Feedback-ID: id4a34324:Fastmail
Date: Fri, 3 Apr 2026 16:41:18 -0400
From: Andres Freund <andres@anarazel.de>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>,
	Peter Geoghegan <pg@bowt.ie>, Tomas Vondra <tv@fuzzy.cz>,
	Nazir Bilal Yavuz <byavuz81@gmail.com>
Subject: Re: AIO / read stream heuristics adjustments for index prefetching
Message-ID: <q46tgxvdbqvbhsvrkgaod7la3jbbecjitpwanwse3jeu4ervdr@z3zqxlcxcgtd>
References: <f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu>
 <CAAKRu_bfwBzg7=Zy88st6gBJf97Wkd3k=+m1ecApn=59SwmKSw@mail.gmail.com>
 <pj4kgtdrevvkfbmlri6p27belctxru7ytyprcb6v74c7zbh3l6@m4dcu2rljedv>
 <CAAKRu_b+tymZQc2vv__k8oPHsVQppg9QmaeQLiupZ3t2AO=z3A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: 
 <CAAKRu_b+tymZQc2vv__k8oPHsVQppg9QmaeQLiupZ3t2AO=z3A@mail.gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/q46tgxvdbqvbhsvrkgaod7la3jbbecjitpwanwse3jeu4ervdr%40z3zqxlcxcgtd>
Precedence: bulk

Hi,

Forgot to send this one earlier:

On 2026-04-03 11:46:59 -0400, Melanie Plageman wrote:
> On Thu, Apr 2, 2026 at 11:47 AM Andres Freund <andres@anarazel.de> wrote:
> >
> > What do you think about the updated patch to achieve that that I posted?
> 
> here is some review on 0005 and 0006 earlier posted
> concrete things:
> -------
> - I’d reorder stream→distance == 0 in read_stream_look_ahead() (and
> issue), I found myself asking why it wasn’t first

It'd not be correct in should_issue_now(). We can't move it before

	/* there is no pending IO that could be issued */
	if (pending_read_nblocks == 0)
		return false;

because then we'd trigger a call to read_stream_start_pending_read(), without
there being a pending read.

We can't move it before

	/* never start more IOs than our cap */
	if (stream->ios_in_progress >= stream->max_ios)
		return false;

because that could lead us to exceeding max_ios.

Greetings,

Andres Freund