Re: Making Lemonade (Floating-point format changes)

On Wed, 15 May 2024 20:08:27 -0000 (UTC)
Thomas Koenig <tkoenig@netcologne.de> wrote:

Michael S <already5chosen@yahoo.com> schrieb:
 

IIRC, you reported something like 200 (or 300?) MFLOPS for your
matrix multiplication benchmark running on a single POWER9 core. 

 

Not too bad. Not too good, either.

Just reran the tests, it gave me somewhere around 405-410 MFlops
on a POWER9 machine running at 2.2 GHz (or so /proc/cpuinfo says).
This is with the standard gfortran matmul routine.
 

I don't think that nowadays /proc/cpuinfo has any relationship to
actual frequency. Most likely with a single core active even the
cheapest POWER9 SKU runs at 3.8 GHz.
If there is no ready-made utility, you can measure it by yourself with
latency-bound loop. Just don't forget that on POWER9 all simple integer
opcodes have latency=2.
If there are any difficulties, I can help.

I got ~150 MFLOPS running on EPYC3 at relatively low frequency (3.6
GHz) using my plug-in replacements for gcc __multf3/__addtf3 

 
Scaled to frequency, the hardware implementation on POWER is then
better by a factor of around four.  Not too bad, actually.
>

If my guess about frequency is correct, then more like factor of 2.6.
Of which, factor of approximately 1.3 has to be attributed to bad
libgcc ABI.
[O.T.]
BTW, on ARM64 libgcc ABI for __multf3/__addtf3 is similarly bad. The
only decent ABI for __multf3/__addtf3 that I encountered experimenting
on godbolt was for RV64. But that a little consolation considering huge
performance gap between the best RV64 and not even the best, but just a
competent iAMD64 or ARM64.
[/O.T.]

Anyway, performance per clock is of limited interest. What matters is
absolute performance (sometimes throughput, sometimes latency) and
performance per watt.
I would guess, that using SMT4 POWER9 can get over 80% of theoretical
throughput, but getting here would take either multiplying really big
matrix or lots of medium ones.
On EPYC3, on the other hand, I don't expect measurable SMT gain. But
relatively to POWER9 EPYC3 has more cores and much lower power
consumption per core.

[..]

I just looked it up - on POWER9, xsaddqp has 12 cycles of latency,
with one result per cycle, POWER10 has 12 to 13 cycles with two
results per cycle. 

>
So, a bottleneck is somewhere else. May be, multiplication? 

 
I messed up the name of the instruction. What I meant was xsmaddqp
(just trips off the tounge, doesn't it?), which on POWER9 actually
has a throughput of 1/13 per cycle, a big, fat instruction,
obviously.  On POWER10, this actually got worse, with performance
dropping to 1/18 per cycle, with a latency of 25 cycles.  Hm,
apparently somebody didn't think it was all that important,
apparently :-(

Sounds like that.
Hopefully it's compensated by better power efficiency. And
unfortunately it's aggravated by lower cost-effectiveness. Or, at least
that what was claimed by poster (luke.l ?) here.