Xeon W memory population (balanced?)

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Bernhard Neuhauser

New Member
Aug 16, 2018
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I noticed that the X11SR-F includes a memory population table on page 30.

It basically claims that the following configurations are optimums:
  • 4x 32GB
  • 8x 32GB
  • 6x 64GB <= why 6x and not 4x?
  • 8x 64GB
My original assumption was that quad channel means that optimums would be 4 and 8 populated dimms.

Questions:
  • Does anyone know why 64GB modules need 6x instead of 4x dimms?
  • What would be the performance impact of only populating 4x 64GB dimm?
The basic idea was to start with 256GB RAM and still have the ability to upgrade in a year if necessary. But now i wonder if it hurts a lot to have an unbalanced memory setup.

I was only able to find in depth information dealing with xeon sp memory configurations: https://lenovopress.com/lp0742.pdf

Page 18 clearly shows that there is a huge variety of memory bandwith possible (17-100% depending on memory setup). But i am sadly unable to find any information dealing with xeon w's 4 channel memory layout.
 

Evan

Well-Known Member
Jan 6, 2016
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Ok I can’t say this is the case for this board and the W CPU’s but I notice HPE also say 8 dimms is good in intel scalable, they do this by balancing 2 dimm per channel over 4 channels (not 6).
Maybe they do similar for the W’s, 2 DPC over 3 channels ?

Anyway for me I would stick to 4 or 8 dimms.

(Scalable has huge jumps of performance from 1-2-3-4 channels and 5-6 less so with 5 wise than 6, pure speculation that it’s similar for W’s)
 

alex_stief

Well-Known Member
May 31, 2016
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Questions:
  • Does anyone know why 64GB modules need 6x instead of 4x dimms?
  • What would be the performance impact of only populating 4x 64GB dimm?
The basic idea was to start with 256GB RAM and still have the ability to upgrade in a year if necessary. But now i wonder if it hurts a lot to have an unbalanced memory setup.
6 DIMMs for Xeon W is not a recommended memory configuration, no matter what the manual states. You could try to use DIMMs with different sizes in order to have the same amount of RAM on each channel, but memory performance will suffer.
The performance impact from populating 4 instead of 8 DIMMs would be negligible. If there was any performance impact at all.
If you want to start with 256GB and have future upgradeability, use 4x64GB.

On the question of how much an "unbalanced" memory configuration will hurt application performance: That depends entirely on the workload you have and how "unbalanced" your memory configuration is.