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help / color / mirror / Atom feedFrom: Thomas Munro <[email protected]>
To: Tomas Vondra <[email protected]>
Cc: Peter Geoghegan <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Georgios <[email protected]>
Cc: Konstantin Knizhnik <[email protected]>
Cc: Dilip Kumar <[email protected]>
Subject: Re: index prefetching
Date: Mon, 21 Jul 2025 12:59:04 +1200
Message-ID: <CA+hUKGLvL408O2ss6YZQsdupryvBqjhuej_hyOYU9fobhdHkTQ@mail.gmail.com> (raw)
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On Sun, Jul 20, 2025 at 1:07 AM Thomas Munro <[email protected]> wrote:
> On Sat, Jul 19, 2025 at 11:23 PM Tomas Vondra <[email protected]> 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....
[1] https://www.postgresql.org/message-id/flat/CA%2BhUKGLPakwZiFUa5fQXpYDpCXvZXQ%3DP3cWOGACCoobh7U2r3A%4...
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Subject: Re: index prefetching
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