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Clarification: Nehalem-EP and DDR3

April 29, 2009

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!

One comment

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