Horton

Re: AIO v2.2

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@postgresql.org
Date: 2025-01-07T20:09:56Z
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 aio: Fix reference to outdated name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

Attachments

On 07/01/2025 18:11, Andres Freund wrote:
> The difference between a handle and a reference is useful right now, to have
> some separation between the functions that can be called by anyone (taking a
> PgAioHandleRef) and only by the issuer (PgAioHandle). That might better be
> solved by having a PgAioHandleIssuerRef ref or something.
> 
> Having PgAioReturn be separate from the AIO handle turns out to be rather
> crucial, otherwise it's very hard to guarantee "forward progress",
> i.e. guarantee that pgaio_io_get() will return something without blocking
> forever.

Right, yeah, I can see that.

>>> typedef enum PgAioHandleState
>>> {
>>> 	/* not in use */
>>> 	AHS_IDLE = 0,
>>>
>>> 	/* returned by pgaio_io_get() */
>>> 	AHS_HANDED_OUT,
>>>
>>> 	/* pgaio_io_start_*() has been called, but IO hasn't been submitted yet */
>>> 	AHS_DEFINED,
>>>
>>> 	/* subject's prepare() callback has been called */
>>> 	AHS_PREPARED,
>>>
>>> 	/* IO has been submitted and is being executed */
>>> 	AHS_IN_FLIGHT,
>>>
>>> 	/* IO finished, but result has not yet been processed */
>>> 	AHS_REAPED,
>>>
>>> 	/* IO completed, shared completion has been called */
>>> 	AHS_COMPLETED_SHARED,
>>>
>>> 	/* IO completed, local completion has been called */
>>> 	AHS_COMPLETED_LOCAL,
>>> } PgAioHandleState;
>>
>> Do we need to distinguish between DEFINED and PREPARED?
> 
> I found it to be rather confusing if it's not possible to tell if some action
> (like the prepare callback) has already happened, or not.  It's useful to be
> able look at an IO in a backtrace or such and see exactly in what state it is
> in.

I see.

> In v1 I had several of the above states managed as separate boolean variables
> - that turned out to be a huge mess, it's a lot easier to understand if
> there's a single strictly monotonically increasing state.

Agreed on that

>> I didn't quite understand the point of the prepare callbacks. For example,
>> when AsyncReadBuffers() calls smgrstartreadv(), the
>> shared_buffer_readv_prepare() callback will be called. Why doesn't
>> AsyncReadBuffers() do the "prepare" work itself directly; why does it need
>> to be in a callback?
> 
> One big part of it is "ownership" - while the IO isn't completely "assembled",
> we can release all buffer pins etc in case of an error. But if the error
> happens just after the IO was staged, we can't - the buffer is still
> referenced by the IO. For that the AIO subystem needs to take its own pins
> etc.  Initially the prepare callback didn't exist, the code in
> AsyncReadBuffers() was a lot more complicated before it.
> 
> 
>> I assume it's somehow related to error handling, but I didn't quite get
>> it. Perhaps an "abort" callback that'd be called on error, instead of a
>> "prepare" callback, would be better?
> 
> I don't think an error callback would be helpful - the whole thing is that we
> basically need claim ownership of all IO related resources IFF the IO is
> staged. Not before (because then the IO not getting staged would mean we have
> a resource leak), not after (because we might error out and thus not keep
> e.g. buffers pinned).

Hmm. The comments say that when you call smgrstartreadv(), the IO handle 
may no longer be modified, as the IO may be executed immediately. What 
if we changed that so that it never submits the IO, only adds the 
necessary callbacks to it?

In that world, when smgrstartreadv() returns, the necessary details and 
completion callbacks have been set in the IO handle, but the caller can 
still do more preparation before the IO is submitted. The caller must 
ensure that it gets submitted, however, so no erroring out in that state.

Currently the call stack looks like this:

AsyncReadBuffers()
-> smgrstartreadv()
   -> mdstartreadv()
     -> FileStartReadV()
       -> pgaio_io_prep_readv()
         -> shared_buffer_readv_prepare() (callback)
         <- (return)
       <- (return)
     <- (return)
   <- (return)
<- (return)

I'm thinking that the prepare work is done "on the way up" instead:

AsyncReadBuffers()
-> smgrstartreadv()
   -> mdstartreadv()
     -> FileStartReadV()
       -> pgaio_io_prep_readv()
       <- (return)
     <- (return)
   <- (return)
-> shared_buffer_readv_prepare()
<- (return)

Attached is a patch to demonstrate concretely what I mean.

This adds a new pgaio_io_stage() step to the issuer, and the issuer 
needs to call the prepare functions explicitly, instead of having them 
as callbacks. Nominally that's more steps, but IMHO it's better to be 
explicit. The same actions were happening previously too, it was just 
hidden in the callback. I updated the README to show that too.

I'm not wedded to this, but it feels a little better to me.

-- 
Heikki Linnakangas
Neon (https://neon.tech)