Re: AIO v2.5

Hi,

On 2025-03-19 13:20:17 -0400, Melanie Plageman wrote:
> On Tue, Mar 18, 2025 at 4:12 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > Attached is v2.10,
>
> I noticed a few comments could be improved in 0011: bufmgr: Use AIO
> in StartReadBuffers()
> [...]

Yep.

> Above and in AsyncReadBuffers()
>
> * To support retries after short reads, the first operation->nblocks_done is
> * buffers are skipped.
>
> can't quite understand this

Heh, yea, it's easy to misunderstand. "short read" in the sense of a partial
read, i.e. a preadv() that only read some of the blocks, not all. I'm
replacing the "short" with partial.

(also removed the superfluous "is")

> * A secondary benefit is that this would allows us to measure the time in
> * pgaio_io_acquire() without causing undue timer overhead in the common,
> * non-blocking, case. However, currently the pgstats infrastructure
> * doesn't really allow that, as it a) asserts that an operation can't
> * have time without operations b) doesn't have an API to report
> * "accumulated" time.
> */
>
> allows->allow
>
> What would the time spent in pgaio_io_acquire() be reported as?

I'd report it as additional time for the IO we're trying to start, as that
wait would otherwise not happen.

> And what is "accumulated" time here? It seems like you just add the time to
> the running total and that is already accumulated.

Afaict there currently is no way to report a time delta to
pgstat. pgstat_count_io_op_time() computes the time since
pgstat_prepare_io_time(). Due to the assertions that time cannot be reported
for an operation with a zero count, we can't just do two
pgstat_prepare_io_time(); ...; pgstat_count_io_op_time();
twice, with the first one passing cnt=0.

Greetings,

Andres Freund