IBM BladeCenter HS22 Product Manual - page 7
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7.
This new processor design comes with some trade-offs in memory capacity, performance, and cost: For
example, more lower-cost/lower-capacity DIMMs mean lower memory speed. Alternatively, fewer higher-
capacity DIMMs cost more but offer higher performance.
Regardless of memory speed, the Xeon 5500 platform represents a significant improvement in memory
bandwidth over the previous Xeon 5400 platform. At 1333MHz, the improvement is almost
500% over the
previous generation. This huge improvement is mainly due to the dual integrated memory controllers and
faster DDR-3 1333MHz memory. Throughput at 800MHz is
25 gigabytes per second (GBps); at
1066MHz it’s
32GBps; and at 1333MHz it’s 35GBps. This improvement translates into improved
application performance and scalability.
Memory interleaving refers to how physical memory is interleaved across the physical DIMMs. A
balanced system provides the best interleaving. A Xeon 5500 processor-based system is balanced when
all memory channels on a socket have the same amount of memory.
In the case of HS22, which has a balanced DIMM layout, it is easy to balance the system for all three
memory frequencies. The recommended DIMM population is shown below, assuming DIMMs with
identical capacities.
Desired Memory Speed
DIMMs per Channel
DIMM Slots to Populate
1333MHz
1
2,4,6,8,10, and 12
1066MHz
2
All slots
800MHz
2
All slots;
clock memory speed to 800MHz
in BIOS
A memory rank is simply a segment of memory that is addressed by a specific address bit. DIMMs
typically have 1, 2 or 4 memory ranks, as indicated by their size designation.
A typical memory DIMM description is
2GB 4Rx8 DIMM
The 4R designator is the rank count for this particular DIMM (R for rank = 4)
The x8 designator is the data width of the rank
It is important to ensure that DIMMs with appropriate number of ranks are populated in each channel for
optimal performance. Whenever possible,
it is recommended to use dual-rank DIMMs in the system.
Dual-rank DIMMs offer better interleaving and hence better performance than single-rank DIMMs. For
instance, a system populated with six 2GB dual-rank DIMMs outperforms a system populated with six
2GB single-rank DIMMs by
7% for SPECjbb2005. Dual-rank DIMMs are also better than quad-rank
DIMMs because
quad-rank DIMMs will cause the memory speed to be down-clocked.
Another important guideline is to populate equivalent ranks per channel. For instance,
mixing one
single-rank DIMM and one dual-rank DIMM in a channel should be avoided.
Note: It is important to ensure that all three memory channels in each processor are populated. The
relative memory bandwidth decreases as the number of channels populated decreases. This is because
the bandwidth of all the memory channels is utilized to support the capability of the processor. So, as the
channels are decreased, the burden to support the requisite bandwidth is increased on the remaining
channels, causing them to become a bottleneck.
For peak performance:
Always populate both processors with equal amounts of memory to ensure a balanced NUMA system.
Always populate all 3 memory channels on each processor with equal memory capacity.
Ensure an even number of ranks are populated per channel.
Use dual-rank DIMMs whenever appropriate.
For optimal
1333MHz performance, populate 6 dual-rank DIMMs (3 per processor).
For optimal
1066MHz performance, populate 12 dual-rank DIMMs (6 per processor).
For optimal
800MHz performance with high DIMM counts, populate 12 dual-rank or quad-rank DIMMs
(6 per processor) and clock memory down to 800MHz in BIOS.
With the above rules, it is not possible to have a performance-optimized system with 4GB, 8GB,
16GB, or 128GB. With 3 memory channels and interleaving rules, configure systems with 6GB, 12GB,
18GB, 24GB, 48GB, 72GB, 96GB, etc., for optimized performance.
Power guidelines:
Fewer larger DIMMs (for example 6 x 4GB DIMMs vs. 12 x 2GB DIMMs will generally have lower
power requirements
x8 DIMMs (x8 data width of rank) will generally draw less power than equivalently sized x4 DIMMs