beowulf in space

Robert G. Brown rgb at phy.duke.edu
Tue Apr 15 12:36:59 EDT 2003


On Tue, 15 Apr 2003, James Cownie wrote:

> 
> >   b) communications latency (bandwidth actually can be as big as you
> >      like or are likely to ever need, since you ARE a satellite, after
> >      all...:-)
> 
> Well, 18 months ago ESA were getting 50Mb optically between satellites 
> 
>   http://www.esa.int/export/esaCP/ESASGBZ84UC_Improving_0.html
> 
> since that is 
> 
> 1) a Moore generation ago :-) (though I think development times are
>    longer in space technology)
> 2) public information
> 
> I expect that the people on the "dark side" who do this can indeed get
> an awful lot of bandwidth...

I think the relevant numbers that indicate the limits of what technology
CAN do from satellites are more likely to come from looking at the
humble satellite dish attached to many homes.  Order of 100 channels in
the television range, some of them HDTV, at a guess order of 100 MB/sec
per second (assume order of a MB/sec per channel).  Or look at phone
satellites.  And that is using only a small part of the spectrum, and
ignores the possibility of multiple channels reusing the same spectrum
with directional links.

I would guess that one could, with some effort downlink some orders of
magnitude more than gigabytes per second, and I meant bytes.  How many
orders (and I meant plural there, too) probably does depend on a lot of
things including distance from earth, ambient atmospheric conditions in
the intervening space between transmitter and receiver, what frequencies
one is using, the number of directional-parallel channels one can
maintain.  A single visible-light laser link, for example, could likely
carry many gigabits per second even allowing for atmospheric distortion.

However, one of the NASA guys on the list probably knows at least the
comsat or tvsat numbers (Jim?).  And as you say, the military probably
has lots of bandwidth down from theirs although how much they aren't
likely to say.  However, they take HIGH resolution pictures in a pretty
much steady stream...

I think we should just accept Jim's statement that near the earth we can
get a "lot" of bandwidth on demand, but that things get dicier for
obvious reasons when you get far away.  Less power, harder to hold a
tight beam, less signal to noise on both ends' receivers, more
retransmissions.  It's pretty astounding that we were able to get the
incredible flyby pictures of Jupiter and the outer planets at all that
we got, given the minute size of the spacecraft and their power
supplies, their extreme distance from the earth, and the decades they
were in space.  Nasa does literally incredible engineering.  Expensive,
sure, but the REALLY expensive missions are the ones where something
breaks and the whole investment (human and otherwise) is wasted.

Let's also not forget who "invented" the beowulf, as well (tip of the
hat to Nasa Goddard, Don and Tom and all the rest:-).  I'm quite certain
that they use beowulfish concepts all the time in their engineering, and
Jim did an excellent job of indicating some of the reasons why.  This
isn't even inconsistent with the original beowulf idea -- sure, one
would be silly to throw a general purpose cluster up into space to do
e.g. my computations, but real optimizing beowulf engineering matches
the design to the task.  Of course they're going to engineer a "cluster"
that matches their precise needs and specifications.  It just isn't
going to be doing work "for earth" -- it will be doing signal processing
and so forth, and even there only when the economics of the available
data bandwidth and/or robust engineering requirements dictate.

If I were engineering a space vehicle, I'd make even the onboard
navigation computer redundant.  This might be more of a "high
availability" model than high performance, but an ideal design might mix
both.  Lots of processors reduces the time required for a
parallelizeable navigation computation AND can make the computer more
robust against the failure of one or more processors -- as long as you
have at least one left, you can complete key computations, just more
slowly.  Heck, from one point of view every compute node is already a
specialized "parallel cluster" -- the system has a CPU and a variety of
bridges, dedicated special purpose processors, and so forth all on
board, so how could NASA NOT make "parallel" environments for
spacecraft.

The one thing they won't do is use off-the-shelf parts, and I can't
blame them.  The damn things break all the time here on earth.  I'm
typing away on a computer that has a dying hard drive, while waiting for
it to rsync a final time with my fingers crossed.  For me it is no
biggie.  A trip to Intrex, a hundred bucks (or more likely warranty
replacement).  In space that's kinda hard.

SO although I'm certain that they use clusters on spacecraft in at least
one sense of the word, I'm equally certain that they are NOT beowulfs,
according to the standard definition.

   rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu



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