Feedback-ID: id4a34324:Fastmail
Date: Tue, 11 Feb 2025 10:36:55 -0500
From: Andres Freund <andres@anarazel.de>
To: Jelte Fennema-Nio <postgres@jeltef.nl>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, torikoshia <torikoshia@oss.nttdata.com>, 
	pgsql-hackers@postgresql.org, rjuju123@gmail.com, Bruce Momjian <bruce@momjian.us>
Subject: Re: RFC: Allow EXPLAIN to Output Page Fault Information
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Hi,

On 2025-02-11 09:59:43 +0100, Jelte Fennema-Nio wrote:
> On Tue, 11 Feb 2025 at 00:53, Andres Freund <andres@anarazel.de> wrote:
> > > The thing is that you'd often get completely misleading stats. Some of the IO
> > > will still be done by the backend itself, so there will be a non-zero
> > > value. But it will be a significant undercount, because the asynchronously
> > > executed IO won't be tracked (if worker mode is used).
> 
> Yeah, makes sense. Like I said, I would be completely fine with not
> showing these numbers at all/setting them to 0 for setups where we
> cannot easily get useful numbers (and this bgworker AIO would be one
> of those setups).

Shrug. It means that it'll not work in what I hope will be the default
mechanism before long.  I just can't get excited for that. In all likelihood
it'll result in bug reports that I'll then be on the hook to fix.


> > Independent to of this, it's probably not good that we're tracking shared
> > buffer hits after io combining, if I interpret this correctly... That looks to
> > be an issue in master, not just the AIO branch.
> 
> You mean that e.g. a combined IO for 20 blocks still sounds only as 1
> "shared read"? Yeah, that sounds like a bug.

Yep.

Greetings,

Andres Freund