public inbox for [email protected]  

help / color / mirror / Atom feed

From: Laurenz Albe <[email protected]>
To: Marc Millas <[email protected]>
To: [email protected] <[email protected]>
Subject: Re: shared buffers
Date: Sat, 26 Apr 2025 00:46:15 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <CADX_1abjKJhk10vWz6S=cwgXsppGXw2VjNnLVByE3EMJzLzD4w@mail.gmail.com>
References: <CADX_1abjKJhk10vWz6S=cwgXsppGXw2VjNnLVByE3EMJzLzD4w@mail.gmail.com>

On Fri, 2025-04-25 at 15:42 +0200, Marc Millas wrote:
> got something strange to me:
> Same db ie. same data, around 1.2TB,one on pg13, one on pg16
> same 16 GB of shared_buffers,
> I am the single user.
> both have track_io_timing on
> 
> on pg13, if I run a big request with explain (analyze,buffers), 
> I see around 6 GB read
> if I do rerun the very same request, no more read(s), all data in the shared buffers cache. fine
> If I check with pg_buffercache what's in it, I see the biggest tables of my request within
> the biggest users (in number of blocks used). All this is fine.
> 
> next, if I do the very same on the pg16 machine, whatever the number of times I rerun the
> explain (analyze, buffers) of the same request, each time, the explain shows the same volume
> of reads. again and again.
> If I check with pg_buffercache, the set of objects stay the same, WITHOUT the objects of my
> request, just like if those objects where sticky.

I can't see the plans, so I can only guess.

Perhaps the v16 plan uses a sequential scan on a table that is more than a quarter of
shared_buffers in size, so that PostgreSQL uses a ring buffer to read it instead of
blowing out more than a quarter of its buffer cache.

Yours,
Laurenz Albe

view thread (2+ messages)

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected]
  Subject: Re: shared buffers
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox