Just brain-storming here... would moving NBuffers to shared memory solve this specific issue? Though I'm pretty sure
thatwould open up a new set of synchronization issues elsewhere, so I'm not sure if there's a net gain.
Jack
>-----Original Message-----
>From: Andres Freund <andres@anarazel.de>
>Sent: Monday, July 14, 2025 11:12 AM
>To: Dmitry Dolgov <9erthalion6@gmail.com>
>Cc: Thom Brown <thom@linux.com>; Ashutosh Bapat
><ashutosh.bapat.oss@gmail.com>; Tomas Vondra <tomas@vondra.me>;
>Thomas Munro <thomas.munro@gmail.com>; PostgreSQL-development <pgsql-
>hackers@postgresql.org>; Jack Ng <Jack.Ng@huawei.com>; Ni Ku
><jakkuniku@gmail.com>
>Subject: Re: Changing shared_buffers without restart
>
>Hi,
>
>On July 14, 2025 10:39:33 AM EDT, Dmitry Dolgov <9erthalion6@gmail.com>
>wrote:
>>> On Mon, Jul 14, 2025 at 10:23:23AM -0400, Andres Freund wrote:
>>> > Those steps are separated in time, and I'm currently trying to
>>> > understand what are the consequences of performing them in
>>> > different order and whether there are possible concurrency issues
>>> > under various scenarios. Does this make more sense, or still not?
>>>
>>> I still don't understand why it'd ever make sense to put a buffer
>>> onto the freelist before updating NBuffers first.
>>
>>Depending on how NBuffers is updated, different backends may have
>>different value of NBuffers for a short time frame. In that case a
>>scenario I'm trying to address is when one backend with the new
>>NBuffers value allocates a new buffer and puts it into the buffer
>>lookup table, where it could become reachable by another backend, which
>>still has the old NBuffer value. Correct me if I'm wrong, but
>>initializing buffer headers + updating NBuffers means clock sweep can
>>now return one of those new buffers, opening the scenario above, right?
>
>The same is true if you put buffers into the freelist.
>
>Andres
>--
>Sent from my Android device with K-9 Mail. Please excuse my brevity.
