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

On Tue, Oct 22, 2024 at 02:54:57PM -0500, Nathan Bossart wrote:
> My colleague Salvatore Dipietro (CC'd) sent me a couple of profiles that
> showed an enormous amount of s_lock() time going to the
> __sync_lock_test_and_set() call in the AArch64 implementation of tas().
> Upon closer inspection, I noticed that we don't implement a custom
> TAS_SPIN() for this architecture, so I quickly hacked together the attached
> patch and ran a couple of benchmarks that stressed the spinlock code. I
> found no discussion about TAS_SPIN() on ARM in the archives, but I did
> notice that the initial AArch64 support was added [0] before x86_64 started
> using a non-locking test [1].
>
> These benchmarks are for a c8g.24xlarge running a select-only pgbench with
> 256 clients and pg_stat_statements.track_planning enabled.
>
> without the patch:
>
> [...]
>
> tps = 74135.100891 (without initial connection time)
>
> with the patch:
>
> [...]
>
> tps = 549462.785554 (without initial connection time)

Are there any objections to proceeding with this change? So far, it's been
tested on a c8g.24xlarge and an Apple M3 (which seems to be too small to
show any effect). If anyone has access to a larger ARM machine, additional
testing would be greatly appreciated. I think it would be unfortunate if
this slipped to v19.

--
nathan