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Re: Buffer locking is special (hints, checksums, AIO writes)

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Noah Misch <noah@leadboat.com>, Kirill Reshke <reshkekirill@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>
Date: 2026-02-09T22:19:06Z
Lists: pgsql-hackers 
Hi,

On 2026-02-09 12:14:47 +0200, Heikki Linnakangas wrote:
> On 09/02/2026 03:52, Andres Freund wrote:
> > On 2026-02-07 14:59:34 +0200, Heikki Linnakangas wrote:
> > > > +/*
> > > > + * Try to set a single hint bit in a buffer.
> > > > + *
> > > > + * This is a bit faster than BufferBeginSetHintBits() /
> > > > + * BufferFinishSetHintBits() when setting a single hint bit, but slower than
> > > > + * the former when setting several hint bits.
> > > > + */
> > > > +bool
> > > > +BufferSetHintBits16(uint16 *ptr, uint16 val, Buffer buffer)
> > > 
> > > This could use some more explanation. The point is that this does "*ptr =
> > > val", if it's allowed to set hint bits. That's not obvious. And "single hint
> > > bit" isn't really accurate, as you could update multiple bits in *ptr with
> > > one call.
> > 
> > Agreed.  I updated it to
> > 
> >   * Try to set hint bits on a single 16bit value in a buffer.
> >   *
> >   * If hint bits are allowed to be set, set *ptr = val, try mark the buffer
> >   * dirty and return true. Otherwise false is returned.
> >   *
> >   * *ptr needs to be a pointer to memory within the buffer.
> >   *
> >   * This is a bit faster than BufferBeginSetHintBits() /
> >   * BufferFinishSetHintBits() when setting hints once in a buffer, but slower
> >   * than the former when setting hint bits multiple times in the same buffer.
> 
> +1. Instead of "try mark the buffer dirty", I'd say just "mark the buffer
> dirty". The only reason it might not to mark the buffer dirty is that it was
> already marked dirty, right? I wouldn't call that a failure.

It's not quite the only reason:

		/*
		 * If we need to protect hint bit updates from torn writes, WAL-log a
		 * full page image of the page. This full page image is only necessary
		 * if the hint bit update is the first change to the page since the
		 * last checkpoint.
		 *
		 * We don't check full_page_writes here because that logic is included
		 * when we call XLogInsert() since the value changes dynamically.
		 */
		if (XLogHintBitIsNeeded() && (lockstate & BM_PERMANENT))
		{
			/*
			 * If we must not write WAL, due to a relfilelocator-specific
			 * condition or being in recovery, don't dirty the page.  We can
			 * set the hint, just not dirty the page as a result so the hint
			 * is lost when we evict the page or shutdown.
			 *
			 * See src/backend/storage/page/README for longer discussion.
			 */
			if (RecoveryInProgress() ||
				RelFileLocatorSkippingWAL(BufTagGetRelFileLocator(&bufHdr->tag)))
				return;

			wal_log = true;
		}

Greetings,

Andres Freund

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. bufmgr: Fix ordering of checks in PinBuffer()

  2. Use UnlockReleaseBuffer() in more places

  3. bufmgr: Make UnlockReleaseBuffer() more efficient

  4. bufmgr: Don't copy pages while writing out

  5. Fix use of wrong variable in _hash_kill_items()

  6. Fix bug due to confusion about what IsMVCCSnapshot means

  7. bufmgr: Switch to standard order in MarkBufferDirtyHint()

  8. bufmgr: Remove the, now obsolete, BM_JUST_DIRTIED

  9. Require share-exclusive lock to set hint bits and to flush

  10. heapam: Don't mimic MarkBufferDirtyHint() in inplace updates

  11. bufmgr: Allow conditionally locking of already locked buffer

  12. bufmgr: Avoid spurious compiler warning after fcb9c977aa5

  13. lwlock: Remove ForEachLWLockHeldByMe

  14. lwlock: Remove support for disowned lwlwocks

  15. bufmgr: Implement buffer content locks independently of lwlocks

  16. bufmgr: Change BufferDesc.state to be a 64-bit atomic

  17. lwlock: Improve local variable name

  18. lwlock: Invert meaning of LW_FLAG_RELEASE_OK

  19. bufmgr: Make definitions related to buffer descriptor easier to modify

  20. heapam: Add batch mode mvcc check and use it in page mode

  21. freespace: Don't modify page without any lock

  22. heapam: Move logic to handle HEAP_MOVED into a helper function

  23. bufmgr: Optimize & harmonize LockBufHdr(), LWLockWaitListLock()

  24. bufmgr: Add one-entry cache for private refcount

  25. bufmgr: Separate keys for private refcount infrastructure

  26. Add pg_atomic_unlocked_write_u64

  27. Rename BUFFERPIN wait event class to BUFFER

  28. bufmgr: Turn BUFFER_LOCK_* into an enum

  29. lwlock: Fix, currently harmless, bug in LWLockWakeup()

  30. bufmgr: Use atomic sub for unpinning buffers

  31. bufmgr: Allow some buffer state modifications while holding header lock

  32. bufmgr: Fix valgrind checking for buffers pinned in StrategyGetBuffer()

  33. bufmgr: Don't lock buffer header in StrategyGetBuffer()

  34. bufmgr: fewer calls to BufferDescriptorGetContentLock

  35. bufmgr: Fix signedness of mask variable in BufferSync()

  36. bufmgr: Introduce FlushUnlockedBuffer

  37. Improve ReadRecentBuffer() scalability