| From: | Soumyadeep Chakraborty <soumyadeep2007(at)gmail(dot)com> |
|---|---|
| To: | Thomas Munro <thomas(dot)munro(at)gmail(dot)com> |
| Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>, "k(dot)jamison(at)fujitsu(dot)com" <k(dot)jamison(at)fujitsu(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Ranier Vilela <ranier(dot)vf(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel Seq Scan vs kernel read ahead |
| Date: | 2020-07-22 19:02:38 |
| Message-ID: | CAE-ML+_ibpDEe6eGbp597GeMH-Z_658HW6WmDVH=XjzASQoH7Q@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, Jul 21, 2020 at 9:33 PM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>
> On Wed, Jul 22, 2020 at 3:57 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > Yeah, that is true but every time before the test the same amount of
> > data should be present in shared buffers (or OS cache) if any which
> > will help in getting consistent results. However, it is fine to
> > reboot the machine as well if that is a convenient way.
>
> We really should have an extension (pg_prewarm?) that knows how to
> kick stuff out PostgreSQL's shared buffers and the page cache
> (POSIX_FADV_DONTNEED).
>
>
+1. Clearing the OS page cache on FreeBSD is non-trivial during testing.
You can't do this on FreeBSD:
sync; echo 3 > /proc/sys/vm/drop_caches
Also, it would be nice to evict only those pages from the OS page cache
that are Postgres pages instead of having to drop everything.
Regards,
Soumyadeep (VMware)
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