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Re: AIO writes vs hint bits vs checksums

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: Andres Freund <andres@anarazel.de>
Cc: pgsql-hackers@postgresql.org, Noah Misch <noah@leadboat.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Thomas Munro <thomas.munro@gmail.com>
Date: 2024-09-25T08:12:27Z
Lists: pgsql-hackers 
Antonin Houska <ah@cybertec.at> wrote:

> Andres Freund <andres@anarazel.de> wrote:
> 
> > What I'd instead like to propose is to implement the right to set hint bits as
> > a bit in each buffer's state, similar to BM_IO_IN_PROGRESS. Tentatively I
> > named this BM_SETTING_HINTS. It's only allowed to set BM_SETTING_HINTS when
> > BM_IO_IN_PROGRESS isn't already set and StartBufferIO has to wait for
> > BM_SETTING_HINTS to be unset to start IO.
> > 
> > Naively implementing this, by acquiring and releasing the permission to set
> > hint bits in SetHintBits() unfortunately leads to a significant performance
> > regression. While the performance is unaffected for OLTPish workloads like
> > pgbench (both read and write), sequential scans of unhinted tables regress
> > significantly, due to the per-tuple lock acquisition this would imply.
> 
> An alternative approach: introduce a flag that tells that the checksum is
> being computed, and disallow setting hint bits when that flag is set. As long
> as the checksum computation takes take much less time than the IO, fewer hint
> bit updates should be rejected.

Well, the checksum actually should not be computed during the IO, so the IO
would still disallow hint bit updates :-(

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Require share-exclusive lock to set hint bits and to flush

  2. heapam: Move logic to handle HEAP_MOVED into a helper function

  3. Add very basic test for kill_prior_tuples

  4. aio: Add README.md explaining higher level design

  5. heapam: Only set tuple's block once per page in pagemode

  6. bufmgr: Use AIO in StartReadBuffers()

  7. bufmgr: Implement AIO read support

  8. aio: Implement support for reads in smgr/md/fd

  9. aio: Add io_method=io_uring

  10. aio: Add io_method=worker

  11. aio: Infrastructure for io_method=worker

  12. aio: Add core asynchronous I/O infrastructure

  13. bufmgr: Make it easier to change number of buffer state bits