[Beowulf] Working for DUG, new thead
pbisbal at pppl.gov
Tue Jun 19 13:59:57 PDT 2018
On 06/19/2018 03:10 PM, Joe Landman wrote:
> On 6/19/18 2:47 PM, Prentice Bisbal wrote:
>> On 06/13/2018 10:32 PM, Joe Landman wrote:
>>> I'm curious about your next gen plans, given Phi's roadmap.
>>> On 6/13/18 9:17 PM, Stu Midgley wrote:
>>>> low level HPC means... lots of things. BUT we are a huge Xeon Phi
>>>> shop and need low-level programmers ie. avx512, careful
>>>> cache/memory management (NOT openmp/compiler vectorisation etc).
>>> I played around with avx512 in my rzf code.
>>> https://github.com/joelandman/rzf/blob/master/avx2/rzf_avx512.c .
>>> Never really spent a great deal of time on it, other than noting
>>> that using avx512 seemed to downclock the core a bit on Skylake.
>> If you organize your code correctly, and call the compiler with the
>> right optimization flags, shouldn't the compiler automatically handle
>> a good portion of this 'low-level' stuff?
> I wish it would do it well, but it turns out it doesn't do a good
> job. You have to pay very careful attention to almost all aspects of
> making it simple for the compiler, and then constraining the
> directions it takes with code gen.
> I explored this with my RZF stuff. It turns out that with -O3, gcc
> (5.x and 6.x) would convert a library call for the power function into
> an FP instruction. But it would use 1/8 - 1/4 of the XMM/YMM register
> width, not automatically unroll loops, or leverage the vector nature
> of the problem.
> Basically, not much has changed in 20+ years ... you annotate your
> code with pragmas and similar, or use instruction primitives and give
> up on the optimizer/code generator.
> When it comes down to it, compilers aren't really as smart as many of
> us would like. Converting idiomatic code into efficient assembly
> isn't what they are designed for. Rather correct assembly. Correct
> doesn't mean efficient in many cases, and some of the less obvious
> optimizations that we might think to be beneficial are not taken. We
> can hand modify the code for this, and see if these optimizations are
> beneficial, but the compilers often are not looking at a holistic
>> I understand that hand-coding this stuff usually still give you the
>> best performance (See GotoBLAS/OpenBLAS, for example), but does your
>> average HPC programmer trying to get decent performance need to
>> hand-code that stuff, too?
> Generally, yes. Optimizing serial code for GPUs doesn't work well.
> Rewriting for GPUs (e.g. taking into account the GPU data/compute flow
> architecture) does work well.
Thanks for the reply. This sounds like the perfect opportunity for me to
rant about Intel's marketing for Xeon Phi vs. GPUs. When GPUs took off
and Intel was formulating their answer to GPUs, they kept saying you
wouldn't need to rewrite your code like you need to for GPUs. You could
just recompile and everything would work on the new MIC processors.
Then when Intel's MIC processors finally did come out, guess what? You
*did* have to rewrite your code to get any meaningful increase in
performance. For example, you'd have to make sure your loops were
data-parallel and use OpenMP or TBB, or Cilk Plus or whatever, to really
take advantage of the MIC. This meant you had to rewrite your code, but
Intel did everything they could to avoid admitting you would need to
rewrite your code. Instead, they used the euphemism 'code modernization'
I often wonder if that misleading marketing is one of the reasons why
the Xeon Phi has already been canned. I know a lot of people who were
excited for the Xeon Phi, but I don't know any who ever bought the Xeon
Phis once they came out.
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