Feedback-ID: id4a34324:Fastmail
Date: Mon, 22 Sep 2025 18:14:12 -0400
From: Andres Freund <andres@anarazel.de>
To: pgsql-hackers@postgresql.org,
	Melanie Plageman <melanieplageman@gmail.com>,
 Thomas Munro <thomas.munro@gmail.com>,
	Heikki Linnakangas <hlinnaka@iki.fi>, Noah Misch <noah@leadboat.com>,
	Robert Haas <robertmhaas@gmail.com>,
 Michael Paquier <michael.paquier@gmail.com>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
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Hi,

On 2025-09-15 19:05:37 -0400, Andres Freund wrote:
> On 2025-08-22 15:44:48 -0400, Andres Freund wrote:
> > The hardest part about this change is that everything kind of depends on each
> > other. The changes are large enough that they clearly can't just be committed
> > at once, but doing them over time risks [temporary] performance regressions.
> >
> >
> >
> >
> > The order of changes I think makes the most sense is the following:
> >
> > 1) Allow some modifications while holding the buffer header spinlock
> >
> > 2) Reduce buffer pin with just an atomic-sub
> >
> >    This needs to happen first, otherwise there are performance regressions
> >    during the later steps.
>
> Here are the first few cleaned up patches implementing the above steps, as
> well as some cleanups.  I included a commit from another thread, as it
> conflicts with these changes, and we really should apply it - and it's
> arguably required to make the changes viable, as it removes one more use of
> PinBuffer_Locked().
>
> Another change included is to not return the buffer with the spinlock held
> from StrategyGetBuffer(), and instead pin the buffer in freelist.c. The reason
> for that is to reduce the most common PinBuffer_locked() call. By definition
> PinBuffer_locked() will become a bit slower due to 0003. But even without 0003
> it 0002 is faster than master. And the previous approach also just seems
> pretty unclean.   I don't love that it requires the new TrackNewBufferPin(),
> but I don't really have a better idea.
>
> I invite particular attention to the commit message for 0003 as well as the
> comment changes in buf_internals.h within.

Robert looked at the patches while we were chatting, and I addressed his
feedback in this new version.

Changes:

- Updated patch description for 0002, giving a lot more background

- Improved BufferDesc comments a fair bit more in 0003

- Reduced the size of 0003 a bit, by using UnlockBufHdrExt() instead of a CAS
  loop in buffer_stage_common() and reordering some things in
  TerminateBufferIO()

- Made 0004 only use the non-looping atomic op in UnpinBufferNoOwner(), not
  MarkBufferDirty(). I realized the latter would take additional complexity to
  make safe (a CAS loop in TerminateBufferIO()).  I am somewhat doubtful that
  there are workloads where it matters...


Greetings,

Andres Freund

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