memory performance question

[zombie67]

I have a quad channel machine, with 8 slots and with two CPUs. Which would have better performance, 1 DIMM in each channel, or 2 DIMMs in each channel? I can never remember this.

TIA!

[Bryan]

Max performance is 1 per channel :D

[Mumps]

Hang on there mister. A lot of the information about memory performance is very dependent on the chipset being used on the motherboard. As examples, both HP and Lenovo server architectures gain speed with more DIMMs.

https://lenovopress.com/lp0742.pdf
https://community.spiceworks.com/topic/1858832-is-it-better-to-have-many-small-or-fewer-large-ram-sticks

So it needs to be clarified what architecture the question is to be applied to. And especially the age of the hardware (leading to DDR2, DDR3, DDR4 or other) will be necessary knowledge.

[zombie67]

Here are the three systems in question:

Dual Xeon CPU E5-2680 v2, C602, DDR3
Dual Xeon CPU E5-2630 v3, C612, DDR4
Dual Xeon CPU E5-2683 v3, C612, DDR4

TIA!

[Bryan]

Rather than the link Mumps gave, THIS (https://lenovopress.com/lp0501.pdf) is the one for the E5 V3/V4. It says that all memory channels should be populated the same so the system is balanced. Basically, 1 stick per channel (4 per CPU) or 2 sticks per channel (8/CPU) will provide the same memory bandwidth as long as all sticks are the same. If the memory channels aren't populated the same (unbalanced) then BW goes down dramatically.

When I built my servers - E5 V4s I was told by SuperMicro that the optimum was 1 stick per memory channel (4/CPU).

[zombie67]

What's going on here? DDR4 and DDR3 are physically different, right?

https://www.dropbox.com/s/fkhs2t235q990mx/Screen%20Shot%202018-04-03%20at%203.11.51%20PM.jpg?dl=0
https://www.dropbox.com/s/1bqj9o8h0x00ab9/Screen%20Shot%202018-04-03%20at%203.11.35%20PM.jpg?dl=0

[zombie67]

It turns out they are really DDR3 according to the label.

Here is another question about quantities of DIMMs. This machine is a HP server, with dual CPUs. 1st CPU is on the mobo with 8 DIMM slots. The 2nd CPU is on a daughter card with 4 DIMM slots. Currently each has four DIMMs, 8gb each. What happens if I populate the remaining 4 slots on the mobo with matching DIMMs? It seems like one CPU would have access to 64gb, and the other only 32gb. Or is the combined 96 GB shared equally somehow? Does that impact performance?

Edit: I found the answer here: http://h20331.www2.hp.com/Hpsub/downloads/Z620_memory-configurations_WP_090712.pdf


Optimize performance
Generally, maximum memory performance is achieved by evenly distributing total desired memory capacity across all operational channels. Proper individual DIMM capacity selection is essential to maximizing performance. On the second CPU, installing the same amount of memory as the first CPU will optimize performance. Refer to the Optimal Memory Configuration tables on the next page for more information.

Frustrating. Did 16gb DIMMs not exist back then or something?

[John P. Myers]

Max performance is 1 per channel :DThis is correct. Especially on server boards