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IBM eX5 Implementation Guide
Populate one DIMM per channel on
every
channel on memory before populating a second
DIMM in
any
channel.
Populate DIMMs at the end of a memory channel first before populating the DIMM closer
to the memory buffer. That is, install to sockets 1, 3, 6, and 8 first.
If you have a mix of DIMM capacities (such as 4 GB and 8 GB DIMMs), insert the largest
DIMMs first (spreading out the DIMMs across every memory channel), then move to next
largest DIMMs, and finish with the smallest capacity DIMMs that you have.
Therefore, where memory performance is key to a successful deployment, the best
configuration is to install 32 or 64 identical DIMMs across eight memory cards and four
processors.
A system with fewer than four installed processors or fewer than eight installed memory cards
has fewer memory channels, therefore less bandwidth and lower performance.
Mixed DIMMs and the effect on performance
Using DIMMs of various capacities (for example, 4 GB and 8 GB DIMMs) is supported. The
capacities of the DIMMs might differ for several reasons:
Not all applications require the full memory capacity that a homogenous memory
population provides.
Cost-saving requirements might dictate using a lower memory capacity for several of the
platform’s DIMMs.
Certain configurations might attempt to use the DIMMs that came with the base platform,
along with optional DIMMs of a separate type.
Figure 3-20 on page 87 illustrates the relative performance of three mixed memory
configurations as compared to a baseline of a fully populated memory configuration. While
these configurations use 4 GB (4R x8) and 2 GB (2R x8) DIMMs as specified, similar trends
to this data are expected when using other mixed DIMM capacities. In all cases, memory is
populated in minimum groups of four, as specified in the following configurations, to ensure
that Hemisphere Mode is maintained.
Figure 3-20 on page 87 shows the following configurations:
Configuration A: Full population of equivalent capacity DIMMs (2 GB). This represents an
optimally balanced configuration.
Configuration B: Each memory channel is balanced with the same memory capacity, but
half of the DIMMs are of one capacity (4 GB), and half of the DIMMs are of another
capacity (2 GB).
Configuration C: Eight DIMMs of one capacity (4 GB) are populated across the eight
memory channels, and four additional DIMMs (2 GB) are installed one per memory buffer,
so that Hemisphere Mode is maintained.
Configuration D: Four DIMMs of one capacity (4 GB) are populated across four memory
channels, and four DIMMs of another capacity (2 GB) are populated on the other four
memory channels, with configurations balanced across the memory buffers, so that
Hemisphere Mode is maintained.
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