Horton

Re: AIO v2.0

Andres Freund <andres@anarazel.de>

View thread
From: Andres Freund <andres@anarazel.de>
To: Noah Misch <noah@leadboat.com>
Cc: pgsql-hackers@postgresql.org, Heikki Linnakangas <hlinnaka@iki.fi>, 陈宗志 <baotiao@gmail.com>
Date: 2024-09-30T14:49:17Z
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 aio: Fix reference to outdated name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 2024-09-17 11:08:19 -0700, Noah Misch wrote:
> > - I am worried about the need for bounce buffers for writes of checksummed
> >   buffers. That quickly ends up being a significant chunk of memory,
> >   particularly when using a small shared_buffers with a higher than default
> >   number of connection. I'm currently hacking up a prototype that'd prevent us
> >   from setting hint bits with just a share lock. I'm planning to start a
> >   separate thread about that.
> 
> AioChooseBounceBuffers() limits usage to 256 blocks (2MB) per MaxBackends.
> Doing better is nice, but I don't consider this a blocker.  I recommend
> dealing with the worry by reducing the limit initially (128 blocks?).  Can
> always raise it later.

On storage that has nontrivial latency, like just about all cloud storage,
even 256 will be too low. Particularly for checkpointer.

Assuming 1ms latency - which isn't the high end of cloud storage latency - 256
blocks in flight limits you to <= 256MByte/s, even on storage that can have a
lot more throughput. With 3ms, which isn't uncommon, it's 85MB/s.

Of course this could be addressed by tuning, but it seems like something that
shouldn't need to be tuned by the majority of folks running postgres.


We also discussed the topic at https://postgr.es/m/20240925020022.c5.nmisch%40google.com
> ... neither BM_SETTING_HINTS nor keeping bounce buffers looks like a bad
> decision.  From what I've heard so far of the performance effects, if it were
> me, I would keep the bounce buffers.  I'd pursue BM_SETTING_HINTS and bounce
> buffer removal as a distinct project after the main AIO capability.  Bounce
> buffers have an implementation.  They aren't harming other design decisions.
> The AIO project is big, so I'd want to err on the side of not designating
> other projects as its prerequisites.

Given the issues that modifying pages while in flight causes, not just with PG
level checksums, but also filesystem level checksum, I don't feel like it's a
particularly promising approach.

However, I think this doesn't have to mean that the BM_SETTING_HINTS stuff has
to be completed before we can move forward with AIO. If I split out the write
portion from the read portion a bit further, the main AIO changes and the
shared-buffer read user can be merged before there's a dependency on the hint
bit stuff being done.

Does that seem reasonable?

Greetings,

Andres Freund