Date: Mon, 30 Dec 2024 11:19:47 -0500
From: Bruce Momjian <bruce@momjian.us>
To: Jelte Fennema-Nio <postgres@jeltef.nl>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, torikoshia <torikoshia@oss.nttdata.com>,
	Pgsql Hackers <pgsql-hackers@postgresql.org>
Subject: Re: RFC: Allow EXPLAIN to Output Page Fault Information
Message-ID: <Z3LII9YEgp3kAZG-@momjian.us>
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On Fri, Dec 27, 2024 at 03:15:40PM +0100, Jelte Fennema-Nio wrote:
> On Tue Dec 24, 2024 at 4:52 PM CET, Tom Lane wrote:
> > torikoshia <torikoshia@oss.nttdata.com> writes:
> > > I have attached a PoC patch that modifies EXPLAIN to include page
> > > fault information during both the planning and execution phases of a
> > > query.
> > 
> > Surely these numbers would be too unstable to be worth anything.
> 
> What makes you think that? I'd expect them to be similarly stable to the
> numbers we get for BUFFERS. i.e. Sure they won't be completely stable,
> but I expect them to be quite helpful when debugging perf issues,
> because large numbers indicate that the query is disk-bound and small
> numbers indicate that it is not.
> 
> These numbers seem especially useful for setups where shared_buffers is
> significantly smaller than the total memory available to the system. In
> those cases the output from BUFFERS might give the impression that that
> you're disk-bound, but if your working set still fits into OS cache then
> the number of page faults is likely still low. Thus telling you that the
> numbers that you get back from BUFFERS are not as big of a problem as
> they might seem.

I certainly would love to see storage I/O numbers as distinct from
kernel read I/O numbers.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Do not let urgent matters crowd out time for investment in the future.