| 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: | 2025-08-14 19:15:02 |
| Message-ID: | CAH2-WzntgDeopLJpyEbUh23Qr1vgoYv5jbFkYsymTScEKxBj7A@mail.gmail.com |
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On Thu, Aug 14, 2025 at 2:53 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> I think this is just an indicator of being IO bound.
Then why does the exact same pair of runs show "I/O Timings: shared
read=194.629" for the sequential table backwards scan (with total
execution time 1132.360 ms), versus "I/O Timings: shared read=352.88"
(with total execution time 697.681 ms) for the random table backwards
scan?
Obviously it is hard to believe that the query with shared
read=194.629 is one that is naturally much more I/O bound than another
similar query that shows shared read=352.88. What "I/O Timings" shows
more or less makes sense to me already -- it just doesn't begin to
explain why *overall query execution* is much slower when scanning
backwards sequentially.
> I'd see what changes if you temporarily reduce
> /sys/block/nvme6n1/queue/max_sectors_kb to a smaller size.
I reduced max_sectors_kb from 128 to 8. That had no significant effect.
> Could you show iostat for both cases?
iostat has lots of options. Can you be more specific?
--
Peter Geoghegan
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