The challenges for the FB-DIMM architecture include latency as well as power and thermal load.
The FB-DIMM architecture adds two types of latency, serialization latency and transmission latency. Serialization latency occurs when data is processed by the AMB; therefore, it is necessary. Transmission latency is measured from the time a read request is initiated to the time the memory controller receives the first frame of read data. Because the FB-DIMM architecture is based on the point-to-point interconnection of AMBs, read requests must travel through n-1 AMBs before reaching the nth AMB. As more DIMMs are added to the channel, the total transmission latency increases. Each FB-DIMM can add 2 to 6 nanoseconds (ns) of transmission latency; therefore, the cumulative transmission latency can be significant in a fully-scaled system. The system adjusts timing to make sure that all DIMMs in a channel share the same latency.
In the future, transmission latency may be minimized by using a technique called variable read latency. With variable latency capability, the round-trip latency from each FB-DIMM is dependent on its distance from the memory controller. This capability will allow latency to be reduced for DIMMs closer to the memory controller.
Power and thermal loads
An FB-DIMM consumes almost 5 Watts more than a typical registered DDR2 DIMM. For servers that support up to 16 FB-DIMMs, this has the potential to increase power consumption by as much as 80 W. however, the actual overall power consumption must be evaluated at the system level. The operation of the AMB also causes the FB-DIMM to get hotter. Therefore a heat spreader is required (Figure 7) to help draw heat away from the FB-DIMM and allow it to be cooled more efficiently by the server's internal fans.
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