Horton

Re: AIO v2.0

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andres Freund <andres@anarazel.de>, pgsql-hackers@postgresql.org
Date: 2024-09-02T10:03:07Z
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 aio: Fix reference to outdated name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 01/09/2024 09:27, Andres Freund wrote:
> The main reason I had previously implemented WAL AIO etc was to know the
> design implications - but now that they're somewhat understood, I'm planning
> to keep the patchset much smaller, with the goal of making it upstreamable.

+1 on that approach.

> To solve the issue with an unbounded number of AIO references there are few
> changes compared to the prior approach:
> 
> 1) Only one AIO handle can be "handed out" to a backend, without being
>     defined. Previously the process of getting an AIO handle wasn't super
>     lightweight, which made it appealing to cache AIO handles - which was one
>     part of the problem for running out of AIO handles.
> 
> 2) Nothing in a backend can force a "defined" AIO handle (i.e. one that is a
>     valid operation) to stay around, it's always possible to execute the AIO
>     operation and then reuse the handle.  This provides a forward guarantee, by
>     ensuring that completing AIOs can free up handles (previously they couldn't
>     be reused until the backend local reference was released).
> 
> 3) Callbacks on AIOs are not allowed to error out anymore, unless it's ok to
>     take the server down.
> 
> 4) Obviously some code needs to know the result of AIO operation and be able
>     to error out. To allow for that the issuer of an AIO can provide a pointer
>     to local memory that'll receive the result of an AIO, including details
>     about what kind of errors occurred (possible errors are e.g. a read failing
>     or a buffer's checksum validation failing).
> 
> 
> In the next few days I'll add a bunch more documentation and comments as well
> as some better perf numbers (assuming my workstation survived...).

Yeah, a high-level README would be nice. Without that, it's hard to 
follow what "handed out" and "defined" above means for example.

A few quick comments the patches:

v2.0-0001-bufmgr-Return-early-in-ScheduleBufferTagForWrit.patch

+1, this seems ready to be committed right away.

v2.0-0002-Allow-lwlocks-to-be-unowned.patch

With LOCK_DEBUG, LWLock->owner will point to the backend that acquired 
the lock, but it doesn't own it anymore. That's reasonable, but maybe 
add a boolean to the LWLock to mark whether the lock is currently owned 
or not.

The LWLockReleaseOwnership() name is a bit confusing together with 
LWLockReleaseUnowned() and LWLockrelease(). From the names, you might 
think that they all release the lock, but LWLockReleaseOwnership() just 
disassociates it from the current process. Rename it to LWLockDisown() 
perhaps.

v2.0-0003-Use-aux-process-resource-owner-in-walsender.patch

+1. The old comment "We don't currently need any ResourceOwner in a 
walsender process" was a bit misleading, because the walsender did 
create the short-lived "base backup" resource owner, so it's nice to get 
that fixed.

v2.0-0008-aio-Skeleton-IO-worker-infrastructure.patch

My refactoring around postmaster.c child process handling will conflict 
with this [1]. Not in any fundamental way, but can I ask you to review 
those patch, please? After those patches, AIO workers should also have 
PMChild slots (formerly known as Backend structs).

[1] 
https://www.postgresql.org/message-id/a102f15f-eac4-4ff2-af02-f9ff209ec66f@iki.fi

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