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

Hi,

On 2025-01-08 12:12:19 -0600, Nathan Bossart wrote:
> 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.

Seems reasonable. It's not surprising that spinning with an atomic operation
doesn't scale very well once a you have more than a handful cores.

Of course the whole way locking works in pg_stat_statements is a scalability
disaster, but that's a bigger project to fix.

Out of curiosity, have you measured whether this has a positive effect without
pg_stat_statements? I think it'll e.g. also affect lwlocks, as they also use
perform_spin_delay().

Greetings,

Andres