Chapter 2. IBM eX5 technology 25
extend the pages across ranks, thereby making the pages effectively larger and therefore
creating more page-hit cycles.
We used three types of memory DIMMs for this analysis:
Four GB 4Rx8 (four ranks using x8 DRAM technology)
Two GB 2Rx8 (two ranks)
One GB 1Rx8 (one rank)
We used the following memory configurations:
Fully populated memory:
– Two DIMMs on each memory channel
– Eight DIMMs per memory card
Half-populated memory:
– One DIMM on each memory channel
– Four DIMMs per memory card (slots 1, 3, 6, and 8; see Figure 3-16 on page 76)
Quarter-populated memory:
– One DIMM on just half of the memory channels
– Two DIMMs per memory card
Although several benchmarks were conducted, this section focuses on the results gathered
using the industry-standard STREAM benchmark, as shown in Figure 2-7.
Figure 2-7 Comparing the performance of memory DIMM configurations using STREAM
Taking the top performance result of 16x 4 GB quad-rank DIMMs as the baseline, we see how
the performance drops to 95% of the top performance with 16x 2 GB dual-rank DIMMs, and
89% of the top performance with 16x 1 GB single-rank DIMMs.
You can see similar effects across the three configurations based on eight DIMMs per
processor and four DIMMs per processor. These results also emphasize the same effect that
is shown in 3.8.3, “Maximizing memory performance” on page 84 for the x3850 X5, where
performance drops away dramatically when all eight memory channels per CPU are not used.
Relative STREAM Triad Throughput
by DIMM population per processor
100
98
55
95
89
52
89
73
42
0 20406080100120
16x 4GB (4R)
8x 4GB (4R)
4x 4GB (4R)
16x 2GB (2R)
8x 2GB (2R)
4x 2GB (2R)
16x 1GB (1R)
8x 1GB (1R)
4x 1GB (1R)
Relative Memory Throughput
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