Challenges; Latency; Power And Thermal Loads - HP 226824-001 - ProLiant - ML750 Introduction Manual

Challenges

The challenges for the FB-DIMM architecture include latency as well as power and thermal load.

Latency

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.
To estimate the power consumption for all current HP ProLiant servers that support FB-DIMMs, use the
power calculators at
http://www.hp.com/configurator/calc/Power Calculator Catalog.xls.
Figure 7. FB-DIMM with full module heat spreader
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