Essential System Administration, 3rd Edition

Managing CPU Resources

CPU usage is usually the first factor that I consider when I am tracking down a performance problem or just trying to assess the current system state in general.^[10]

Nice Numbers and Process Priorities

Most Unix systems use a priority-based round-robin scheduling algorithm to distribute CPU resources among multiple competing processes. All processes have an execution priority assigned to them, an integer value that is dynamically computed and updated on the basis of several factors. Whenever the CPU is free, the scheduler selects the most favored process to begin or resume executing; this usually corresponds to the process with the lowest priority number, because lower numbers are defined as more favored than higher ones in typical implementations.

Although there may be a multitude of processes simultaneously present on the system, only one process actually uses the CPU processor at any given time (assuming the system has only a single CPU). Once a process begins running, it continues to execute until it needs to wait for an I/O operation to complete, receives an interrupt from the kernel, or otherwise gives up control of the CPU, or until it exhausts the maximum execution time slice (or quantum ) defined on that system (10 milliseconds is a common value). Once the current process stops executing, the scheduler again selects the most favored process on the system and starts or resumes it. The process of changing the current running process is called a ...