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AMD touts LRDIMM memory for x86 servers

Pushing up to 384GB per socket

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It looks like Advanced Micro Devices is first to market with support for load reduced DIMM DDR3 main memory for x86 and quite possibly all kinds of servers, and is trotting out Inphi, the maker of the isolation memory buffer chip that is at the heart of this technology.

With LRDIMM memory, you take out the register on a DDR3 memory module and plunk in a buffer chip that allows the memory chips on the module to run at a higher clock speed and also allows for more memory chips to be put on each channel.

When the Opteron 4200s and 6200s were launched last November, AMD said that it would support unregistered, registered, and load reduced DDR3 memory modules with the chips. To add LRDIMM support, the main memory controllers on the Opterons had to be tweaked so they could talk this new chatter and support the higher clock speeds on the memory. What might not have been obvious at the time was that the extra memory that AMD was offering was only available through the use of LRDIMM memory.

As it stands, says John Fruehe, director of product marketing for server, embedded, amd FireStream products at AMD, if you use unregistered DIMMs, you top out at 64GB per socket, and registered DIMMs are at the 256GB per socket upper limit, twice the maximum memory as the prior generation of Opteron 6100 processors. By moving to LRDIMMs, you can boost that memory capacity to 384GB per socket without having to resort to special memory extender electronics that add costs to the server.

At the moment, AMD has put the Inphi iMB buffer chip through the testing paces and has certified it as compliant with its Opteron 6200s. While the Opteron 4200s can technically support LRDIMM memory, AMD doesn't expect customers to do that. But they might, given the low-power server buyers and builders that like the Opteron 4200 chips and that for a given amount of memory, LRDIMMs burn less juice.

Paul Washkewicz, vice president of marketing at Inphi, tells El Reg that if you compare a 1.5 volt RDIMM with 16GB of capacity, you can move to a 1.35 volt LRDIMM with 32GB of capacity and burn 20 per cent less juice. And Washkewicz says that Samsung, Elpida, Hynix, and Micron are getting close to price parity between RDIMM and LRDIMM memory at identical capacity, so you don't have to pay a big premium, either.

Intel is expected to support LRDIMM memory in its forthcoming "Sandy Bridge-EP" Xeon E5 processors, which launch this quarter. ®

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