Horton

Re: AIO v2.3

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@postgresql.org, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Noah Misch <noah@leadboat.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>
Date: 2025-02-10T22:48:38Z
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 aio: Fix reference to outdated name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 Thu, Jan 23, 2025 at 5:29 PM Andres Freund <andres@anarazel.de> wrote:
+/* caller will issue more io, don't submit */
+#define READ_BUFFERS_MORE_MORE_MORE (1 << 3)

> - Heikki doesn't love pgaio_submit_staged(), suggested pgaio_kick_staged() or
>  such. I don't love that name though.

Problem statement: You want to be able to batch I/O submission, ie
make a single call to ioring_enter() (and other mechanisms) to start
several I/Os, but the code that submits is inside StartReadBuffers()
and the code that knows how many I/Os it wants to start now is at a
higher level, read_stream.c and in future elsewhere.  So you invented
this flag to tell StartReadBuffers() not to call
pgaio_submit_staged(), because you promise to do it later, via this
staging list.  Additionally, there is a kind of programming rule here
that you *must* submit I/Os that you stage, you aren't allowed to (for
example) stage I/Os and then sleep, so it has to be a fairly tight
piece of code.

Would the API be better like this?:  When you want to create a batch
of I/Os submitted together, you wrap the work in pgaio_begin_batch()
and pgaio_submit_batch(), eg the loop in read_stream_lookahead().
Then bufmgr wouldn't need this flag: when it (or anything else) calls
smgrstartreadv(), if there is not currently an explicit batch then it
would be submitted immediately, and otherwise it would only be staged.
This way, batch construction (or whatever word you prefer for batch)
is in a clearly and explicitly demarcated stretch of code in one
lexical scope (though its effect is dynamically scoped just like the
staging list itself because we don't want to pass explicit I/O
contexts through the layers), but code that doesn't call those and
reaches AsyncReadBuffer() or whatever gets an implicit batch of size
one and that's also OK.  Not sure what semantics nesting would have
but I doubt it matters much.

> Things that need to be fixed / are fixed in this:
> - max pinned buffers should be limited by io_combine_limit, not * 4
> - overflow distance
> - pins need to be limited in more places

I have patches for these and a few more things and will post in a
separate thread shortly because they can be understood without
reference to this AIO stuff and that'll hopefully be more digestible.

+    /*
+     * In some rare-ish cases one operation causes multiple reads (e.g. if a
+     * buffer was concurrently read by another backend). It'd be much better
+     * if we ensured that each ReadBuffersOperation covered only one IO - but
+     * that's not entirely trivial, due to having pinned victim buffers before
+     * starting IOs.
+     *
+     * TODO: Change the API of StartReadBuffers() to ensure we only ever need
+     * one IO.

Likewise.

+    /* IO finished, but result has not yet been processed */
+    PGAIO_HS_COMPLETED_IO,
+
+    /* IO completed, shared completion has been called */
+    PGAIO_HS_COMPLETED_SHARED,
+
+    /* IO completed, local completion has been called */
+    PGAIO_HS_COMPLETED_LOCAL,

(Repeating something I mentioned in off-list bikeshedding)  I wondered
if it might be clearer to use the terminology "terminated" for the
work that PostgreSQL has to do after an I/O completes, instead of
overloading/subdividing the term "completed".  We already "terminate"
an I/O when smgr I/O completes in pre-existing bufmgr terminology, and
this feels like a sort of generalisation of that notion.  In this AIO
world, some work is done by the backend that receives the completion
notification from the kernel, and some is done by the backend that
submitted the I/O in the first place, a division that doesn't exist
with simple synchronous system calls.  I wonder if it would be clearer
to use terms based on those two roles, rather than "shared" and
"local", leading to something like:

PGAIO_HS_COMPLETED,
PGAIO_HS_TERMINATED_BY_COMPLETER,
PGAIO_HS_TERMINATED_BY_SUBMITTER,