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New Dual-CPU Xeon System: What type of memory?

Tags:
  • Video Encoding
  • Processors
  • Memory
  • CPUs
  • Xeon
  • Dual
Last response: in Memory
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December 22, 2013 6:47:34 AM

I'm building a new machine which will consist of dual Xeon processors, most likely E5-2630 v2. The machine will have 8 DIMMS: either 4x4GB for each CPU, or 4x8 GB for each CPU. I don't need ECC memory. I was planning on regular DDR3 1866 for this machine, but this article makes me unsure:
http://www.anandtech.com/show/6068/lrdimms-rdimms-super...
From this article, it seems that if you put too many large memory modules in a single system, the increased capacitance will slow the memory dramatically.

My question for the power builders is whether or not this is a real problem. If I were to use regular 64-GB of unbuffered DIMMS (4 8-GB DIMMS for each Xeon CPU), how much slower would they be for memory-intensive apps (VIdeo encoding) then if I used fully buffered DIMMS? What memory should I use?

More about : dual cpu xeon system type memory

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a b } Memory
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December 22, 2013 8:01:59 AM

I'm running 8 x 8GB unbuffered DIMMs on my Asus P9X79 Pro board and have no issues. All of the ram is detected. I was getting some minor instability initially, but this may have been due to a UEFI firmware issue, as I had to manually fix the RAM voltage for all channels. No *increase* was needed, but the system was a bit unstable if the voltage was not fixed in the UEFI console to 1.5V.

My sticks are rated as DDR3-1600, and they have no problem holding that frequency when the slots on my board are fully populated. Based on my experience, your RAM should not slow down at all.
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December 22, 2013 8:47:30 AM

Okay, then we agree. That's what I thought. But then that means the article I posted is either very wrong, or I'm mis-understanding. Why do people buy RDIMMS (Registered DIMM) when they build servers? They cost about 33 percent more. What is the advantage?

Also, and this might be a dumb question, but how can one tell for sure that your board is running DDR3-1600 at full speed? Can the system throttle down the memory speed (like it does with CPU clock)? Are there tools that will report this? Linux tools?
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a b } Memory
a b à CPUs
December 22, 2013 9:56:37 AM

I use CPU-Z mostly, but there are lots of tools out there that will report memory speeds, timings, and voltages. Not sure about Linux though, as I mainly use Windows, with a little bit of OSX thrown in to be able to provide rudimentary support for Apple systems where I work. As far as I'm aware, RAM clock speeds cannot be dynamically adjusted like a CPU can. The frequency, timing, and voltage adjustments must be made in the BIOS/UEFI console, and only take effect after a reboot is performed.

With registered RAM modules, a register is placed between the DRAM modules themselves, and the memory controller. It reduces the electrical load on the memory controller, increasing stability of the system with more RAM sticks installed. Registered modules are more expensive because they have additional components, and are produced in lower volume than non-registered parts. There is also a slight performance penalty when using registered modules. Both reads and writes to registered RAM is buffered by one clock cycle from the bus, to the module itself.

Also keep in mind when building that system, that the motherboard you buy must support registered RAM. Motherboards not designed to support registered modules will not work when you try to install registered parts.

I have to ask though, is stability of paramount importance here? The performance penalty is minimal, and they cost considerably more, especially since you plan to buy quite a few of them. The need for registered RAM is found more in server systems where uptime has greater significance over performance and cost. Unless there is some other requirement or circumstance I am not aware of, non-registered parts might be a better way to go.
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