Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

View thread
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>
Date: 2025-03-12T17:06:03Z
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 aio: Fix reference to outdated name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 2025-03-11 20:57:43 -0700, Noah Misch wrote:
> > I think we'll really need to do something about this for BSD users regardless
> > of AIO. Or maybe those OSs should fix something, but somehow I am not having
> > high hopes for an OS that claims to have POSIX confirming unnamed semaphores
> > due to having a syscall that always returns EPERM... [1].
> 
> I won't mind a project making things better for non-root BSD users.  I do
> think such a project should not block other projects making things better for
> everything else (like $SUBJECT).

Oh, I strongly agree.  The main reason I would like it to be addressed that
I'm pretty tired of having to think about open/netbsd whenever we update some
default setting.


> > > On Mon, Mar 10, 2025 at 02:23:12PM -0400, Andres Freund wrote:
> > > > Attached is v2.6 of the AIO patchset.
> > >
> > > > - 0005, 0006 - io_uring support - close, but we need to do something about
> > > >   set_max_fds(), which errors out spuriously in some cases
> > >
> > > What do we know about those cases?  I don't see a set_max_fds(); is that
> > > set_max_safe_fds(), or something else?
> > 
> > Sorry, yes, set_max_safe_fds(). The problem basically is that with io_uring we
> > will have a large number of FDs already allocated by the time
> > set_max_safe_fds() is called. set_max_safe_fds() subtracts already_open from
> > max_files_per_process allowing few, and even negative, IOs.
> > 
> > I think we should redefine max_files_per_process to be about the number of
> > files each *backend* will additionally open.  Jelte was working on related
> > patches, see [2]
> 
> Got it.  max_files_per_process is a quaint setting, documented as follows (I
> needed the reminder):
> 
>         If the kernel is enforcing
>         a safe per-process limit, you don't need to worry about this setting.
>         But on some platforms (notably, most BSD systems), the kernel will
>         allow individual processes to open many more files than the system
>         can actually support if many processes all try to open
>         that many files. If you find yourself seeing <quote>Too many open
>         files</quote> failures, try reducing this setting.
> 
> I could live with
> v6-0003-Reflect-the-value-of-max_safe_fds-in-max_files_pe.patch but would lean
> against it since it feels unduly novel to have a setting where we use the
> postgresql.conf value to calculate a value that becomes the new SHOW-value of
> the same setting.

I think we may update some other GUCs, but not sure.


> Options I'd consider before that:

> - Like you say, "redefine max_files_per_process to be about the number of
>   files each *backend* will additionally open".  It will become normal that
>   each backend's actual FD list length is max_files_per_process + MaxBackends
>   if io_method=io_uring.  Outcome is not unlike
>   v6-0002-Bump-postmaster-soft-open-file-limit-RLIMIT_NOFIL.patch +
>   v6-0003-Reflect-the-value-of-max_safe_fds-in-max_files_pe.patch but we don't
>   mutate max_files_per_process.  Benchmark results should not change beyond
>   the inter-major-version noise level unless one sets io_method=io_uring.  I'm
>   feeling best about this one, but I've not been thinking about it long.

Yea, I think that's something probably worth doing separately from Jelte's
patch.  I do think that it'd be rather helpful to have jelte's patch to
increase NOFILE in addition though.


> > > > +static void
> > > > +maybe_adjust_io_workers(void)
> > >
> > > This also restarts workers that exit, so perhaps name it
> > > start_io_workers_if_missing().
> > 
> > But it also stops IO workers if necessary?
> 
> Good point.  Maybe just add a comment like "start or stop IO workers to close
> the gap between the running count and the configured count intent".

It's now
/*
 * Start or stop IO workers, to close the gap between the number of running
 * workers and the number of configured workers.  Used to respond to change of
 * the io_workers GUC (by increasing and decreasing the number of workers), as
 * well as workers terminating in response to errors (by starting
 * "replacement" workers).
 */


> > > > +{
> > > ...
> > > > +		/* Try to launch one. */
> > > > +		child = StartChildProcess(B_IO_WORKER);
> > > > +		if (child != NULL)
> > > > +		{
> > > > +			io_worker_children[id] = child;
> > > > +			++io_worker_count;
> > > > +		}
> > > > +		else
> > > > +			break;				/* XXX try again soon? */
> > >
> > > Can LaunchMissingBackgroundProcesses() become the sole caller of this
> > > function, replacing the current mix of callers?  That would be more conducive
> > > to promptly doing the right thing after launch failure.
> > 
> > I'm not sure that'd be a good idea - right now IO workers are started before
> > the startup process, as the startup process might need to perform IO. If we
> > started it only later in ServerLoop() we'd potentially do a fair bit of work,
> > including starting checkpointer, bgwriter, bgworkers before we started IO
> > workers.  That shouldn't actively break anything, but it would likely make
> > things slower.
> 
> I missed that.  How about keeping the two calls associated with PM_STARTUP but
> replacing the assign_io_workers() and process_pm_child_exit() calls with one
> in LaunchMissingBackgroundProcesses()?

I think replacing the call in assign_io_workers() is a good idea, that way we
don't need assign_io_workers().

Less convinced it's a good idea to do the same for process_pm_child_exit() -
if IO workers errored out we'll launch backends etc before we get to
LaunchMissingBackgroundProcesses(). That's not a fundamental problem, but
seems a bit odd.

I think LaunchMissingBackgroundProcesses() should be split into one that
starts aux processes and one that starts bgworkers. The one maintaining aux
processes should be called before we start backends, the latter not.


> In the event of a launch failure, I think that would retry the launch
> quickly, as opposed to maybe-never.

That's a fair point.


> > > > +			/*
> > > > +			 * It's very unlikely, but possible, that reopen fails. E.g. due
> > > > +			 * to memory allocations failing or file permissions changing or
> > > > +			 * such.  In that case we need to fail the IO.
> > > > +			 *
> > > > +			 * There's not really a good errno we can report here.
> > > > +			 */
> > > > +			error_errno = ENOENT;
> > >
> > > Agreed there's not a good errno, but let's use a fake errno that we're mighty
> > > unlikely to confuse with an actual case of libc returning that errno.  Like
> > > one of EBADF or EOWNERDEAD.
> > 
> > Can we rely on that to be present on all platforms, including windows?
> 
> I expect EBADF is universal.  EBADF would be fine.

Hm, that's actually an error that could happen for other reasons, and IMO
would be more confusing than ENOENT. The latter describes the issue to a
reasonable extent, EBADFD seems like it would be more confusing.

I'm not sure it's worth investing time in this - it really shouldn't happen,
and we probably have bigger problems than the error code if it does. But if we
do want to do something, I think I can see a way to report a dedicated error
message for this.


> EOWNERDEAD is from 2006.
> https://learn.microsoft.com/en-us/cpp/c-runtime-library/errno-constants?view=msvc-140
> says VS2015 had EOWNERDEAD (the page doesn't have links for older Visual
> Studio versions, so I consider them unknown).

Oh, that's a larger list than I'd have though.


> https://github.com/coreutils/gnulib/blob/master/doc/posix-headers/errno.texi
> lists some OSs not having it, the newest of which looks like NetBSD 9.3
> (2022).  We could use it and add a #define for platforms lacking it.

What would we define it as?  I guess we could just pick a high value, but...


Greetings,

Andres Freund