24 IBM eX5 Implementation GuideFigure 2-6 Relative memory performance based on DIMM placement (one processor and two memory cards shown)2.3.3 Memory rankingThe underlying speed of the memory as measured in MHz is not sensitive to memorypopulation. (In Intel Xeon 5500 processor-based systems, such as the x3650 M2, if rulesregarding optimal memory population are not followed, the system BIOS clocks the memorysubsystem down to a slower speed. This situation is not the case with the x3850 X5.)Unlike Intel 5500 processor-based systems, more ranks are better for performance in thex3850 X5. Therefore, quad-rank memory is better than dual-rank memory, and dual-rankmemory is better than single-rank memory. Again, the frequency of the memory as measuredin MHz does not change depending on the number of ranks used. (Intel 5500-based systems,such as the x3650 M2, are sensitive to the number of ranks installed. Quad-rank memory inthose systems always triggers a stepping down of memory speed as enforced by the BIOS,which is not the case with the eX5 series.)Performance test between ranksWith the Xeon 7500 and 6500 processors, having more ranks gives better performance. Thebetter performance is the result of the addressing scheme. The addressing scheme can1 Each processor:2 memory controllers2 DIMMs per channel8 DIMMs per MCMem Ctrl 1 Mem Ctrl 21.02 Mem Ctrl 1 Mem Ctrl 2Each processor:2 memory controllers1 DIMM per channel4 DIMMs per MC0.94Mem Ctrl 1Memory cardDIMMsChannelMemory bufferSMI linkMemory controller3 Mem Ctrl 1 Mem Ctrl 2Each processor:2 memory controllers2 DIMMs per channel4 DIMMs per MC0.61Relativeperformance4 Mem Ctrl 1 Mem Ctrl 2Each processor:2 memory controllers1 DIMM per channel2 DIMMs per MC0.585 Mem Ctrl 1 Mem Ctrl 2Each processor:1 memory controller2 DIMMs per channel8 DIMMs per MC0.516 Mem Ctrl 1 Mem Ctrl 2Each processor:1 memory controller1 DIMM per channel4 DIMMs per MC0.477 Mem Ctrl 1 Mem Ctrl 2Each processor:1 memory controller2 DIMMs per channel4 DIMMs per MC0.318 Mem Ctrl 1 Mem Ctrl 2Each processor:1 memory controller1 DIMM per channel2 DIMMs per MC0.2910.940.610.51 0.470.31 0.290.5800.10.20.30.40.50.60.70.80.911 2 3 4 5 6 7 8ConfigurationRelative memory performanceMemory configurations
