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A memory primer for new BL460cGen8 blades with Intel Ivy Bridge Processors.

chuckk281 on ‎09-23-2013 12:25 PM

A customer recently asked about the advantages/disadvantages associated with LRDIMMs--particularly with the 1866MHz. LRDIMMs we’ve just launched with our latest BL460c Gen8 with Ivy Bridge update. I believe LRDIMMs introduce a slight amount of latency, but is there anything else?


First of all what are LRDIMMs and what advantage do they bring? Load reduced DIMM (LRDIMM) is another new memory technology. Designed with a buffer chip (or chips) to replace the register used in RDIMMs to help minimize loading, the LRDIMM is targeted to increase overall server system memory capacity and speed using a memory buffer chip or chips as opposed to a register.


lrdimm_d.jpg             ldrimm_buffer.jpg


So, Greg Park, one of our field engineers, went over and discussed this with Eric Pope (engineering lead for memory). Here are his comments:


The advantages of using LRDIMMs really come down to capacity at higher throughput.  A quad-rank RDIMM (Registered DIMM) is limited to operate at 1066 speeds and drops to 800MHz. at 2DPC (DIMMS per channel) and isn’t supported at 3DPC.  A quad-rank LRDIMM can now run at 1866MHz. speeds at 1DPC and 2DPC.  At 3DPC, it drops to 1333MHz., but it does support 3DPC.


For this particular customer, the 3DPC consideration isn’t even in the cards on the BL460c Gen8.  The main advantage they get with LRDIMMs on the BL460c Gen8 is capacity.  The only way to get to 512GB on memory in a BL460c Gen8 is with 32GB LRDIMMs.  If this is your direction, I’d definitely recommend the 1866MHz. version due to the significant speed bump from 1333MHz. to 1866MHz.


If you are considering 256GB with either 8x 32GB LRDIMMs or 16x 16GB RDIMM, then you’ll see similar performance.  When mostly idle, the latency and power numbers will be similar.  When heavily utilized, the LRDIMM solution consumes about 5% more power and adds about 6-7% to the latency.  Throughput will be almost identical.  If your application is highly sensitive to latency, then I’d recommend the RDIMM solution.  If you’re more worried about future expansion, then the LRDIMM solution may be the right way to go. Another factor to consider is the relative pricing of LRDIMMs vs RDIMMs.  While memory prices are subject to changing market conditions, LRDIMMs are generally priced higher than RDIMMs from a $/GB perspective. Take all these factors into the decision-making process when choosing what memory technology is right for your business needs.


Here are some numbers we’ve measured that show the actual comparisons.  We published numbers similar to this in the Gen8 configuration whitepaper, but these are new with 1866MHz. support.  I highlighted the two rows I was comparing for the 256GB system capacity.





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