os that rather uses the gpu?

Robert Myers rbmyersusa at gmail.com
Fri Jul 16 23:49:34 UTC 2010

On Fri, Jul 16, 2010 at 6:36 PM, Christofer C. Bell <
christofer.c.bell at gmail.com> wrote:

> On Thu, Jul 15, 2010 at 10:56 PM, Robert Myers <rbmyersusa at gmail.com>wrote:
>> This is a technical forum, not a debating society.  Science is awash
>> already in useless flops.
> You're showing your ignorance, Robert.  FLOPS is a very useful measure and
> more is always good, but I don't think you know what it means.  It means
> FLoating Point Operations Per Second.  It's nothing more than a measure of
> computational speed and says nothing about the accuracy of the models being
> computed.
>> In one of these long-running public debates, I pointed out to a national
>> decision-maker how relatively meaningless and frequently wrong NOAA's
>> hurricane season predictions have been.  Guess what?  The most recent
>> forecast was wrapped in all kinds of weasel words.  That won't stop them
>> from drawing their paychecks and burning megawatts producing useless
>> pictures.
> If the forecast is wrong, the either the model was inaccurate or the input
> data was wrong.  It says absolutely nothing about the value of computing the
> answer fast (what FLOPS provides).  While I totally get you're going to
> somehow try to refute the idea that "having a fast computer is good" and
> thus, I can only imagine, make the case that "a slow computer is even
> better," you're not going to convince anyone.

Whether you like it or not, and whether, even more important, you understand
it or not, the architecture of a computer has a big impact on the kinds of
models you can implement with any degree of efficiency on the computer.
 Hanging lots of flops on inadequate bandwidth is popular because you can
get a high ranking on the Top 500 list with a minimal expenditure, but it
also means that you can't do an important calculation like a global FFT at
anything like reasonable efficiency.  Flops are almost free.  Bandwidth is
expensive.  Unfortunately, what is almost free (flops) can be nearly useless
for, say, an FFT unless you have the bandwidth to move data around globally
fast enough.

Skimping on global bandwidth so that you can't do global operations
effectively forces you to approximations that require only local
computation, that's the kind of modeling we get, and the fact that there is
an inherent disconnect between the way the equations behave and the way the
discretized model behaves gets swept under the rug.  Nonlinear systems
interact globally at all scales at every time step.  If your discretization
scheme forces an inaccurate representation of that global interaction, then
you shouldn't ever trust what's coming out of your simulation of nonlinear

Attempting to simulate hurricanes is about as nonlinear as it gets.  When
your weatherman on TV knowingly talks about tropical waves as a cause of
concern, he's talking about a large scale flow pattern that might or might
not go through some tortuous nonlinear processes that will turn it into a
hurricane.  Modelling the interaction of hugely different physical scales
accurately is essential to accurate prediction, and modern "supercomputers"
are notably and chronically short of the bandwidth required to do so.

> Anyway, technical forum or not, it's a forum made up of people and that
> makes it a community.  To get along in a community, it helps to not be an
> asshole.  While you may not care about being an asshole, do note that
> "getting along in a community" does influence if anyone takes you
> seriously.  And you seem very much to want to be taken seriously.

Calling other people names helps to build a community?  From your pompous
and condescending tone unbacked by actual knowledge, I'd be inclined to
label you in the same way you have chosen to label me, with the addition of
the word "ignorant" in front of it, but I don't think name-calling and
labeling ever helps.

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