Horton

Re: AIO v2.0

Ants Aasma <ants.aasma@cybertec.at>

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From: Ants Aasma <ants.aasma@cybertec.at>
To: Andres Freund <andres@anarazel.de>
Cc: Jakub Wartak <jakub.wartak@enterprisedb.com>, Thomas Munro <thomas.munro@gmail.com>, pgsql-hackers@postgresql.org, Heikki Linnakangas <hlinnaka@iki.fi>, 陈宗志 <baotiao@gmail.com>
Date: 2025-01-10T10:33:39Z
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 aio: Fix reference to outdated name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, 9 Jan 2025 at 22:53, Andres Freund <andres@anarazel.de> wrote:

<Edited to highlight interesting numbers>

> Workstation w/ 2x Xeon Gold 6442Y:
>
>            march                     mem        result
>           native                     100        246.13766ms @ 33.282 GB/s
>           native                  100000        456.08080ms @ 17.962 GB/s
>
> Zen 4 laptop (AMD Ryzen 7 PRO 7840U):
>            march                     mem        result
>           native                     100        95.15734ms @ 86.089 GB/s
>           native                  100000        187.68125ms @ 43.648 GB/s
>
> Workstation w/ 2x Xeon Gold 5215:
>            march                     mem        result
>           native                     100        283.48674ms @ 28.897 GB/s
>           native                  100000        689.78023ms @ 11.876 GB/s
>
> That's quite the drastic difference between amd and intel. Of course it's also
> comparing a multi-core server uarch (lower per-core bandwidth, much higher
> aggregate bandwidth) with a client uarch.

In hindsight building the hash around mulld primitive was a bad decision
because Intel for whatever reason decided to kill the performance of it:

vpmulld          latency throughput
                         (values/cycle)
Sandy Bridge     5       4
Alder Lake      10       8
Zen 4            3       16
Zen 5            3       32

Most top performing hashes these days seem to be built around AES
instructions.

But I was curious why there is such a difference in streaming results.
Turns out that for whatever reason one core gets access to much less
bandwidth on Intel than on AMD. [1]

This made me take another look at your previous prewarm numbers. It looks
like performance is suspiciously proportional to the number of copies of
data the CPU has to make:

config                        checksums   time in ms   number of copies
buffered io_engine=io_uring   0           3883.127     2
buffered io_engine=io_uring   1           5880.892     3
direct io_engine=io_uring     0           2067.142     1
direct io_engine=io_uring     1           3835.968     2

To me that feels like there is a bandwidth bottleneck in this workload and
checksumming the page when the contents is not looked at just adds to
consumed bandwidth, bringing down the performance correspondingly.

This doesn't explain why you observed slowdown in the select case, but I
think that might be due to the per-core bandwidth limitation. Both cases
might pull in the same amount of data into the cache, but without checksums
it is spread out over a longer time allowing other work to happen
concurrently.

[1] https://chipsandcheese.com/p/a-peek-at-sapphire-rapids#%C2%A7bandwidth


> The difference between the baseline CPU target and a more modern profile is
> also rather impressive.  Looks like some cpu-capability based dispatch would
> likely be worth it, even if it didn't matter in my case due to -march=native.

Yes, along with using function attributes for the optimization flags to avoid
the build system hacks.

--
Ants