Horton

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: 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>
Date: 2025-09-15T23:05:37Z
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. Require share-exclusive lock to set hint bits and to flush

  2. lwlock: Remove ForEachLWLockHeldByMe

  3. bufmgr: Implement buffer content locks independently of lwlocks

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

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

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

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

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

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

  10. bufmgr: Separate keys for private refcount infrastructure

  11. Add pg_atomic_unlocked_write_u64

  12. Rename BUFFERPIN wait event class to BUFFER

  13. bufmgr: Turn BUFFER_LOCK_* into an enum

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

  15. bufmgr: Use atomic sub for unpinning buffers

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

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

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

  19. bufmgr: fewer calls to BufferDescriptorGetContentLock

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

  21. bufmgr: Introduce FlushUnlockedBuffer

  22. Improve ReadRecentBuffer() scalability

Attachments

Hi,

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.

I'm not sure I like the TODOs added in 0003 and removed in 0004, but squashing
the changes doesn't really seem better either.

Greetings,

Andres Freund