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help / color / mirror / Atom feedFrom: Peter Geoghegan <[email protected]>
To: Andres Freund <[email protected]>
Cc: Tomas Vondra <[email protected]>
Cc: Alexandre Felipe <[email protected]>
Cc: Thomas Munro <[email protected]>
Cc: Nazir Bilal Yavuz <[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: Sat, 28 Feb 2026 14:32:35 -0500
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On Fri, Feb 27, 2026 at 7:37 PM Andres Freund <[email protected]> wrote:
> I do continue to wonder if we ought to pass down some hints from the planner
> about how much data an indexscan is likely to read to influence readahead
> aggressiveness.
As you know, I already have some of that, but it isn't particularly
well developed. I agree that pushing the hinting mechanism further and
using something like your proposed READ_STREAM_SLOW_START mechanism
(which you mention in your later email from today) seems most
promising.
> I do agree it's right beign concerned about the increase in index fetches with
> such mark/restore cases.
> Is this, by any chance, with starting the server and running these queries in
> that order? Are you repeating these runs within one server start, evicting
> the buffers inbetween?
Yes, I repeat the runs 3 times, evicting/prewarming as needed each
time. I go with the fastest run.
> Are you using huge pages? I see rather differing performance results
> with/without when not prefetching.
Yes, I always use huge_pages (and never use transparent huge pages).
> It really shouldn't be sensitive - that query will never be able to reuse
> heapam pages within a query, and evicting a clean buffer isn't that expensive.
Could have been my previous failure to use "cpupower idle-set -D 0" to
get maximally stable performance for my microbenchmarks. As discussed
privately, I've added this as yet another step in the script that
automates all this for me/that prepares my system to run a
microbenchmark.
--
Peter Geoghegan
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Subject: Re: index prefetching
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