do the number of memory sticks make a difference in virtualization efficiency?

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Thuleman

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Consider a dual socket motherboard that will accept 24 dimms. Let's say you populate that board with two E5-2660 CPUs which have 8 cores each and under "memory specification, # of Memory Channels" it says that there are 4 channels.

How does the channel number relate to the population of dimms, and to virtualization efficiency? Or does it?

Let's consider 192 GB of RAM, would it be best to populate all 24 dimms (8 GB x 24), or just 12 (16 GB x 12) in terms of performance? Obviously it makes more sense to use 16 GB sticks to leave space for future expansion, but does it matter in terms of how well the CPU works with memory and ESXi?
 
I'm only aware of what happens when you go over a certain amount the memory speed steps down. In our R720s if I went over 16 banks the speed went from 1600mhz to 1333. And drops to 800 the higher you go.
 
Define "virtualization efficiency". If you mean VM density, then you'll always want to max RAM regardless the speed it runs at.
 
SpaceHonkey said:
Define "virtualization efficiency". If you mean VM density, then you'll always want to max RAM regardless the speed it runs at.
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Yeah I should have been more clear about it, I didn't mean VM density but the speed at which memory can be accessed and whether physically "fragmented" memory, meaning 24x 8GB vs say 12x 16GB makes any difference at all in terms of how quickly the memory can be accessed.

The question then too is whether the difference in access times is actually significant at all.
 
The only time it really matters is if the memory speed has to drop..and most workloads won't even notice that.
 
If you have a CPU that supports 4-way memory interleave (E5-26xx) then you really want to install memory sticks in multiples of 4 to get maximum memory speeds.

If you have a dual-CPU setup using 4-way interleave (dual E5-26xx) then you want to install in multiples of 8 (4 per CPU).

If you install 12x 16GB to get 192Gb on such a motherboard then you are installing 6 DIMMs/CPU and will not get maximum memory interleave performance.

A noted by OHalo above you do sometimes get speed decreases if you exceed certain limits on the moery bus. In practice, however, this is usually offset by being able to run the memory a a better CAS Latency, e.g., DDR3 1600 memory with CAS 11 running at DDR3 1066 speed usually resets to CAS 9 and achieves similar net memory bandwidth. The net effect is not noticeable under most workloads.

The exact details of what you need to install and what (if any) speed degradation you get due to multiple DIMMs/channel depends entirely on specifics of your MB and characteristics of the memory sticks themselves (# or ranks). There is no way to give you a generic answer.
 
Its always recommend though that if you have a 2, 3, or 4 channel system, that you install matching memory in that type of kit, like a dual or quad channel memory kit
 
KILL____ said:
Its always recommend though that if you have a 2, 3, or 4 channel system, that you install matching memory in that type of kit, like a dual or quad channel memory kit
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This. Analyzing beyond this is not worthwhile. Focus on getting as much RAM as you can afford.
 
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