Network RAM revisited

Joel Jaeggli joelja at darkwing.uoregon.edu
Thu May 29 03:43:52 EDT 2003


2GB dimms reg ecc dimms seem to still be a factor of 8 or so more 
expensive than 1gb dimms. but spending ~$13000 or so per node for memory 
makes spending $600 for a mainboard and $400ea for cpu's seem like a 
comparitvly minor part of the cost. even with niceish cases to protect 
your expensive ram, and a blown out myrinet setup you could probably slip 
in under 1.25-1.33x the cost of the ram.

joelja

On Thu, 29 May 2003, Greg Lindahl wrote:

> On Wed, May 28, 2003 at 07:29:52PM -0400, Robert G. Brown wrote:
> 
> >  Weather, cosmology, there are a few
> > "enormous" problems where researchers ALWAYS want to work bigger than
> > current physical limits and budgets permit and can still get useful
> > results even with the penalties imposed by disk or other VM extensions.
> 
> Yes, but those are bad examples.  The more memory you use in a 72-hour
> weather forecast (more memory means more input data, higher resolution
> output), the more cpu time it takes to make the computation -- the cpu
> time increases as memory**(4/3). In reality, weather forecasting is
> actually limited by our ability to insert fudge factors for local
> physics, i.e. things that aren't resolved on the grid. You need ~ 7
> cells in 1 km to resolve a thunder head. Current production runs are
> at 10-20km cells. Those pesky weather satellites produce plenty of
> high-resolution input. So memory size is not a problem, and the data
> is big enough that you can use 100-200 cpus with Myrinet. Not too
> shabby.
> 
> Now if you have a serial code, sure, you might be able to use network
> RAM. But parallelizing a weather code is old hat.
> 
> By the way, Opteron mobos make your arguments about big memory
> motherboards a bit obsolete... there are 2 cpu motherboards with 2x4 =
> 8 total DIMM slots. I'm not sure what 4 cpu motherboards will do, but
> it might be 4x4 = 16. These 2 cpu motherboards are not that
> expensive... the Khapri is gold-plated, but its competition is not.
> Bigger DIMMs are expensive, but they are always falling.
> 
> I always thought it was kind of a shame that motherboards used to have
> such low limits on memory. While I was at UVa I wanted to build a
> small cluster with 1 Tbyte of memory for big-memory apps. The figure
> of merit was that the cluster was going to have a total cost less than
> 2x the price of just the memory. If Digital had made a low end Alpha
> motherboard that could address big memory, I could have done it, but
> their low end chipset didn't carry out enough address pins. Grrr.
> 
> -- greg
> 
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Joel Jaeggli	      Academic User Services   joelja at darkwing.uoregon.edu    
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