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From: Chao Li <[email protected]>
To: Arseniy Mukhin <[email protected]>
Cc: Joel Jacobson <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Thu, 23 Oct 2025 16:16:27 +0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAE7r3MLivh1sHWF06hrVXkiQbw-KChPcQsh+9CheXprm5vRVMQ@mail.gmail.com>
References: <[email protected]>
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> On Oct 21, 2025, at 00:43, Arseniy Mukhin <[email protected]> wrote:
> 
> 
> 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.
> 

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’s pos equals to beforeWritePos, then we do “directly advancement”
    * We can change to if a listener’s 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=1, Tests=0,  3 wallclock secs ( 0.01 usr  0.01 sys +  0.10 cusr  0.29 csys =  0.41 CPU)
Result: FAIL
```

I didn’t 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/

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