Feedback-ID: ic6394509:Fastmail
MIME-Version: 1.0
Date: Wed, 12 Nov 2025 21:37:51 +0100
From: "Joel Jacobson" <joel@compiler.org>
To: "Arseniy Mukhin" <arseniy.mukhin.dev@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Message-Id: <074cfe3f-b59f-4c42-8dab-f845f691ae0e@app.fastmail.com>
In-Reply-To: 
 <CAE7r3MJyitsHSd9A-y42Cmzs5gmG=zJH2pqMytSVzmzF4e_K9A@mail.gmail.com>
References: <6899c044-4a82-49be-8117-e6f669765f7e@app.fastmail.com>
 <165530.1752362320@sss.pgh.pa.us>
 <02a7cd37-e2fc-4212-8b19-f8c239c95fb8@app.fastmail.com>
 <e396ecf3-4227-4918-b9ff-e9568dcebcf0@app.fastmail.com>
 <b6a427fe-2ed6-4568-85d9-207a68172617@app.fastmail.com>
 <CAK80=jhmE40KVqQ3ho37MArS7cAED1p9m7uikDxcnDmqdW7t8A@mail.gmail.com>
 <96f00bf1-cc9d-4520-9d02-9e14e7767c88@app.fastmail.com>
 <30c2aa7d-dd6c-4b68-a2e4-f217a1a34acf@app.fastmail.com>
 <CA+hUKGLrMGkWDB0cwTa0RqD+AF7O-Ywgck8aVYKwOQnZgYRRug@mail.gmail.com>
 <0b4d402a-9ac2-4aa8-acf8-8231dbe579ea@app.fastmail.com>
 <c8e84bb2-d2e5-4a77-b547-3a93c5778e7e@app.fastmail.com>
 <3095599.1758644879@sss.pgh.pa.us>
 <0dc6a2cc-5216-4dc1-9dd2-430cafc6095b@app.fastmail.com>
 <E7D8A5CD-C445-4713-B6B7-6CC1D3A71161@gmail.com>
 <baf71275-9083-41be-ae13-f80821541a1a@app.fastmail.com>
 <52CC167F-763B-4ECA-B0B4-DAB381816828@gmail.com>
 <d35a114d-204a-4b40-9284-06749532b724@app.fastmail.com>
 <9186C6D0-F7A9-482A-9183-89E530B57E36@gmail.com>
 <da0996!!!!!!d8-ecf9-490d-bbd0-2a5bdb5b115a@app.fastmail.com>
 <D1CF7CA5-213A-4FC1-86DB-052D58904A0A@gmail.com>
 <cad78ff4-aae9-4c1c-8e16-ff0ccd4957f1@app.fastmail.com>
 <1073593.1759423179@sss.pgh.pa.us>
 <4bd5e6c4-6fa7-44bb-869d-59a32a331fa8@app.fastmail.com>
 <CAFY6G8dap-bCnAnMG-2Gzew8yv2Vbi9gsx9+yszKMmd57ygfvA@mail.gmail.com>
 <85828f29-e72e-4400-94f3-9a69bc8dc239@app.fastmail.com>
 <2495353.1759860890@sss.pgh.pa.us>
 <8aeae418-92a6-4bbd-9c06-9574c79e59f7@app.fastmail.com>
 <2531672.1759868124@sss.pgh.pa.us>
 <474efa78-337c-41cd-a73a-f845a0115109@app.fastmail.com>
 <2749343.1759949176@sss.pgh.pa.us>
 <8bfca2be-1ec0-4e15-aafb-0b7b661fe936@app.fastmail.com>
 <9eba307f-f2fb-48f0-9507-2e197f39ef9e@app.fastmail.com>
 <cde2a91d-d47a-43b5-ab27-f98aa739d38e@app.fastmail.com>
 <8c71183a-0d28-4bcf-a806-78446ff95404@app.fastmail.com>
 <1009807.1760476747@sss.pgh.pa.us>
 <1F7227F5-C33D-4E2C-8511-33F1468590D0@gmail.com>
 <0a5a20d3-4621-46b3-b2ab-903f63a20dea@app.fastmail.com>
 <6F913129-ABEF-4004-AAF3-F22FC34!29AE8@gmail.com>
 <cbabbb45-ec27-47f1-a8f3-eb29073e9aef@app.fastmail.com>
 <ceab2c54-3463-4029-a7d8-895a617bb457@app.fastmail.com>
 <1547585.1760645808@sss.pgh.pa.us>
 <b8df8040-1b15-4a1a-b5f6-74acebbd2d0c@app.fastmail.com>
 <CAE7r3MLivh1sHWF06hrVXkiQbw-KChPcQsh+9CheXprm5vRVMQ@mail.gmail.com>
 <14865EB6-0BF4-462B-9072-10BDAC10C052@gmail.com>
 <CAE7r3MK-3AOdh1mpZ8hw9h6F_i0D5RMoAy7CttnfCJRpB8GJDA@mail.gmail.com>
 <0BCA1C2D-B92C-459E-B1A6-6D06BA4C62CF@gmail.com>
 <55d24cbb-e9ef-491f-a99b-b3dbd7cecdf9@app.fastmail.com>
 <38574cad-e90d-47b7-a015-753bb6bbc360@app.fastmail.com>
 <66631FB7-5BEA-4ED5-A694-9AD8B9CCFEE8@gmail.com>
 <4b7b49a5-5e1a-44a8-93e0-60457d15cb1d@app.fastmail.com>
 <82DEA2B6-6FC5-4A79-BDE3-1FD72F104A6E@gmail.com>
 <38de1036-d8cf-420c-b845-edb5a946b191@app.fastmail.com>
 <87E40BF8-8877-4DBD-9040-99AF8A4E6358@gmail.com>
 <d29ae0a8-b28c-4450-bd9f-c56657e57f45@app.fastmail.com>
 <DF4BDA30-CC41-4BAF-9852-E399C3F273EC@gmail.com>
 <7556f0d4-03fd-451a-bd34-5f62b424319a@app.fastmail.com>
 <290910DE-9A03-4AE6-B348-073D5DA96ACC@gmail.com>
 <CAE7r3MJxxycYwDFJdTJ+HmzeNJ_JGKyr4HpT5njN1ouVX56OUg@mail.gmail.com>
 <4B243750-12BE-4C16-A769-A803268F40E3@gmail.com>
 <CAE7r3MJ0VoAJzdLzX0dgfPJpBJxW+wg_pYaCi6mQJi47+qukhg@mail.gmail.com>
 <4605CAD6-69D5-4082-B96C-91FC0DE5399D@gmail.com>
 <dc5ebc6f-6dde-499a-8d6b-c32172b84186@app.fastmail.com>
 <bacdfabb-a2a8-4379-ac97-adfc302d626a@app.fastmail.com>
 <294e1641-d658-4d43-8671-60e8ff860532@app.fastmail.com>
 <26d4dd6a-cfc5-4efc-9704-9cd3216ed712@app.fastmail.com>
 <7456ec96-7a9c-45a0-988e-ba1c7f9ec937@app.fastmail.com>
 <CAE7r3MJyitsHSd9A-y42Cmzs5gmG=zJH2pqMytSVzmzF4e_K9A@mail.gmail.com>
Subject: Re: Optimize LISTEN/NOTIFY
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Archived-At: 
 <https://www.postgresql.org/message-id/074cfe3f-b59f-4c42-8dab-f845f691ae0e%40app.fastmail.com>
Precedence: bulk

