DDR3 memory technology

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roughly the same for both DDR2 and DDR3 memory. This means that 27 to 28 nanoseconds of
memory latency are relatively fixed. Any improvements in memory latency must come elsewhere.
DDR3 achieves improvements in latency through its faster data rate and the elimination of Fully
Buffered DIMMs.
There are two different measurements of memory latency for a system— unloaded latency and loaded
latency.
• Unloaded latency is the fastest possible time for the processor to retrieve data from the memory
subsystem. It is measured when the system is idle. The timing and electrical properties of the
memory subsystem determine unloaded latency.
• Loaded latency is a measurement of the latency when the memory subsystem is saturated with
memory requests. With loaded latency, many additional factors come into play, including the
number of memory controllers in the memory subsystem; controller efficiency in handling queued
requests; and memory interleaving. Loaded latency gives a truer representation of the memory
subsystem’s capabilities in real-world environments.
Table 8 compares the unloaded and loaded latencies of DDR2 and DDR3 memory in Intel-based 2P
ProLiant servers.
Table 8: Memory latency of DDR2 and DDR3 in Intel-based ProLiant servers
2P ProLiant G5
DDR2 at 667 MT/s
2P ProLiant G6
DDR3 at 800 MT/s
2P ProLiant G6
DDR3 at 1333 MT/s
2P ProLiant Gen8
DDR3 at 1600 MT/s
Unloaded latency
126 ns 80 ns 70 ns 65 ns
Loaded latency
147 ns
140 ns
100 ns
124 ns
Achieving highest performance with DDR3 memory
DDR3 memory delivers improved performance over DDR2 memory. With the NUMA architectures, the
way you install DDR3 DIMMs in the system is important.
Maximizing system throughput
The key to maximizing system throughput is to populate as many of the system memory channels as
possible. This helps to ensure that the memory bandwidth of all channels is available to the system.
With 2P ProLiant G6 servers based on the Intel Xeon 5500 series processors, this means installing a
minimum of six DIMM modules, one in each memory channel.
Minimizing memory latency
You can optimize memory latency, particularly loaded memory latency, by running at the highest
data rate. For systems that are capable of supporting the higher data rates, achieving this memory
speed depends on the number of and the rank of the DIMMs installed in each channel.
Using balanced memory configurations
For nearly all application environments, the optimal configuration for DDR3 memory is to balance
installed memory across memory channels and across processors. Balancing installed memory across
memory channels on a processor optimizes channel and rank interleaving, ensuring maximum
memory throughput.