Re: use a non-locking initial test in TAS_SPIN on AArch64

On Wed, Jan 08, 2025 at 06:07:44PM -0500, Tom Lane wrote:
> 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?

This was on a c8g.24xlarge (Neoverse-V2, Armv9.0-a) [0].

> 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.

Great.  This matches what I saw on an M3.

[0] https://github.com/aws/aws-graviton-getting-started/blob/main/README.md#building-for-graviton

-- 
nathan