While your computer is running, counters in the operating system are incremented to track various system activities.
System activities that are tracked are as follows:
Central processing unit (CPU) utilization
Buffer usage
Disk and tape input/output (I/O) activity
Terminal device activity
System call activity
Context switching
File access
Queue activity
Kernel tables
Interprocess communication
Paging
Free memory and swap space
Kernel memory allocation (KMA)