Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Noah Misch <noah@leadboat.com>
Cc: pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Jelte Fennema-Nio <postgres@jeltef.nl>, Antonin Houska <ah@cybertec.at>
Date: 2025-04-01T16:51:53Z
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. aio: Fix assertion, clarify README

  2. aio: Fix reference to outdated name

  3. aio: Fix possible state confusions due to interrupt processing

  4. aio: Improve debug logging around waiting for IOs

  5. aio: Fix crash potential for pg_aios views due to late state update

  6. Increase BAS_BULKREAD based on effective_io_concurrency

  7. localbuf: Add Valgrind buffer access instrumentation

  8. aio: Make AIO more compatible with valgrind

  9. aio: Avoid spurious coverity warning

  10. tests: Fix incompatibility of test_aio with *_FORCE_RELEASE

  11. tests: Cope with WARNINGs during failed CREATE DB on windows

  12. aio: Add errcontext for processing I/Os for another backend

  13. aio: Add README.md explaining higher level design

  14. aio: Minor comment improvements

  15. aio: Add test_aio module

  16. aio: Add pg_aios view

  17. docs: Add acronym and glossary entries for I/O and AIO

  18. Enable IO concurrency on all systems

  19. read_stream: Introduce and use optional batchmode support

  20. docs: Reframe track_io_timing related docs as wait time

  21. bufmgr: Use AIO in StartReadBuffers()

  22. bufmgr: Implement AIO read support

  23. aio: Add WARNING result status

  24. Let caller of PageIsVerified() control ignore_checksum_failure

  25. pgstat: Allow checksum errors to be reported in critical sections

  26. Add errhint_internal()

  27. localbuf: Track pincount in BufferDesc as well

  28. aio, bufmgr: Comment fixes/improvements

  29. Fix mis-attribution of checksum failure stats to the wrong database

  30. aio: Implement support for reads in smgr/md/fd

  31. aio: Add io_method=io_uring

  32. aio: Add liburing dependency

  33. aio: Rename pgaio_io_prep_* to pgaio_io_start_*

  34. aio: Pass result of local callbacks to ->report_return

  35. aio: Be more paranoid about interrupts

  36. Redefine max_files_per_process to control additionally opened files

  37. aio: Change prefix of PgAioResultStatus values to PGAIO_RS_

  38. bufmgr: Improve stats when a buffer is read in concurrently

  39. aio: Add io_method=worker

  40. aio: Infrastructure for io_method=worker

  41. aio: Add core asynchronous I/O infrastructure

  42. aio: Basic subsystem initialization

  43. tests: Expand temp table tests to some pin related matters

  44. localbuf: Introduce FlushLocalBuffer()

  45. localbuf: Introduce TerminateLocalBufferIO()

  46. localbuf: Fix dangerous coding pattern in GetLocalVictimBuffer()

  47. localbuf: Introduce StartLocalBufferIO()

  48. localbuf: Introduce InvalidateLocalBuffer()

  49. Allow lwlocks to be disowned

  50. Make jsonb casts to scalar types translate JSON null to SQL NULL.

  51. bufmgr/smgr: Don't cross segment boundaries in StartReadBuffers()

  52. Use aux process resource owner in walsender

  53. bufmgr: Return early in ScheduleBufferTagForWriteback() if fsync=off

Hi,

On 2025-04-01 09:07:27 -0700, Noah Misch wrote:
> On Tue, Apr 01, 2025 at 11:55:20AM -0400, Andres Freund wrote:
> > WRT the locking issues, I've been wondering whether we could make
> > LWLockWaitForVar() work that purpose, but I doubt it's the right approach.
> > Probably better to get rid of the LWLock*Var functions and go for the approach
> > I had in v1, namely a version of LWLockAcquire() with a callback that gets
> > called between LWLockQueueSelf() and PGSemaphoreLock(), which can cause the
> > lock acquisition to abort.
>
> What are the best thing(s) to read to understand the locking issues?

Unfortunately I think it's our discussion from a few days/weeks ago.

The problem basically is that functions like LockBuffer(EXCLUSIVE) need to be able
to non-racily

a) wait for in-fligth IOs
b) acquire the content lock

If you just do it naively like this:

    else if (mode == BUFFER_LOCK_EXCLUSIVE)
    {
        if (pg_atomic_read_u32(&buf->state) &_IO_IN_PROGRESS)
            WaitIO(buf);
        LWLockAcquire(content_lock, LW_EXCLUSIVE);
   }

you obviously could have another backend start new IO between the WaitIO() and
the LWLockAcquire().  If that other backend then doesn't consume the
completion of that IO, the current backend could end up endlessly waiting for
the IO.  I don't see a way to avoid with narrow changes just to LockBuffer().


We need some infrastructure that allows to avoid that issue.  One approach
could be to integrate more tightly with lwlock.c. If

1) anyone starting IO were to wake up all waiters for the LWLock

2) The waiting side checked that there is no IO in progress *after*
   LWLockQueueSelf(), but before PGSemaphoreLock()

The backend doing LockBuffer() would be guaranteed to have the chance to wait
for the IO, rather than the lwlock.


But there might be better approaches.

I'm not really convinced that using generic lwlocks for buffer locking is the
best idea. There's just too many special things about buffers. E.g. we have
rather massive NUMA scalability issues due to the amount of lock traffic from
buffer header and content lock atomic operations, particuly on things like the
uppermost levels of a btree.  I've played with ideas like super-pinning and
locking btree root pages, which move all the overhead to the side that wants
to exclusively lock such a page - but that doesn't really make sense for
lwlocks in general.


Greetings,

Andres Freund