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Subject: Re: Changing shared_buffers without restart
From: Yuhang Qiu <iamqyh@gmail.com>
In-Reply-To: 
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Date: Fri, 11 Sep 2026 18:56:28 +0800
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>,
 pgsql-hackers@postgresql.org,
 Heikki Linnakangas <heikki.linnakangas@databricks.com>,
 Haoyu Huang <haoyu.huang@databricks.com>,
 Tomas Vondra <tomas@vondra.me>,
 Peter Eisentraut <peter@eisentraut.org>,
 Thomas Munro <thomas.munro@gmail.com>,
 Dmitry Dolgov <9erthalion6@gmail.com>,
 Robert Haas <robertmhaas@gmail.com>,
 Andres Freund <andres@anarazel.de>,
 Jakub Wartak <jakub.wartak@enterprisedb.com>
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To: Palak Chaturvedi <chaturvedipalak1911@gmail.com>
Archived-At: 
 <https://www.postgresql.org/message-id/8E7D0939-ADE1-405A-91D2-0139A6E322B3%40gmail.com>
Precedence: bulk

Hi Palak,

> Yes. v20260908-0016 rechecks BM_TAG_VALID under LockBufHdr() and does
> UnlockBufHdr() + continue instead of asserting when the tag was
> cleared concurrently.
>=20
> Right, that line predates the MADV_REMOVE range fix. Removed it in
> v20260908-0015.

Thanks. 0015 and 0016 LGTM.

I also tested the patches and did some more review.

With shared_buffers=3D32MB and max_shared_buffers left at its default, =
parallel
workers fail to start, even without any resize:
    FATAL:  failed to initialize shared_buffers to 16384
    CONTEXT:  parallel worker

RestoreGUCState() resets the GUC to its 128MB boot value, which exceeds
MaxNBuffers and fails the check hook.

There is also a performance issue in the grow path:
PGSharedMemoryEnsureAllocated() gets the whole structure's range, rather =
than
just the added range. Even a small increase makes MADV_POPULATE_WRITE =
walk the
existing range again, adding overhead for large buffer pools. Could we =
limit
this to the page-aligned [current_end, new_end), as in the shrink path?

Two minor points:
* check_shared_buffers() allows equality with MaxNBuffers, so "must be =
less than"
  should be "must not exceed".
* In proc.c, "specifid there" should be "specified there".

Some issues raised in earlier reviews remain unaddressed. Could you =
include a
status list with the next update, separating resolved and unresolved =
items?
That would help reviewers who are new to the thread or haven't followed =
it for
a while.

Best regards,
Yuhang Qiu