IBM PureFlex System and IBM Flex System Products and Technology

207

Table 5-51 shows the required placement of memory DIMMs for the p260 and the p24L,

depending on the number of DIMMs installed.

Table 5-51 DIMM placement: p260 and p24L

Use of mixed DIMM sizes

All installed memory DIMMs do not have to be the same size. However, keep the following

groups of DIMMs the same size:

򐂰 Slots 1-4

򐂰 Slots 5-8

򐂰 Slots 9-12

򐂰 Slots 13-16

5.4.9 Active Memory Expansion

The optional Active Memory Expansion feature is a POWER7 technology that allows the

effective maximum memory capacity to be much larger than the true physical memory.

Applicable to AIX 6.1 or later, this innovative compression and decompression of memory

content using processor cycles allows memory expansion of up to 100%.

This memory expansion allows an AIX 6.1 or later partition to do more work with the same

physical amount of memory. Conversely, a server can run more partitions and do more work

with the same physical amount of memory.

Active Memory Expansion uses processor resources to compress and extract memory

contents. The trade-off of memory capacity for processor cycles can be an excellent choice.

However, the degree of expansion varies based on how compressible the memory content is.

Have adequate spare processor capacity available for the compression and decompression.

Tests in IBM laboratories using sample workloads showed excellent results for many

workloads in terms of memory expansion per additional processor used. Other test workloads

had more modest results.

Number of DIMMs

Processor 0 Processor 1

DIMM 1

DIMM 2

DIMM 3

DIMM 4

DIMM 5

DIMM 6

DIMM 7

DIMM 8

DIMM 9

DIMM 10

DIMM 11

DIMM 12

DIMM 13

DIMM 14

DIMM 15

DIMM 16

2 x x

4

x x x x

6

x x x x x x

8

x x x x x x x x

10

x x x x x x x x x x

12

x x x x x x x x x x x x

14

x x x x x x x x x x x x x x

16

x x x x x x x x x x x x x x x x

208 IBM PureFlex System and IBM Flex System Products and Technology

You have a great deal of control over Active Memory Expansion usage. Each individual AIX

partition can turn on or turn off Active Memory Expansion. Control parameters set the amount

of expansion wanted in each partition to help control the amount of processor used by the

Active Memory Expansion function. An IBM Public License (IPL) is required for the specific

partition that is turns memory expansion on or off. After being turned on, there are monitoring

capabilities in standard AIX performance tools, such as lparstat, vmstat, topas, and svmon.

Figure 5-40 represents the percentage of processor used to compress memory for two

partitions with various profiles. The green curve corresponds to a partition that has spare

processing power capacity. The blue curve corresponds to a partition constrained in

processing power.

Figure 5-40 Processor usage versus memory expansion effectiveness

Both cases show a knee of the curve relationship for processor resources required for

memory expansion:

򐂰 Busy processor cores do not have resources to spare for expansion.

򐂰 The more memory expansion that is done, the more processor resources are required.

The knee varies, depending on how compressible the memory contents are. This variability

demonstrates the need for a case by case study to determine whether memory expansion

can provide a positive return on investment. To help you perform this study, a planning tool is

included with AIX 6.1 Technology Level 4 or later. The tool allows you to sample actual

workloads and estimate both how expandable the partition memory is and how much

processor resource is needed. Any Power System model runs the planning tool.

% CPU

utilization

for

expansion

Amount of memory expansion

1 = Plenty of spare

CPU resource available

2 = Constrained CPU

resource – already

running at significant

utilization

1

2

Very cost effective