[Beowulf] Why one might want a bunch o' processors under your desk.
James.P.Lux at jpl.nasa.gov
Fri May 6 17:49:25 PDT 2005
Today I was running a lot of antenna models, using a method of moments code
called NEC4 (in FORTRAN).
Just to describe the computational task for context:
The antenna I am modeling is 9 patches, in a square grid, the middle one of
which is excited.
Basically, it breaks the antenna into a whole bunch of segments (1981 of
them in my model), calculates the interactions between them (i.e. if you
have a voltage in segment i, what current does that induce in segment j)
making a square matrix some 2000x2000. It then solves for the currents
given an excitation, giving you a vector of some 1981 currents. The sum of
the radiated fields is calculated at points covering a hemisphere (8326 of
them in my case).
The process is, as one might imagine, highly compute intensive (i.e.
there's not much disk access going on, after the 10 line file describing
the geometry has been read in). The matrix math routines have been (I've
been told) highly optimized for the Pentium architecture.
On a P4 1.7GHz, it uses 64MB of RAM, and takes about 21 seconds to fill the
matrix (that's the calculating the interactions part), and about 49 seconds
to calculate the currents. Calculating the actual far field pattern takes
about 126 seconds. (Interestingly, I ran it on my new HP Tablet too, which
has a Pentium M 1GHz(nom) processor.. the first two steps were about the
same speed (slightly faster), but the last was much slower, and the fan was
merrily spinning... I suspect it slowed down because it got too hot (speed
reported as 590 MHz, not 1000 MHz)
OK.. both of these are Windows (2000 or XP), but run times on comparable
Linux systems are about the same (not much OS activity going on in either
In any case, this is a long enough run time (3.5 minutes) that it's not
interactive. It's definitely a "start it and go get coffee down the hall"
This is for one frequency. Now, say I wanted to run the model for, say,
100 different frequencies (which I do). We're looking at 350 minutes, or
the better part of a day.
Or, more importantly, I want to assess the effects of small changes in the
orientation and position of the elements (what are my construction
tolerances?). Maybe a Monte Carlo analysis, changing parameters using some
random numbers. Can I arrange them differently and get more tolerance?
In any case, this is a problem ripe for parallelizing. One could
parallelize the pattern computation (i.e. calculate the matrix once and
split the 8000 points among multiple processors). Or, for the parametric
studies, calculate different frequencies (which requires redoing the
matrix, since everything is wavelength dependent) on different processors.
It's a design problem ripe for interaction too. There's a lot of
parameters I can change (size and shape of the patches, segmentation,
spacing, etc.), so running a "try all possible values of all variables
overnight" strategy won't work. Equally poor would be a "submit massive
batch job to the JPL DELL 1024 processor cluster", mostly because the
design space probably spans several thousand parameter combinations. I
want to try a few things, then try some more, and use my experience to
guide the process, not depend on a optimizing program, for which I'd have
to come up with a goal function that is sort of ill-defined.
Oh yeah, what I REALLY want to do is simulate an antenna with several
hundred patches, not just 9.
What I DON'T want to do is rewrite (or even recompile) the antenna modeling
code. It works, it's been validated, it's been optimized (to a certain
extent), and besides, my job is to use the code, not to rewrite it for
And yes, there are approximations, better modeling codes, etc.
available. But again, I'd like to avoid having to track them down,
validate them, and so forth. I want to run my tried and true (but slow)
I suspect that I am not alone. There are probably hundreds of people who
have similar kinds of problems, and would be well served by a desktop or
James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
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