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How to check for memory over-commitment in AME

Question

In LPARs that utilize the Power 7 (and later) feature of Active Memory Expansion (AME), assessing memory resources is a more complex task compared with dedicated memory systems. How can the memory in such a system be evaluated?

Answer

Introduction

Active Memory Expansion (AME) allows for the compression of memory pages to increase the system's effective virtual address space. At high usage, unused computational memory is moved to the compressed pool instead of being paged-out to paging space. This is typically employed in environments with excess CPU resources and are somewhat constrained on physical memory. Active Memory Expansion is feature that has been introduced in POWER7/POWER7+ systems with a minimum level of AIX 6.1 TL4 SP2.

AME Scenarios

After planning and configuring the system with the amepat tool, there are some scenarios that might require a change of AME configuration:

1. Virtual memory exceeds Target Memory Expansion Size


When this scenario is present, the system is over-committed and will start paging out to disk. From a configuration stand-point, rerun the amepat tool to either increase the Expansion Factor or to increase the size of physical memory.

2. Virtual memory exceeds assigned physical memory and less than Target Memory Expansion Size (with no deficit)


This is the ideal scenario when using AME as the compressed pool is able to satisfy the memory demands of the LPAR.

3. Virtual memory exceeds assigned physical memory and less than Target Memory Expansion Size (with deficit)


When the system is unable to compress memory pages to meet the Target Memory Expansion Size, there will be a deficit and pages that exceed the allocated memory are moved to paging space. Not all memory pages are subject to compression (pinned pages or client pages) and therefore, a deficit is present. Rerun the amepat tool to either decrease the Expansion Factor or to increase the size of physical memory.

4. Virtual memory is below assigned physical memory


While there isn't a problem with over-commitment with this setup, it is not benefiting from AME. Rerun the amepat tool to decrease the allocated physical memory and evaluate the current Expansion Factor.

Tools to use with a live example
The following tools on AIX can be used to determine the current status of an AME-enabled LPAR (with a live example from the IBM Redbook IBM PowerVM Virtualization Managing and Monitoring):

# amepat

1. Comparing the Virtual Memory Size (MB) to the Target Expanded Memory Size, we find that the system is not over-committed logically.
2. Due to the Deficit Memory Size (MB), the system will start utilizing paging space due to the inability to compress more memory.

# vmstat -c

Comparing the avm value (in 4k pages) to the tmem value (MB) will tell us if the system is logically over-committed.

1. Observing the dxm will show us the deficit in 4k pages.

# svmon -O summary=AME

Comparing the virtual column to the size column shows no issue with logical memory over-commitment.

1. The dxm column shows the deficit in 4k pages

For more information regarding AME, please refer to the IBM Redbook IBM PowerVM Virtualization Managing and Monitoring (sg247590):
http://www.redbooks.ibm.com/abstracts/sg247590.html

Source: IBM Technote