Challenges
The challenges for the FB-DIMM architecture include latency and power use (thermal load).
Memory latency is the delay from the time the data is requested to the time when the data is available
from the memory controller. The FB-DIMM architecture increases this latency in two ways: serialization
and transmission. Serialization latency is added to memory access when the data is serialized prior to
transmission, and then de-serialized after it reaches its destination. Additionally, the transmission
latency, which exists for all memory technologies, increases due to the point-to-point interconnection
of AMBs. 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.
An FB-DIMM consumes almost 5 Watts more than a typical registered DDR2 DIMM. The operation of
the AMB also causes the FB-DIMM to get hotter. Therefore, a heat spreader is required to help draw
heat away from the FB-DIMM so it can be cooled more efficiently by the server's internal fans
(Figure 16). These concerns are driving the design of AMBs that use 15 to 20 percent less power,
even though some manufacturers claim up to 40 percent power reduction.
Figure 16. FB-DIMM with full module heat spreader
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