Re: Remove Instruction Synchronization Barrier in spin_delay() for ARM64 architecture

On Mon, Aug 11, 2025 at 04:12:48PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart@gmail.com> writes:
>> On Thu, Jun 19, 2025 at 12:10:52PM -0700, Salvatore Dipietro wrote:
>>> We can notice that with low concurrency (1,2,4) results are similar
>>> while with medium concurrency (8,16)
>>> the No-ISB approach can introduce some regression especially on
>>> smaller instances. However, we can see some significant
>>> positive performance impact with high concurrency (>=32) settings on
>>> large instances (up to 8.76x on m7g.16xl with 256 concurrency).
> 
>> Given these mixed results, it's unclear to me how exactly we should
>> proceed.  Perhaps there is another approach that reduces the regressions to
>> a negligible level while still producing gains at higher levels of
>> concurrency.  Or maybe we can convince ourselves that these regressions
>> aren't worth worrying about, but that seems like a bit of a stretch to me.
> 
> I agree; I'm also worried that this may be optimizing for one
> particular ARM implementation and have negative effects on others.

Based on this discussion, I am marking the patch as Returned with Feedback.

-- 
nathan