Understanding the Linux Kernel

10.1. Scheduling Policy

The scheduling algorithm of traditional Unix operating systems must fulfill several conflicting objectives: fast process response time, good throughput for background jobs, avoidance of process starvation, reconciliation of the needs of low- and high-priority processes, and so on. The set of rules used to determine when and how selecting a new process to run is called scheduling policy.

Linux scheduling is based on the time-sharing technique already introduced in Section 5.4.3 in Chapter 5: several processes are allowed to run "concurrently," which means that the CPU time is roughly divided into "slices," one for each runnable process.^[1] Of course, a single processor can run only one process at any given instant. If a currently running process is not terminated when its time slice or quantum expires, a process switch may take place. Time-sharing relies on timer interrupts and is thus transparent to processes. No additional code needs to be inserted in the programs in order to ensure CPU time-sharing.

^[1] Recall that stopped and suspended processes cannot be selected by the scheduling algorithm to run on the CPU.

The scheduling policy is also based on ranking processes according to their priority. Complicated algorithms are sometimes used to derive the current priority of a process, but the end result is the same: each process is associated with a value that denotes how appropriate it is to be assigned to the CPU.

In Linux, process priority is dynamic. ...