Re: AIO writes vs hint bits vs checksums

Antonin Houska <ah(at)cybertec(dot)at> wrote:

> Andres Freund <andres(at)anarazel(dot)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