IBM System x3850 X5 Implementation Manual page 95

Use these population guidelines:
Each CPU requires a minimum of two DIMMs to operate.
All processors must be identical.
Only configurations of two, three, or four processors are supported.
The number of installed processors dictates what memory cards can be used:
– Two installed processors enable four memory cards.
– Three installed processors enable six memory cards.
– Four installed processors enable all eight memory cards.
A processor must be installed in socket 1 or 2 for the system to successfully boot.
A processor is required in socket 3 or 4 to use PCIe slots 1 - 4. See Figure 3-7 on page 66.
When installing three or four processors, use a QPI Wrap Card Kit (part number 49Y4379)
to improve performance. The kit contains two wrap cards. See 3.4.2, "QPI Wrap Card" on
page 66.
When using a MAX5 memory expansion unit, as shown in Figure 3-12 on page 70, you
maximize performance when you have four installed processors because there are four
active QPI links to the MAX5. However, configurations of two and three processors are still
supported.
Consider the X7542 processor for CPU frequency-dependent workloads because it has
the highest core frequency of the available processor models.
If high processing capacity is not required for your application but high memory bandwidth
is required, consider using four processors with fewer cores or a lower core frequency
rather than two processors with more cores or a higher core frequency. Having four
processors enables all memory channels and maximizes memory bandwidth. We
describe this situation in 3.8, "Memory" on page 76.
Chapter 3. IBM System x3850 X5 and x3950 X5
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