Horton

Re: Changing shared_buffers without restart

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
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>
Date: 2025-07-14T15:11:36Z
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 →

  1. Remove PG_MMAP_FLAGS from mem.h

  2. Improve runtime and output of tests for replication slots checkpointing.

  3. Revert support for improved tracking of nested queries

  4. Use exported symbols list on macOS for loadable modules as well

  5. Add support for basic NUMA awareness

  6. Avoid unnecessary copying of a string in pg_restore.c

  7. aio: Infrastructure for io_method=worker

  8. Improve InitShmemAccess() prototype

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
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