If there is one area where a wealth of performance statistics can be found, it is in the area of monitoring memory utilization. Due to the inherent complexity of today's demand-paged virtual memory operating systems, memory utilization statistics are many and varied. It is here that the majority of a system administrator's work with resource management takes place.
The following statistics represent a cursory overview of commonly-found memory management statistics:
- Page Ins/Page Outs
These statistics make it possible to gauge the flow of pages from system memory to attached mass storage devices (usually disk drives). High rates for both of these statistics can mean that the system is short of physical memory and is thrashing, or spending more system resources on moving pages into and out of memory than on actually running applications.
- Active/Inactive Pages
These statistics show how heavily memory-resident pages are used. A lack of inactive pages can point toward a shortage of physical memory.
- Free, Shared, Buffered, and Cached Pages
These statistics provide additional detail over the more simplistic active/inactive page statistics. By using these statistics, it is possible to determine the overall mix of memory utilization.
- Swap Ins/Swap Outs
These statistics show the system's overall swapping behavior. Excessive rates here can point to physical memory shortages.
Successfully monitoring memory utilization requires a good understanding of how demand-paged virtual memory operating systems work. While such a subject alone could take up an entire book, the basic concepts are discussed in Chapter 4, Physical and Virtual Memory
. This chapter, along with time spent actually monitoring a system, gives you the the necessary building blocks to learn more about this subject.