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Subject: Re: Optimize LISTEN/NOTIFY
From: Chao Li <li.evan.chao@gmail.com>
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 <CAE7r3MLivh1sHWF06hrVXkiQbw-KChPcQsh+9CheXprm5vRVMQ@mail.gmail.com>
Date: Thu, 23 Oct 2025 16:16:27 +0800
Cc: Joel Jacobson <joel@compiler.org>,
 Tom Lane <tgl@sss.pgh.pa.us>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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To: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
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 <https://www.postgresql.org/message-id/14865EB6-0BF4-462B-9072-10BDAC10C052%40gmail.com>
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> On Oct 21, 2025, at 00:43, Arseniy Mukhin =
<arseniy.mukhin.dev@gmail.com> wrote:
>=20
>=20
> I managed to reproduce the race with v20-alt3. I tried to write a TAP
> test reproducing the issue, so it was easier to validate changes.
> Please find the attached TAP test. I added it to some random package
> for simplicity.
>=20

With alt3, as we have acquired the notification lock after reading every =
message to update the POS, I think we can do a little bit more =
optimization:

The notifier: in SignalBackend()
    * Now we check if a listener=E2=80=99s pos equals to beforeWritePos, =
then we do =E2=80=9Cdirectly advancement=E2=80=9D
    * We can change to if a listener=E2=80=99s pos is between =
beforeWritePos and afterWritePos, then we can do the advancement.

The listener: in asyncQueueReadAllNotifications():
    * With alt3, we only lock and update pos
    * We can do more. If current pos in shared memory is after that =
local pos, then meaning some notifier has done an advancement, so it can =
stop reading.

I tried to run your TAP test on my MacBook, but failed:

```
t/008_listen-pos-race.pl .. Dubious, test returned 32 (wstat 8192, =
0x2000)
No subtests run

Test Summary Report
-------------------
t/008_listen-pos-race.pl (Wstat: 8192 (exited 32) Tests: 0 Failed: 0)
  Non-zero exit status: 32
  Parse errors: No plan found in TAP output
Files=3D1, Tests=3D0,  3 wallclock secs ( 0.01 usr  0.01 sys +  0.10 =
cusr  0.29 csys =3D  0.41 CPU)
Result: FAIL
```

I didn=E2=80=99t spend time debugging the problem. If you can figure the =
problem, maybe I can run the test from my side.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/