| From: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Tomas Vondra <tomas(at)vondra(dot)me>, Peter Geoghegan <pg(at)bowt(dot)ie>, Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Georgios <gkokolatos(at)protonmail(dot)com>, Konstantin Knizhnik <knizhnik(at)garret(dot)ru>, Dilip Kumar <dilipbalaut(at)gmail(dot)com> |
| Subject: | Re: index prefetching |
| Date: | 2025-08-28 21:50:57 |
| Message-ID: | CA+hUKGKL3MRvEftAE+kwBuL2PLg2CwUoHEMr=-KSvsWN8pHq9w@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Fri, Aug 29, 2025 at 7:52 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2025-08-28 19:08:40 +0200, Tomas Vondra wrote:
> > From the 2x regression (compared to master) it might seem like that, but
> > even with the increased distance it's still slower than master (by 25%). So
> > maybe the "error" is to use AIO in these cases, instead of just switching to
> > I/O done by the backend.
>
> If it's slower at a higher distance, we're missing something.
Enough io_workers? What kind of I/O concurrency does it want? Does
wait_event show any backends doing synchronous IO? How many does [1]
want to run for that test workload and does it help?
FWIW there's a very simple canned latency test in a SQL function in
the first message in that thread (0005-XXX-read_buffer_loop.patch),
just on the off-chance that it's useful as a starting point for other
ideas. There I was interested in IPC overheads, latch collapsing and
other effects, so I was deliberately stalling on/evicting a single
block repeatedly without any readahead distance, so I wasn't letting
the stream "hide" IPC overheads.
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