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Re: [Xen-users] Re: xen benchmark results on nehalem

Grant McWilliams <grantmasterflash@xxxxxxxxx> writes:

> But would having MORE ram every make a machine slower? I can understand that
> it might not make it faster but hard to believe it's going to slow down the
> machine!

Quite often, especially with registered ecc ddr2 and registered ecc ddr3
there is a trade off between the number of modules (which effects how much
ram you can have at a given price point)  and the speed of those modules.
I am given to understand that the monumental failure that was FBDIMM technology
mitigated that somewhat, of course at the cost of a ridiculous amount
of power.  

The dell reps trying to sell to one of my clients the other day were explaining
that the ram would only run at 1333mhz if we only filled a few of the slots.
If we filled all 18 slots, the ram would only run at 800mhz.  I tried to
explain to my clients that ram speed didn't really matter for what they were
doing, because they hadn't gotten to the point yet where all their frequently
used data was cached, and that their biggest performance win would be buying
as much 800mhz ram as they could afford.   

So there is a trade off.   Like I said, it's nothing like the speed difference
between slow ram and disk, but it is something to worry about if you are 
running HPC apps or other stuff that was designed so it doesn't hit disk 
hardly at all.

If you care about memory speed, you might want to check out the STREAM 
benchmark.  See: http://www.cs.virginia.edu/stream/  

(And yea, I understand all the cs guys are freaking out 'cause ram isn't getting
faster at the speed CPUS are getting faster.   But in my world, that 
won't matter at all until we are entirely off of ram, or at least until 
SSDs finish crushing hard drives.)  

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