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

LRDIMM, Server System Friend

By Chao Xu on 07-29-2011 at 5:23 PM

In previous LRDIMM Files, Doug Daniels gave a good introduction of LRDIMM and its basic features. I will focus more about LRDIMM unique advantages on server systems and talk about how LRDIMM dramatically increases server system memory capacity while simplifying server system design.   

The days of Billy boy saying 640K memory are long gone. Most of the desktops and laptops go with 4GB of memory and some with 8GB or 16GB. Memory is going up, up and up. This is just the scenario of PCs.  In the world of computer hardware, servers are sexy. They're more profitable than desktop and notebook PCs.  Memory plays a critical role in server systems as rapidly increasing data traffic drives demand for higher memory capacities. 

Today, typical DDR3 registered DIMMs (RDIMMs) get 8GB (Giga Byte) per module by using 2Gb x4 (Giga bit) DRAM dies and dual- rank configuration. By using dual registers on module, it can double the number ranks and give you 16GB capacity. But there is no free lunch in this world. 16GB quad- rank RDIMM also doubles the command/address loadings (capacitors) and data loadings (capacitors) to the memory controller. Due to this double loading effects, quad-rank RDIMM can only run at the lower speed. So server designers are constantly facing the trade-off between performance (speed) and capacity (loadings) by using RDIMMs. Then here comes our server memory friend – LRDIMMs.

LRDIMMs are a JEDEC standard. LR stands for Load Reduced, and that's exactly what the buffer on the DIMM does. Basically the memory buffer (MB) on the DIMM modules faked a single “large” DRAM chip with a heap of smaller ones. The effect of this? You can make a 32GB LRDIMM from 144 2Gbx4 DRAM chips (Octal-rank x4 ECC module, JEDEC R/C-E LRDIMM) or from 72 4Gbx4 DRAM chips (Quad-rank x4 ECC module, JEDEC R/C-C LRDIMM) and the system only sees 1 x 32GB DRAM chip. This gives the server memory designers a lot of freedom to optimize the system performance, cost without scarifying the memory capacity. They can choose cheaper 2Gbx4 DRAM chips instead of 4Gbx4 DRAM chips and get the same 32GB capacity module. And the memory controller sees the same electrical loadings. This makes the life of the memory controller much easier so you don’t need to worry about the details behind the memory buffer.  That is exactly what a friend does, make your life easier!

By using LRDIMMs, servers can easily double, triple, quadruple memory capacities on current systems compared with traditional RDIMMs. A current dual CPU server can house up to 12 32GB DRAM modules, which would provide some 384GB of memory, doubling the previous capacity of 192GBs per server. A next generation dual CPU server which will be launched later this year can house up to 24 64GB DRAM modules, which would provide some 1.5TB of memory.  This much memory will definitely speed up your Facebook activities with a blink of an eye.

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