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I recently acquired an HP Z800 with two Intel Xeon X5650 (Gulftown) 6 core processors. The person that configured the system chose 16GB (8 x 2GB DDR3-1333). I'm assuming this person was unaware these processors have 3 memory channels and to optimize memory performance one should choose memory in multiples of three. Based on this information, I have a question:

By entering the BIOS, can I disable the bank on each processor that has the single memory module? If so, will this have any adverse effects or behave differently than physically removing the modules? I ask due to the fact that I prefer to store the extra memory in the system if it truly behaves as if the memory is not even there. Also, I see this as an opportunity to test 12GB vs. 16GB to see if there is a noticeable difference.

Note: According to http://www.delltechcenter.com/page/04-08-2009+-+Nehalem+and+Memory+Configurations?t=anon, the current configuration reduces the overall data transfer speed to 1066 and in addition, the memory bandwidth goes down by about 23%.

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3 Answers 3

If you're that bothered about memory performance (that's a fast CPU and fast memory in a fast machine, do you need the best if can offer?) just reduce it to 12 x 1GB memory, unless you need that 13th-16th GB of memory you'll be fine, we have lots of perfectly adequate 12GB nehalem servers.

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Thanks for the response. Yes; I am looking for the best it can offer (using everything I currently have in the system). For example, I don't want to purchase more memory as you suggest. I simply want to make the adjustment using what is currently in the machine. That is why I asked the question about disabling memory via the BIOS. Plus, I think it would be fun to test 12GB vs. 16GB... –  Bob Jun 13 '10 at 0:25
    
Sorry, I hadn't read it right, just move to 6 x 2 (I was responding on an iphone so hadn't scrolled up - doh!), that'll be faster than the 8 x 2. –  Chopper3 Jun 13 '10 at 9:25

The document you link to is for the X55xx series of processors (Nehalem-EP). If you actually do have a X56xx processor (Westmere-EP), then the rules have changed somewhat.

Intel improved the memory controller so that you can now double-stack each bank and still run at 1333. I'm sure this will still be slightly dependent on individual setups, but before you carry on it would be worth confirming if your RAM is in fact running at 1333 or at 1066 - don't take it for granted.

https://sp.ts.fujitsu.com/dmsp/docs/wp-westmere-ep-memory-performance-ww-en.pdf

The HP ref document that Helvick linked to supports this hypothesis - it's only once you start using 8- and 16-GB DIMMs, which cannot run at 1333, that your ram system speed slows down.

So your ram is still, in all likelihood, running at 1333, but you're not interleaving as much as you could because you have unevenly balanced ranks. I suspect the performance hit will be minimal :)

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good points there, the changes with Westmere over the original Nehalems make the platform more forgiving. –  Helvick Jun 13 '10 at 21:57
    
You guys have provided some great info. I definitely have Westmere procs and the RAM is 1333. I wonder how all this will pan out if I enable NUMA as Helvick suggested. I'll be running RHEL 5.5 and the primary function of this machine is VMware Workstation. Thanks for the assistance. –  Bob Jun 13 '10 at 22:49
    
NUMA should be enabled by default (although I'm not familiar with the Z800, so maybe it's not). Interleaving is another matter entirely, and the individual options will depend on the system you have. –  Daniel Lawson Jun 14 '10 at 1:32

The HP reference document for configuring the Z800's memory config is here. The easiest way for you to maximize performance is to remove two of the DIMM's and configure for 12GB (6x2GB) balanced across the CPU's as per the document. That should allow the controllers in the X5650's to drive the memory at 1333Mhz rather than the 1066 that you're currently getting. I don't believe you can disable the DIMM's selectively in the BIOS but I'm not familiar with the Z800 so it might be an option. I'm pretty sure that there is a NUMA memory interleaving option that will be set to disabled by default - depending on the workload you plan to run on the system you may see better performance if that is enabled.

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