmail(dot)com> |
| Subject: | Re: index prefetching |
| Date: | 2025-07-21 00:59:04 |
| Message-ID: | CA+hUKGLvL408O2ss6YZQsdupryvBqjhuej_hyOYU9fobhdHkTQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Sun, Jul 20, 2025 at 1:07 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> On Sat, Jul 19, 2025 at 11:23 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
> > The thing that however concerns me is that what I observed was not the
> > distance getting reset to 1, and then ramping up. Which should happen
> > pretty quickly, thanks to the doubling. In my experiments it *never*
> > ramped up again, it stayed at 1. I still don't quite understand why.
>
> Huh. Will look into that on Monday.
I suspect that it might be working as designed, but suffering from a
bit of a weakness in the distance control algorithm, which I described
in another thread[1]. In short, the simple minded algorithm that
doubles on miss and subtracts one on hit can get stuck alternating
between 1 and 2 if you hit certain patterns. Bilal pinged me off-list
to say that he'd repro'd something like your test case and that's what
seemed to be happening, anyway? I will dig out my experimental
patches that tried different adjustments to escape from that state....
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