OCDIMM: Scaling the DRAM Memory Wall Using WDM Based Optical Interconnects

We present OCDIMM (optically connected DIMM), a CPU-DRAM interface that takes advantage of multiwavelength optical interconnects. We show that OCDIMM has at least three key benefits when compared to alternatives such as FBDIMM (fully buffered DIMM), which is used in products from Sun (U. Nawathe et al., 2008) and Intel. First, replacing the multi-hop store-and-forward network in the FBDIMM architecture by a WDM (wavelength-division multiplexing) based optical interconnect results in significantly lower latency (up to 50% reduction in some configurations). Second, it is scalable to much higher capacities (such as 32 DIMMs per channel) with only a modest degradation in latency. Third, due to the higher data rate of an optical interface and the concurrency offered by multiple wavelengths, OCDIMM offers up to a 90% improvement in memory bandwidth. Most importantly, these benefits can be obtained using off-the-shelf DRAM devices, by making simple modifications to the DIMM circuit board and the memory controller.