[Beowulf] AMD and AVX512
joe.landman at gmail.com
Mon Jun 21 13:46:30 UTC 2021
On 6/21/21 9:20 AM, Jonathan Engwall wrote:
> I have followed this thinking "square peg, round hole."
> You have got it again, Joe. Compilers are your problem.
Erp ... did I mess up again?
System architecture has been a problem ... making a processing unit
10-100x as fast as its support components means you have to code with
that in mind. A simple `gfortran -O3 mycode.f` won't necessarily
generate optimal code for the system ( but I swear ... -O3 ... it says
it on the package!)
Way back at Scalable, our secret sauce was largely increasing IO
bandwidth and lowering IO latency while coupling computing more tightly
to this massive IO/network pipe set, combined with intelligence in the
kernel on how to better use the resources. It was simply a better
architecture. We used the same CPUs. We simply exploited the design
End result was codes that ran on our systems with off-cpu work (storage,
networking, etc.) could push our systems far harder than competitors.
And you didn't have to use a different ISA to get these benefits. No
recompilation needed, though we did show the folks who were interested,
how to get even better performance.
Architecture matters, as does implementation of that architecture.
There are costs to every decision within an architecture. For AVX512,
along comes lots of other baggage associated with downclocking, etc.
You have to do a cost-benefit analysis on whether or not it is worth
paying for that baggage, with the benefits you get from doing so. Some
folks have made that decision towards AVX512, and have been enjoying the
benefits of doing so (e.g. willing to pay the costs). For the general
audience, these costs represent a (significant) hurdle one must overcome.
Here's where awesome compiler support would help. FWIW, gcc isn't that
great a compiler. Its not performance minded for HPC. Its a reasonable
general purpose standards compliant (for some subset of standards)
compilation system. LLVM is IMO a better compiler system, and its
clang/flang are developing nicely, albeit still not really HPC focused.
Then you have variants built on that. Like the Cray compiler, Nvidia
compiler and AMD compiler. These are HPC focused, and actually do quite
well with some codes (though the AMD version lags the Cray and Nvidia
compilers). You've got the Intel compiler, which would be a good general
compiler if it wasn't more of a marketing vehicle for Intel processors
and their features (hey you got an AMD chip? you will take the slowest
code path even if you support the features needed for the high
performance code path).
Maybe, someday, we'll get a great HPC compiler for C/Fortran.
e: joe.landman at gmail.com
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Beowulf