Chapter 7. Monitoring and Tuning Memory Use

The memory of a RS/6000 is almost always full of something. If the currently executing programs don't take up all of memory, AIX retains in memory the text pages of programs that ran earlier and the files they used. It doesn't cost anything, because the memory would be unused anyway. In many cases, the program or file pages will be used again, which reduces disk I/O.

This caching technique improves the efficiency of the system but can make it harder to determine the actual memory requirement of a workload.

This chapter describes the ways in which memory use can be measured and modified. It contains the following major sections:

• "How Much Memory Is Really Being Used" lists the commands that report memory use and gives their advantages and disadvantages.
• "Memory-Leaking Programs" describes one of the most common causes of memory overcommitment.
• "Analyzing Patterns of Memory Use with BigFoot" describes the features and use of the BigFoot tool, which reports patterns of memory reference.
• "Assessing Memory Requirements via the rmss Command" documents the techniques for using the Reduced-Memory System Simulator tool.
• "Tuning VMM Memory Load Control" describes the situations in which such tuning might be appropriate and the methods of tuning VMM memory load control with the schedtune command.
• "Tuning VMM Page Replacement" explains the methods and effects of changing the VMM page-replacement thresholds.

Readers who are not familiar with AIX virtual-memory management may want to look at "Performance Overview of the Virtual Memory Manager (VMM) " before continuing.