Configuring and using DDR3 memory with HP ProLiant Gen8 Servers
16
• CAS latency. CAS (Column Address Strobe) latency represents the basic DRAM response time. It is specified as the
number of clock cycles (e.g. 6, 7, 11) that the controller must wait after asserting the Column Address signal before
data is available on the bus. CAS latency plays a larger role in determining the unloaded latency than loaded latency.
Figure 7 shows both unloaded and loaded latency numbers for various DDR3 DIMMs when used in one DIMM per channel
configurations. As this chart illustrates, the idle latency is almost the same for every DIMM type and capacity. This is
because the primary component of idle latency is the memory system overhead of performing a basic memory read or
write operation, which is the same for all DIMM types regardless of their speed.
Figure 7. Idle and Loaded Latencies for various DDR3 DIMMs on 2P HP ProLiant Gen8 servers
Factors influencing memory throughput
Factors affecting memory throughput include the number of memory channels populated, number of ranks on the
channel, channel interleaving, and the speed at which the memory runs.
Number of memory channels and throughput
The largest impact on throughput is the number of memory channels populated. By interleaving memory access across
multiple memory channels, the integrated memory controllers are able to increase memory throughput significantly.
Optimal throughput and latency are achieved when all channels of each installed CPU are populated identically.
As Figure 8 shows, adding a second DIMM to the system (and thus populating the second memory channel) essentially
doubles system read throughput. Gains in throughput for each additional DIMM installed are almost linear until all eight
memory channels are populated.
0
20
40
60
80
100
120
140
160
2 GB
1Rx8
1333 U
4GB
2Rx8
1333 U
8GB
2Rx8
1333 U
4GB
1Rx4
1333 R
4GB
1Rx4
1600 R
8GB
2Rx4
1333 R
8GB
1Rx4
1600 R
16GB
2Rx4
1333 R
16GB
2Rx4
1600 R
32GB
4Rx4
1333 L
Latency (ns)
Latency by DIMM type and capacity at 1DPC
HP ProLiant Gen8 servers
Idle Latency
Loaded Latency