| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Tomas Vondra <tomas(at)vondra(dot)me>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, 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: | 2025-08-29 01:52:48 |
| Message-ID: | CAH2-WzmCirQup_t-wCn72izH_B+koHku0pxYtNunWjdc_Ommow@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Thu, Aug 28, 2025 at 9:10 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> Same. Tomas, could you share what you applied?
Tomas posted a self-contained patch to the list about an hour ago?
> > I'm not sure that Thomas'/your patch to ameliorate the problem on the
> > read stream side is essential here. Perhaps Andres can just take a
> > look at the test case + feature branch, without the extra patches.
> > That way he'll be able to see whatever the immediate problem is, which
> > might be all we need.
>
> It seems caused to a significant degree by waiting at low queue depths. If I
> comment out the stream->distance-- in read_stream_start_pending_read() the
> regression is reduced greatly.
IIUC, that is very roughly equivalent to what the patch actually does.
The fastest configuration of all, independent of io_method, is
"enable_indexscan_prefetch=off". So it's hard to believe that the true
underlying problem is low queue depth. Though I certainly don't doubt
that higher queue depths will help *when io_method=worker*.
--
Peter Geoghegan
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