IBM BladeCenter HX5 Implementation Guide

Chapter 2. IBM eX5 technology 27
on each of the attached memory buffers), DIMMs must be installed in quantities of four per
processor to enable Hemisphere Mode.
In addition, because eight DIMMs per processor are required for using all memory channels,
eight DIMMs per processor need to be installed at a time for optimized memory performance.
Failure to populate all eight channels on a processor can result in a performance reduction of
approximately 50%.
Hemisphere Mode does not require that the memory configuration of each CPU is identical.
For example, Hemisphere Mode is still enabled if CPU 0 is configured with 8x 4 GB DIMMs,
and processor 1 is configured with 8x 2 GB DIMMs. Depending on the application
characteristics, however, an unbalanced memory configuration can cause reduced
performance because it forces a larger number of remote memory requests over the
inter-CPU QPI links to the processors with more memory.
We summarize these points:
򐂰 There are two memory buffers per memory channel, two channels per memory controller,
and two controllers per processor. Each memory channel must contain
at least one DIMM
to enable Hemisphere Mode.
򐂰 Within a processor, both memory controllers need to contain identical DIMM
configurations to enable Hemisphere Mode. Therefore, for best results, install at least
eight DIMMs per processor.
Industry-standard tests run on one Xeon 7500 processor with various memory configurations
have shown that there are performance implications if Hemisphere Mode is not enabled. For
example, for a configuration with eight DIMMs installed and spread across both memory
controllers in a processor and all memory buffers (see Figure 2-8), there is a drop in
performance of 16% if Hemisphere Mode is not enabled.
Figure 2-8 Example memory configuration
For more information about Hemisphere Mode installation options, see the following sections:
򐂰 IBM System x3850 X5: 3.8.2, “DIMM population sequence” on page 79
򐂰 IBM System x3690 X5: “Two processors with memory mezzanine installed” on page 135
򐂰 IBM BladeCenter HX5: 5.10.2, “DIMM population order” on page 196
Intel Xeon 7500 processor
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMM
DIMMDIMM
Buffer Buffer Buffer Buffer
Memory
controller
Memory
controller

IBM BladeCenter HX5 Specifications

General

Brand	IBM
Model	BladeCenter HX5
Category	Server
Language	English