Re: Improving spin-lock implementation on ARM.

On Tue, Dec 1, 2020 at 9:01 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I did what I could in this department. It's late and I'm not going to
> have time to run read/write benchmarks before bed, but here are some
> results for the "pgbench -S" cases. I tried to match your testing
> choices, but could not entirely:
>
> * Configure options are --enable-debug, --disable-cassert, no other
> special configure options or CFLAG choices.
>
> * I have not been able to find a way to make Apple's compiler not
> use the LSE spinlock instructions, so all of these correspond to
> your LSE cases.
>
> * I used shared_buffers = 1GB, because this machine only has 16GB
> RAM so 32GB is clearly out of reach. Also I used pgbench scale
> factor 100 not 1000. Since we're trying to measure contention
> effects not I/O speed, I don't think a huge test case is appropriate.
>
> * I still haven't gotten pgbench to work with -j settings above 128,
> so these runs use -j equal to half -c. Shouldn't really affect
> conclusions about scaling. (BTW, I see a similar limitation on
> macOS Catalina x86_64, so whatever that is, it's not new.)
>
> * Otherwise, the first plot shows median of three results from
> "pgbench -S -M prepared -T 120 -c $n -j $j", as you had it.
> The right-hand plot shows all three of the values in yerrorbars
> format, just to give a sense of the noise level.
>
> Clearly, there is something weird going on at -c 4. There's a cluster
> of results around 180K TPS, and another cluster around 210-220K TPS,
> and nothing in between. I suspect that the scheduler is doing
> something bogus with sometimes putting pgbench onto the slow cores.
> Anyway, I believe that the apparent gap between HEAD and the other
> curves at -c 4 is probably an artifact: HEAD had two 180K-ish results
> and one 220K-ish result, while the other curves had the reverse, so
> the medians are different but there's probably not any non-chance
> effect there.
>
> Bottom line is that these patches don't appear to do much of
> anything on M1, as you surmised.

Great, thank you for your research!

------
Regards,
Alexander Korotkov