Horton

Re: AIO v2.5

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>
Cc: Thomas Munro <thomas.munro@gmail.com>, Antonin Houska <ah@cybertec.at>, pgsql-hackers@postgresql.org, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Jelte Fennema-Nio <postgres@jeltef.nl>
Date: 2025-03-25T19:39:56Z
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 aio: Fix reference to outdated name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

On Tue, Mar 25, 2025 at 02:58:37PM -0400, Andres Freund wrote:
> On 2025-03-25 08:58:08 -0700, Noah Misch wrote:
> > While having nagging thoughts that we might be releasing FDs before io_uring
> > gets them into kernel custody, I tried this hack to maximize FD turnover:
> > 
> > static void
> > ReleaseLruFiles(void)
> > {
> > #if 0
> > 	while (nfile + numAllocatedDescs + numExternalFDs >= max_safe_fds)
> > 	{
> > 		if (!ReleaseLruFile())
> > 			break;
> > 	}
> > #else
> > 	while (ReleaseLruFile())
> > 		;
> > #endif
> > }
> > 
> > "make check" with default settings (io_method=worker) passes, but
> > io_method=io_uring in the TEMP_CONFIG file got different diffs in each of two
> > runs.  s/#if 0/#if 1/ (restore normal FD turnover) removes the failures.
> > Here's the richer of the two diffs:
> 
> Yikes. That's a very good catch.
> 
> I spent a bit of time debugging this. I think I see what's going on - it turns
> out that the kernel does *not* open the FDs during io_uring_enter() if
> IOSQE_ASYNC is specified [1].  Which we do add heuristically, in an attempt to
> avoid a small but measurable slowdown for sequential scans that are fully
> buffered (c.f. pgaio_uring_submit()).  If I disable that heuristic, your patch
> above passes all tests here.

Same result here.  As an additional data point, I tried adding this so every
reopen gets a new FD number (leaks FDs wildly):

--- a/src/backend/storage/file/fd.c
+++ b/src/backend/storage/file/fd.c
@@ -1304,5 +1304,5 @@ LruDelete(File file)
 	 * to leak the FD than to mess up our internal state.
 	 */
-	if (close(vfdP->fd) != 0)
+	if (dup2(2, vfdP->fd) != vfdP->fd)
 		elog(vfdP->fdstate & FD_TEMP_FILE_LIMIT ? LOG : data_sync_elevel(LOG),
 			 "could not close file \"%s\": %m", vfdP->fileName);

The same "make check" w/ TEMP_CONFIG io_method=io_uring passes with the
combination of that and the max-turnover change to ReleaseLruFiles().

> I don't know if that's an intentional or unintentional behavioral difference.
> 
> There are 2 1/2 ways around this:
> 
> 1) Stop using IOSQE_ASYNC heuristic
> 2a) Wait for all in-flight IOs when any FD gets closed
> 2b) Wait for all in-flight IOs using FD when it gets closed
> 
> Given that we have clear evidence that io_uring doesn't completely support
> closing FDs while IOs are in flight, be it a bug or intentional, it seems
> clearly better to go for 2a or 2b.

Agreed.  If a workload spends significant time on fd.c closing files, I
suspect that workload already won't have impressive benchmark numbers.
Performance-seeking workloads will already want to tune FD usage high enough
to keep FDs long-lived.  So (1) clearly loses, and neither (2a) nor (2b)
clearly beats the other.  I'd try (2b) first but, if complicated, quickly
abandon it in favor of (2a).  What other considerations could be important?