IBM eX5 Implementation Guide

226 IBM eX5 Implementation Guide

processing nodes in a high-performance cluster, database servers, or print servers for

graphics printers require the best memory performance possible.

Remember these considerations when installing memory in the System x3850 X5:

򐂰 Due to the fact that no single processor has direct access to all PCIe slots, the QPI links

that are used to communicate between the two processors are not only used for memory

calls, but also for interfacing to PCIe adapters.

򐂰 When installing memory for multiple processors and not all memory cards are installed,

only processing threads that are assigned to processor without local memory experience

a 50% increase in memory latency. For a server with heavy I/O processing, the additional

memory traffic also affects the efficiency of addressing PCIe adapters.

򐂰 Any memory that is installed on memory cards for an uninstalled or defective processor is

not seen by the other processors on the server.

򐂰 For nonuniform memory access (NUMA)-aware operating systems when multiple

processors are installed, you must install the same amount of memory on each memory

card of each installed processor. See 3.8.2, “DIMM population sequence” on page 79 for

details.

򐂰 You can achieve the best processor performance when memory is installed to support

Hemisphere Mode. Hemisphere Mode is the required memory configuration to allow two

3850 X5 servers to scale. It is possible to have processor 1 and 4 in Hemisphere Mode,

and not processor 2 and 3, to permit scaling. In this type of installation, having processor 1

and 4 in Hemisphere Mode improves the memory access latency for all processors. To

determine the DIMM population for Hemisphere Mode, see 3.8.2, “DIMM population

sequence” on page 79.

򐂰 You might consider installing all DIMMs of the same field-replaceable unit (FRU) to avoid

conflicts that might prevent NUMA compliance or Hemisphere Mode support. These

problems are most likely to occur when multiple DIMM sizes are installed. The server

supports installing multiple-sized DIMMs in each rank, but the configuration becomes

complex and difficult to maintain. Operating systems that depend on NUMA compliance

inform you when the server is not NUMA-compliant. However, nothing informs you that the

processors are not in Hemisphere Mode.

򐂰 It is better to install more smaller DIMMs than fewer larger DIMMs to ensure that all of the

memory channels and buffers of each processor have access to the same amount of

memory. This approach allows the processors to fully use their interleave algorithms to

access memory faster by spreading access over multiple paths.

6.3.1 Testing the memory DIMMs

The DIMM components on a server are the most sensitive to static discharge. Improper

handling of memory is the most likely cause of DIMM failures. Working in an extremely dry

location dramatically increases the possibility that you will build up a static charge. Always

use an electrostatic discharge (ESD) strap connected between you and a grounding point to

reduce static buildup.

The best practice when installing memory is to run a memory quick test in diagnostics to

ensure that all of the memory is functional. The following reasons describe why memory

might not be functional:

򐂰 Wrong DIMM for the type of server that you have. Ensure that only IBM-approved DIMMs

are installed in your server.

򐂰 The DIMM is not fully installed. Ensure that the DIMM clips are in the locked position to

prevent the DIMM from pulling out of its slot.