Horton

Re: Changing shared_buffers without restart

Greg Burd <greg@burd.me>

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From: "Burd, Greg" <greg@burd.me>
To: Dmitry Dolgov <9erthalion6@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, 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: 2025-07-14T14:22:17Z
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

> On Jul 14, 2025, at 10:01 AM, Dmitry Dolgov <9erthalion6@gmail.com> wrote:
> 
>> On Mon, Jul 14, 2025 at 09:42:46AM -0400, Andres Freund wrote:
>> What on earth would be the point of putting a buffer on the freelist but not
>> make it reachable by the clock sweep? To me that's just nonsensical.
> 
> To clarify, we're not talking about this scenario as "that's how it
> would work after the resize". The point is that to expand shared buffers
> they need to be initialized, included into the whole buffer machinery
> (freelist, clock sweep, etc.) and NBuffers has to be updated. 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?

Hello, first off thanks for working on the intricate issues related to resizing
shared_buffers.

Second, I'm new in this code so take that in account but I'm the person trying
to remove the freelist entirely [1] so I have reviewed this code recently.

I'd initialize them, expand BufferDescriptors, and adjust NBuffers.  The
clock-sweep algorithm will eventually find them and make use of them.  The
buf->freeNext should be FREENEXT_NOT_IN_LIST so that StrategyFreeBuffer() will
do the work required to append it the freelist after use.  AFAICT there is no
need to add to the freelist up front.


best.

-greg

[1] https://postgr.es/m/flat/E2D6FCDC-BE98-4F95-B45E-699C3E17BA10%40burd.me