Re: Optimize LISTEN/NOTIFY

Joel Jacobson <joel@compiler.org>

From: "Joel Jacobson" <joel@compiler.org>

To: "Tom Lane" <tgl@sss.pgh.pa.us>

Cc: pgsql-hackers <pgsql-hackers@postgresql.org>

Date: 2025-07-15T20:56:57Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Optimize LISTEN/NOTIFY via shared channel map and direct advancement.
- 282b1cde9ded 19 (unreleased) landed
Fix incorrect logic for caching ResultRelInfos for triggers
- 39dcfda2d23a 19 (unreleased) cited

Attachments

0001-Subject-Optimize-LISTEN-NOTIFY-signaling-for-scalabi-v3.patch (application/octet-stream) patch v3-0001

On Tue, Jul 15, 2025, at 09:20, Joel Jacobson wrote:
> On Sun, Jul 13, 2025, at 01:18, Tom Lane wrote:
>> "Joel Jacobson" <joel@compiler.org> writes:
>>> The attached proof-of-concept patch proposes a straightforward
>>> optimization for the single-listener case. It introduces a shared-memory
>>> hash table mapping (dboid, channelname) to the ProcNumber of a single
>>> listener.
>>
>> What does that do to the cost and parallelizability of LISTEN/UNLISTEN?
>
> Good point. The previous patch would effectively force all LISTEN/UNLISTEN
> to be serialized, which would at least hurt parallelizability.
>
> New benchmark confirm this hypothesis.
>
> New patch attached that combines two complementary approaches, that together
> seems to scale well for both common-channel and unique-channel scenarios:

Thanks to the FreeBSD animal failing, I see I made a shared memory blunder.
New squashed patch attached.

/Joel