Webcast of HP, Samsung, ANSYS, Intel and Inphi presentation at IDF 2011 for HPC applications
 

LRDIMM, Memory System Virtualization

By Chao Xu on 09-30-2011 at 2:13 PM

LRDIMM was one of biggest highlights at IDF this year. As mentioned in an Executive Brief titled, "Managing Memory Bandwidth Scaling Challenges with LRDIMM and Memory Buffering," "LRDIMM benefits a broad segment of the capacity enterprise server and workstation market in terms of higher capacity with performance and cost optimization, while allowing organizations of all sizes to take full advantage of multi-core and virtualization technologies". In summary, LRDIMM provides the richest features for memory system virtualization.

Cloud computing makes productivity easier by giving everyone endless ways to work and collaborate from anywhere at any time and on any device. I can own way less and do way more. Virtualization is one of the key technologies to enable the computing cloud. LRDIMM technology gives the same power for enterprise servers to implement memory system virtualizations.

The enterprise server memory systems are mainly determined by two interface technologies. One is the memory controller interface, i.e. host interface. The other is the DRAM interface. Usually the host interface is “matched” with the DRAM interface in different DRAM generations, such as DDR2, DDR3 etc. But this “match” is only good for a certain amount of time. These two interfaces are developed in different pace and eventually they are out of sync. LRDIMM technology will bridge this gap and separate the host interface from the DRAM interface and virtualizes the server memory system!

Enough talk about the concepts. Let me give you some concrete examples. One of LRDIMM features is called “rank multiplication.” Usually the host memory controller can only supply limited number of “chip select” signals in each generation due to package pins limitation. This “chip select” signals determine how many physical memory ranks can be installed on the server memory system, i.e. memory capacity. Due to DRAM technology advances, more memory chip dies can be stacked in one package to increase memory capacity. The number of ranks in one package chip is growing faster than that host memory controller can access. LRDIMM memory buffers hide the DRAM interface from the host interface and implements the host interface as small number of ranks, while implementing the DRAM interface as 2x or 4x number of ranks, i.e., virtualization of host interface from DRAM physical interface.

Another very useful feature of LRDIMM technology is called “Nibble Merge”. There are two main different DRAM interface. One is “x4” and the other is “x8”. “x4” DRAM chip has one differential strobe signal pair controls 4 data lines. “x8” DRAM chip has one differential strobe signal pair controls 8 data lines. There are different attributes for each technology. Different applications need different interfaces. In RDIMM, after the memory module is manufactured, the host memory controller has no choice to select “x4” or “x8” and has to “match” the DRAM interface. In LRDIMM technology, memory buffers separate the host interface from the DRAM interface. Memory buffer implements the host interface which can be “x4” or “x8” regardless of the DRAM interface and selectable by a register bit. This feature provides flexibility for the system memory controller to optimize the system power and performance based on applications.

There are many additional unique and intelligent features in LRDIMM memory buffers, such as Inphi’s iMB™, which is not only fully compliant with JEDEC standard, but also has rich proprietary features which can be used in the cloud computing and virtualizations!

Return to top