[HPGMG Forum] Do we want the benchmark to go into intrinsics?

Jed Brown jed at jedbrown.org
Tue Apr 29 17:58:02 UTC 2014

Sam Williams <swwilliams at lbl.gov> writes:
> The kiviats were constructed with code as is.  HPCG defenders promise
> better performance with optimization.  

This is largely fantasy because of the memory bandwidth limitations.
This is clear from the performance model.

> Showing HPGMG optimized performance and performance characteristics
> looks better than vanilla code/HPCG doesn't address the argument.  It
> only says optimization is beneficial.

Optimization will inevitably be more beneficial for HPGMG because it
exercises much more than memory bandwidth.  This reflects real apps
(almost all of which benefit from optimization).

> I think the reference implementation should be "good".  I'm much more
> curious as to why xlc fails so badly on HPGMG-FE while it does pretty
> good on HPGMG-FV.

It has some tight local loops with nontrivial indexing.  It is easier
for compilers to optimize for x86 because the chips are so much smarter
(out-of-order, branch prediction, etc), but those compilers have bigger
markets and are tend to be better (not to mention standards compliance
and miscompilation rate).

I know a lot of applications that see 5x lower efficiency when running
on BG/Q than on Edison.
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