Re: Improving spin-lock implementation on ARM.

Krunal Bauskar <krunalbauskar(at)gmail(dot)com> writes:
> On Mon, 30 Nov 2020 at 10:14, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> The results I posted at [1] seem to contradict this for Apple's new
>> machines.

> For the results you saw on Mac-Mini was LSE enabled by default.

Hmm, I don't know how to get Apple's clang to admit what its default
settings are ... anybody?

However, it does accept "-march=armv8-a+lse", and that seems to
not be the default, because I get different results from my spinlock-
pounding test than I did yesterday. Abbreviating into a table:

--- CFLAGS=-O2 --- --- CFLAGS="-O2 -march=armv8-a+lse" ---

TPS HEAD CAS patch HEAD CAS patch

clients=1 2127 2174 2612 2722
clients=2 1816 859 892 950
clients=4 714 519 610 468
clients=8 - - 108 185

Unfortunately, that still doesn't lead me to think that either LSE
or CAS are net wins on this hardware. It's quite clear that LSE
makes the uncontended case a good bit faster, but the contended case
is a lot worse, so is that really a tradeoff we want?

> * I would also suggest if possible try with higher scalability (more than 4
> to check if with increase scalability CAS out-perform).

As I said yesterday, running more than 4 processes is just going
to bring the low-performance cores into the equation, which is likely
to swamp any interesting comparison. I did run the test with "-c 8"
today, as shown in the right-hand columns, and the results seem
to bear that out.

regards, tom lane