Memory speed vs size?

EE,

I have a Dell R610 that can fit 96GB with 8GB sticks (KTD - PE313/8G)

I'm not sure how to tell whether it's dual or quad rank but my question is that either one when fully populated into the slots will reduce speed from 1333 down to 800hz

My question is, is this a noticable difference in memory performance and what can the drawbacks be? We don't exactly need 96GB per server and can increase the speed to 1333 at 48GB but have no bench testing or examples to confirm a noticable benefit

Please assist, thanks
snyderkvAsked:
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andyalderCommented:
The chart entitled "Relative Performance for Different Memory Speeds" in http://globalsp.ts.fujitsu.com/dmsp/Publications/public/wp-westmere-ep-memory-performance-ww-en.pdf is very useful; it shows a performance drop from 1.0 to 0.62 when they ran STREAM memory bandwidth sensitive test but only a drop from 1.0 to 0.91 for SPECint. They also deliberately messed up the channel balancing in "Relative Performance for Different Interleaving Levels" graph and again a huge drop in performance using STREAM but far less drop when using the more real world app SPECint test.
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Glen KrinskySystems AdministratorCommented:
You will not be able to tell the difference, but you will see the bechmark difference.  What is that you hope to gain?
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CallandorCommented:
Do you have Xeon 5500 series cpus?  The Xeon 5500 series uses a an onboard memory controller and QPI transport for communication between processors.  This document on p.16 says 1066 memory has 20-25% higher throughput than 800 memory, and 1333 memory has 3.5-9% higher throughput than 1066 memory: ftp://ftp.hp.com/pub/c-products/servers/options/Memory-Config-Recommendations-for-Intel-Xeon-5500-Series-Servers-Rev1.pdf
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andyalderCommented:
Just like the white papers on Intel's site the HP white paper doesn't address the speed difference using a real world application but uses a memory performance test. Generally applications perform better if they have more RAM to play with regardless of the extra RAM possibly slowing the bus down.
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snyderkvAuthor Commented:
Cool thanks for the info and the links. I'll check them out

Funny though, the BIOS says 1333 @ 96G despite what the documentation says. I'm guessing it's running slower but displaying it's rated output.

Thanks
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snyderkvAuthor Commented:
Thanks Andy. But I mean we use maybe <10% of the RAM available so I'm sure there's RAM sitting idle?
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andyalderCommented:
BIOS often reports DIMMs ratings, depends on server model. If you're not going to be using the RAM then better to take it out, as noted it does slow it down if fitted but not used.
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snyderkvAuthor Commented:
Roger
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