Horton

Re: Proposal: Limitations of palloc inside checkpointer

Xuneng Zhou <xunengzhou@gmail.com>

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From: Xuneng Zhou <xunengzhou@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Korotkov <aekorotkov@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Maxim Orlov <orlovmg@gmail.com>, Andres Freund <andres@anarazel.de>, Ekaterina Sokolova <e.sokolova@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2025-08-07T07:16:58Z
Lists: pgsql-hackers

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Hi, Tom!

Thanks for catching this.

On Thu, Aug 7, 2025 at 2:58 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Alexander Korotkov <aekorotkov@gmail.com> writes:
> > I'm going to push this if no objections.
>
> I looked at these patches while preparing release notes, and
> found an oversight.  CheckpointerShmemInit does
>
>         CheckpointerShmem->max_requests = Min(NBuffers, MAX_CHECKPOINT_REQUESTS);
>
> but CheckpointerShmemSize still does
>
>     size = add_size(size, mul_size(NBuffers, sizeof(CheckpointerRequest)));
>
> So if NBuffers is more than MAX_CHECKPOINT_REQUESTS, we will allocate
> extra CheckpointerRequest array entries that we will never use,
> wasting shared memory.  Admittedly the amount is small relative to the
> shared buffers themselves, but at the very least this is confusing.
>
> The comment in CheckpointerShmemSize needs adjustment, too.

I attached a patch to fix it.

Best,
Xuneng

Commits

  1. Fix checkpointer shared memory allocation

  2. Limit checkpointer requests queue size

  3. Process sync requests incrementally in AbsorbSyncRequests