[TriLUG] 32+ GB RAM PC advice

Kevin Flanagan kevin at flanagannc.net
Sun Sep 12 20:51:19 EDT 2004


I recently saw an AMD presentation, they way that they access memory has
to do with the need for the 3rd and 4th processors.  RAM is local to the
processors, then there's a "bridge" (my word, not theirs) between the
pairs of processors.   When you get to >16GB of RAM this bridge is
required, or if you have >2 Processors.  Their claim is that this bridge
is so fast that it's not a delay.  When using only 2 processors and
<16GB of RAM it sure does appear that it would be faster.  I don't know
enough about the details to say for sure, but to me it sure looks like a
dual proc 16GB system would have really high speed access to RAM, and
with 64bits it should really fly.



Kevin

On Sun, 2004-09-12 at 12:03, Carlos J. Cela wrote:

> Ed,
> 
> Thank you for your detailed answer. I have been using Fedora Core for a 
> while, so I guess Opteron/Fedora Core will be the combination of choice 
> here...I have noticed that you can configure 32GB RAM only if you have 4 
> processors installed in the machine....do you know if in a 32GB RAM/Quad 
> Opteron any of the 4 processors can access the full range of memory? I 
> mean, if my application is single-threaded, can I use the 32GB od RAM or 
> just to 1/4th of it? is all of the memory shared? if so, why the '4 
> processors installed' requirement?
> 
> Thanks,
> Carlos-
> 
> 
> Ed Hill wrote:
> 
> >Hi Carlos,
> >
> >For large memory systems (>4GB RAM) the Xeons are awful.  Instead, you
> >should consider Opterons, Itaniums, or other hardware.  We have a number
> >large-memory systems within our network
> >
> >  http://acesgrid.org/technical_blueprint.html
> > 
> >including:
> >
> >  + Dell dual-Itaniums w/ 16GB RAM
> >  + dual-Opterons: (mostly Tyan 2885 MB w/ 16GB RAM)
> >  + an Altix 350 w/ 16 CPUs and 32GB RAM
> >
> >and they perform *far* better than the Xeons on predominantly memory-
> >bandwidth-limited scientific applications.
> >
> >Dual (up to 16GB RAM) and quad (up to 32GB RAM) Opteron systems can be
> >purchased and/or assembled for a few thousand dollars and they scale
> >*remarkably* well.  I'm very fond of our Opterons.  We see almost linear
> >(that is, per-CPU) speed-ups with many applications including our main
> >ocean/atmospheric modeling software.  And Linux (we use both Fedora Core
> >2 for x86_64 and RHEL v3 for x86_64) runs quite nicely on the Opterons.
> >
> >For more than 32GB RAM, you'll have to look into SGI Altix or other
> >high-end hardware.  For truly huge simulations it is often best to get
> >time at a center such as:
> >
> >  http://www.ncsc.org/
> >  http://www.psc.edu/
> >
> >rather than trying to buy your own.  As an academic, you can usually get
> >grants of computing time on their (often huge) systems quite easily.
> >
> >Good luck with your simulations!
> >
> >Ed
> >
> >  
> >

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