cooling

Robert G. Brown rgb at phy.duke.edu
Wed Apr 24 09:07:51 EDT 2002


On Tue, 23 Apr 2002, Robert B Heckendorn wrote:

> We are looking at the facilities issues in installing a beowulf on the
> order of 500 nodes.  What facilities is telling us is that it is going
> to almost cost us more to buy the cooling for the machine than to buy
> machine itself.  How are people making the air conditioning for their
> machines affordable?  Have we miscalculated the HVAC loads?  Are we
> being over charged?  

No, this is one of the miracles of modern beowulfery.  Our new facility
in the physics department here is a modest sized room, perhaps 5mx13m.
It has 75 KW of power in umpty 20A and 15A (120VAC) circuits.  It has a
heat exchange unit in one end of the room (unfortunately we were unable
to commandeer the small room next door which would have put it and its
noise out of the space itself) that is about 3mx3mx3m (to the ceiling,
anyway) and that eats an extra half-meter or more on the sides in wasted
space (across from the door, fortunately), making the first 3m+ of the
room unusable for anything but entrance and AC.

The room did require a certain amount of prep -- old floor out, new
floor in, asbestos removal, paint.  It did require fairly extensive
wiring for all of the nodes -- a couple of large power distribution
panels, power poles every couple of meters where they can service
clusters of racks, a nifty thermal kill for the room power (room temp
hits a preset of say, 30-35C and bammo, all nodes are just shut down the
hard way).  It did require a certain number of overhead cable trays and
so forth.  Still, I believe that the AC alone (one capable of removing
75 KW continuously) dominated the cost of the $150K renovation.  It was
so expensive that we had to reall work to convince the University to do
it at all, and share the space with another department to ensure that it
is filled as much as possible.

Right now we are probably balancing along at the point where the number
of nodes in the room equals the cost of renovation -- we probably have
on the order of $150K worth of systems racked up and shelved.  However,
we are also ordering new nodes and upgrades pretty steadily as grants
and so forth come in, and will likely have well over $250K worth of
hardware in the room by the end of the year (which will translate into
order 250 CPUs -- even buying duals, our nodes (without myrinet and with
only some nodes on gigabit ethernet) are costing roughly $1K/cpu in a 2U
dual athlon rackmount configuration.

By the time the room is FULL (or as full as we can get it), probably in
a couple of years, it should have order of 500 cpus (we're highball
estimating 150W per CPU, although we're hoping for an average that is
more like 100W -- high end Athlons draw about 70W loaded all by
themselves, and then there is the rest of the system).  At that point
our node investment will likely exceed our renovation expense by 3 to 1
or better, and of course the value to the University in grant-funded
research enabled by all of those nodes will be higher still -- every
postdoc or faculty person grant-supported by research done with the
cluster will probably net the university $30K or more in indirect costs.

Overall, I therefore think that this is a solid win for the University
and an investment essential to keeping the University current and
competitive in its theoretical physics (and statistics, the group with
whom we share) research.  The University has at this point some two or
three similar facilities in several buildings on campus.  Computer
science has an even (much) larger cluster/server facility that it shares
with e.g. math (which has at least one large cluster doing imaging
research supported by petrochemical companies).  I believe that they are
considering the construction of an even larger centralized facility to
put genomic research and some biomed engineering clusters in.

In a way it this is wistfully interesting.  Old Guys (tm) will remember
well the days of totally centralized compute resources, where huge,
expensive facilities housed rows of e.g. IBM 370s.  There were high
priests who cared for and fed these beasts, acolytes who scurried in and
out, and one prayed to them in the form of Fortran IV card decks with
HASP job control prologue/epilogues and awaited the granting of your
prayers in the form of a green-barred lineprinter output (charged per
page including the bloody header page) placed into the box labelled with
your last name initial.  It was all very solemn, expensive, and
ritualized.

Then first the minicomputer, then the PC, liberated us from all of that.
An IBM PC didn't run as fast as a 370, but time on the 370 was billed at
something like $1/minute of CPU and time on the PC, even at a capital
cost of $5K for the PC itself (yes, they were expensive little pups) was
amortized out over YEARS (at 1440 minutes/day).  Even using the PC as a
terminal to the 370 allowed one to edit remotely instead of on a
timeshare basis (billed at $1/minute, damn it!) and saved one loads of
connect time (hence money).  And then came Sun workstations, faster PCs,
linux and somewhere in there computing became almost completely
decentralized with a client/server paradigm -- yes, there were a few
centralized servers, but most actual computation and presentation was
done at the desktop.

Even early beowulfs were largely spread out and not horribly
centralized.  An 8 node or 16 node system could fit in an office, a 32
node or even 64 node shelved beowulf could fit in a small server room.
The beauty of them was that you bought one for YOUR research, you didn't
share it (time or otherwise), and once you figured out how to put it all
together it didn't require much care and feeding, certainly not at the
high priest/acolyte stage (although cooling even 32 nodes starts to be
serious business).

Alas, we now seem to have come full circle.  Beowulfs are indeed COTS
supercomputers, but high density beowulfs are rackmounted and put in
centralized, expensive, often shared server rooms and strongly resemble
those centralized computers from which we once were freed.

I exaggerate the woe, of course.  The whole cluster NOW is transparently
accessible at gigabit speeds from your desktop across campus (and
wouldn't be any MORE accessible if you were sitting at a workstation in
the room with it listening to 80db worth of AC roar in your ear), linux
is excruciatingly stable (when it isn't unstable as hell, of course:-),
and once you get the nodes installed and burned in a human needs to
actually visit the cluster room only once in a long while.  We've
replaced the high priests and acolytes with sysadmin wizards and
application/programming gurus but this is a welcome change, actually
(they may appear similar but philosophically they are very different
indeed:-).  Still, the centralization threatens to a greater or lesser
extent the freedom -- it puts control much more into the hands of
administrators, costs more, involves more people in decisions.

Not much to do with the original question, sure, but I needed a little
philosophical ramble to start my day.  Now I have to write an hour exam
for my kiddies, which is less fun.  Last day of class, though, which IS
fun!  Hooray!

(It isn't only the students that anticipate summer...;-)

   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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