Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Noah Misch <noah@leadboat.com>, 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: 2025-04-30T20:00:35Z
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 aio: Fix reference to outdated name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,

After a bit more private back and forth with Alexander I have found the issue
- and it's pretty stupid:

pgaio_io_wait_for_free() does what it says on the tin. For that, after a bunch
of other things, finds the oldest in-flight IO and waits for it.

		PgAioHandle *ioh = dclist_head_element(PgAioHandle, node,
											   &pgaio_my_backend->in_flight_ios);

		switch (ioh->state)
		{
...
			case PGAIO_HS_COMPLETED_IO:
			case PGAIO_HS_SUBMITTED:
				pgaio_debug_io(DEBUG2, ioh,
							   "waiting for free io with %d in flight",
							   dclist_count(&pgaio_my_backend->in_flight_ios));
...
				pgaio_io_wait(ioh, ioh->generation);
				break;


The problem is that, if the log level is low enough, ereport() (which is
called by pgaio_debug_io()), processes interrupts.  The interrupt processing
may end up execute ProcessBarrierSmgrRelease(), which in turn needs to wait
for all in-flight IOs before the IOs are closed.

Which then leads to the
			elog(PANIC, "waiting for own IO in wrong state: %d",
				 state);

error.

The waiting for in-flight IOs before closing FDs only happens with io-uring,
hence this only triggering with io-uring.


A similar set of steps can lead to the "no free IOs despite no in-flight IOs"
ERROR that Alexander also observed - if pgaio_submit_staged() triggers a debug
ereport that executes ProcessBarrierSmgrRelease() in an interrupt, we might
wait for all in-flight IOs during IO submission, triggering the error.


I'm somewhat amazed that Alexander could somewhat reliably reproduce this - I
haven't been able to do so once using Alexander's recipe.  I did find a
reproducer though:

c=32;pgbench -n -c$c -j$c -P1 -T1000 -f <(echo 'SELECT sum(abalance) FROM pgbench_accounts;')
c=1;pgbench -n -c$c -j$c -P1 -T1000 -f <(echo 'DROP DATABASE IF EXISTS foo;CREATE DATABASE foo;')

trigger both issues quickly if run with
  log_min_messages=debug2
  io_method=io_uring
and not when a non-debug log level is used.


I'm not yet sure how to best fix it - locally I have done so by pgaio_debug()
do a HOLD_INTERRUPTS()/RESUME_INTERRUPTS() around the call to ereport.  But
that doesn't really seem great - otoh requiring various pieces of code to know
that anything emitting debug messages needs to hold interrupts etc makes for
rare and hard to understand bugs.

We could just make the relevant functions hold interrupts, and that might be
the best path forward, but we don't really need to hold all interrupts
(e.g. termination would be fine), so it's a bit coarse grained.  It would need
to happen in a few places, which isn't great either.

Other suggestions?


Thanks again for finding and reporting this Alexander!

Greetings,

Andres Freund