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Is 1 DIMM per channel better or worst than 2 DIMMs per channel on Skylake Xeon?

drkirkby
Beginner
946 Views
QUICK SUMMARY  / QUESTIONS
1) I have a Dell 7920 Precision workstation, with two CPU sockets, and 24 DIMM slots.  I only have one CPU, so only 12 DIMM slots may be used.
2) Is it best to install DIMMs in sets of 4, as stated in the manual, or perhaps in sets of 6?
3) Is there any advantage/disadvantage in using 1 DIMM per channel (1 DPC) or 2 DIMMs per channel (2 DPC)
MORE BACKGROUND
The online manual shows various memory population configurations for 1 and 2 CPU setups
https://www.dell.com/support/manuals/en-uk/precision-7920-workstation/precision_7920_om_pub/memory-c...
I attached a bit of the table here, in case Dell remove it.
I think the manual contains only a small subset of the configurations that actually work. For example.
1) At the smallest configuration, the table a single 8 GB DIMM may be used and shows no possibility of using a single 32 GB DIMM. Yet Dell's technical support told me a single 32 GB DIMM would work, and I indeed find that's true.
2) For a configuration with 64 GB RAM, it shows two possibilities - 8 x 8 GB (2 DPC)  or 4 x 16 GB (1 DPC). Yet, as stated above, I know a pair of 32 GB DIMMs work okay. Would you expect 8 x 8 GB (2 DPC)  or 4 x 16 GB (1 DPC) to work better than the other? Both meet the suggestion of using DIMMs in sets of 4.
3) For a 128 GB configuration, the manual shows only one possibility - 8 x 16 GB (2 DPC). I'm rather hoping 4 x 32 GB would work too. In which case it would be 1 DPC, so would that be any better/worst than 8 x 16 GB (2 DPC)?
Given there are 6 memory channels in Skylake, I'm a bit puzzled Dell say its optimal to install in groups of 4. I would have thought groups of 6 to make more sense. That seems to be born out by this graph from Dell

https://downloads.dell.com/manuals/all-products/esuprt_solutions_int/esuprt_solutions_int_solutions_...

showing the relative performance of 1, 2,, 3, 4, 5 and 6 DIMMs. Note 1 is poorest, but 2 & 5 is pretty poor too.

1=0.17
2=0.35
3=0.51
4=0.69
5=0.36
6=1.0
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1 Solution
n_scott_pearson
Super User Retired Employee
917 Views

IMHO, having only one DIMM per channel can be more reliable than having two DIMMs per channel, but it should yield similar (if not identical) performance. When you have two DIMMs installed in a channel, these DIMMs need to be identical in size and performance. In this case, we refer to them as being a 'matched pair'.

How many DIMMs are considered a 'set' is determined by the number of memory channels that the processor supports. If it supports only 4 memory channels, then there are 4 DIMMs in a set and I would expect to see 8 DIMM sockets on the board. If the processor supports 6 memory channels, then there are 6 DIMMs in a set and I would expect to see 12 DIMM sockets. Again, even if you call these DIMMs a 'set', it is important for the DIMMs in the second 'set' form matched pairs with the corresponding DIMMs in the first 'set'.

...S

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2 Replies
n_scott_pearson
Super User Retired Employee
918 Views

IMHO, having only one DIMM per channel can be more reliable than having two DIMMs per channel, but it should yield similar (if not identical) performance. When you have two DIMMs installed in a channel, these DIMMs need to be identical in size and performance. In this case, we refer to them as being a 'matched pair'.

How many DIMMs are considered a 'set' is determined by the number of memory channels that the processor supports. If it supports only 4 memory channels, then there are 4 DIMMs in a set and I would expect to see 8 DIMM sockets on the board. If the processor supports 6 memory channels, then there are 6 DIMMs in a set and I would expect to see 12 DIMM sockets. Again, even if you call these DIMMs a 'set', it is important for the DIMMs in the second 'set' form matched pairs with the corresponding DIMMs in the first 'set'.

...S

drkirkby
Beginner
876 Views
Thank you for the help. I have a somewhat better understanding now.
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