On Wed, Nov 12, 2025, at 17:57, Arseniy Mukhin wrote:
> On Tue, Nov 11, 2025 at 7:35=E2=80=AFPM Joel Jacobson <joel@compiler.o=
rg> wrote:
>> I'm therefore attaching v24 again, but renamed to v26.
>
> Thank you for the new version!

Thanks for reviewing!

> I read direct advancement part of v26, one point about it:
>
> The comment in SignalBackend says:
>
>            * Listening backends that are not advancing and are station=
ary at
>            * a position somewhere in the range we just wrote, can safe=
ly be
>            * direct advanced to the new queue head, since we know that=
 they
>            * are not interested in our messages.
>            */
>
> IIUC it's impossible for the listener to stop somewhere in between
> queueHeadBeforeWrite and queueHeadAfterWrite. If the listener has
> managed to read the first notification from the notifier, it means the
> notifier transaction is complete and the listener should stop only
> after reading all notifications (so we should always see pos =3D
> queueHeadAfterWrite or further).

Here is what I think can happen:

If the notifications written by the notifier fills the current page,
it updates QUEUE_HEAD, and if a listening backend then
enters asyncQueueReadAllNotifications at this time,
it will set its local `head` variable to the current QUEUE_HEAD,
and when the notifier continues filling the next page,
it will again update QUEUE_HEAD, and PreCommit_Notify
will overwrite queueHeadAfterWrite with the QUEUE_HEAD.

Sequence of events:

1. In the notifier, PreCommit_Notify calls asyncQueueAddEntries,
which updates QUEUE_HEAD when the page is full,
(and sets queueHeadAfterWrite to this value).

2. At this time, a listener wakes up and asyncQueueAddEntries
reads the current QUEUE_HEAD value and stores it
in its local `head` variable, and starts reading up to this pos.

3. In the notifier, PreCommit_Notify calls asyncQueueAddEntries
the second time, which updates QUEUE_HEAD,
and sets queueHeadAfterWrite to the final value
before returning.

For this reason, I think the listener could actually stop
in between queueHeadBeforeWrite and queueHeadAfterWrite,
since it's local `head` variable could get the intermediary
QUEUE_HEAD value, when a page is full.

/Joel