From: Tom Lane <tgl@sss.pgh.pa.us>
To: Nathan Bossart <nathandbossart@gmail.com>
cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@postgresql.org,
        dipiets@amazon.com
Subject: Re: use a non-locking initial test in TAS_SPIN on AArch64
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Comments: In-reply-to Nathan Bossart <nathandbossart@gmail.com>
	message dated "Wed, 08 Jan 2025 16:01:19 -0600"
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Date: Wed, 08 Jan 2025 18:07:44 -0500
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Nathan Bossart <nathandbossart@gmail.com> writes:
> AFAICT TAS_SPIN() is only used for s_lock(), which doesn't appear to be
> used by LWLocks.  But I did retry my test from upthread without
> pg_stat_statements and was surprised to find a reproducible 4-6%
> regression.

On what hardware?

I just spent an hour beating on my M4 Pro (the 14-core variant)
and could not detect any outside-the-noise effect of this patch,
with or without pg_stat_statements loaded.  There does seem to be
a small fraction-of-a-percent-ish benefit.  But the run-to-run
variation with 60-second "pgbench -S" tests is a couple of percent,
so I can't say that that's real.

I do feel pretty sure that the patch doesn't hurt on this
class of hardware.

			regards, tom lane