Clarification: Nehalem-EP and DDR3

I have seen a lot of contrasting comments about Nehalem-EP and memory speed on the community groups - especially in the area of supported speed ratings: often in the context of comparison to Opteron's need to reduce supported DIMM speed ratings based on slot population. While it is true Nehalem's 3-channel design allows for a mixture of performance (800/1066/1333) and capacity, it does not allow for both.

Here are the rules (from Intel's "Intel Xeon Processor 5500 Series Datasheet, Volume 2") based on DIMM per Channel (DPC):

• 1-DPC = Support DDR3-1333 (if DIMM supports DDR3-1333)
  
  
  
  • KVR1333D3D4R9S/4G - $169/ea
  
  
  • 12GB/CPU max. @ $507/CPU (24GB/system max.)
  
  
  
  


• 2-DPC = Support DDR3-1066 (if all DIMMs are rated DDR3-1066 or higher)
  
  
  
  • KVR1066D3D4R7S/4G - $138/ea
  
  
  • 24GB/CPU max. @ $828/CPU (48GB/system max.)
  
  
  • KVR1066D3Q4R7S/8G - $1,168/ea
  
  
  • 48GB/CPU max. @ $7,008/CPU (96GB/system max.)
  
  
  • "96GB Memory (12x8GB), 1066MHz Dual Ranked RDIMMs for 2 Processors,Optimized [add $15,400]" - Dell
  
  
  
  


• 3-DPC = Support DDR3-800 only (if all DIMMs are rated DDR3-800 or higher)
  
  
  
  • KVR1066D3D4R7S/4G - $138/ea
  
  
  • 36GB/CPU max. @ $1,242/CPU (72GB/system max.)
  
  
  • "144GB Memory (18x8GB), 800MHz Dual Ranked RDIMMs for 2 Processors,Optimized [add $22,900]" - Dell
  
  
  
  

When the IMC detects the presence of 1, 2 or 3 DIMMs, these speed limits are imposed, regardless of the capabilities of the DIMM. A couple of other notable exceptions exist:

• When one 4-rank DIMM is used, it must be populated in DIMM slot0 of a given channel (farthest from CPU);


• Mixing of 4-rank DIMMs in one channel and 3-DIMMs in other channel (3-DPC) on the same CPU socket is not allowed - forcing BIOS to disable on the 4-rank channel;


• RDIMM
  
  
  
  • Single-rank DIMM: 1-DPC, 2-DPC or 3-DPC
  
  
  • Dual-rank DIMM: 1-DPC, 2-DPC or 3-DPC
  
  
  • Quad-rank DIMM: 1-DPC or 2-DPC
  
  
  
  


• UDIMM
  
  
  
  • Single-rank DIMM: 1-DPC or 2-DPC
  
  
  • Dual-rank DIMM: 1-DPC or 2-DPC
  
  
  • Quad-rank DIMM: n/a
  
  
  
  

Speed freaks be warned!