The scheduler is the component of the kernel that selects which process to run next. The scheduler (or process scheduler, as it is sometimes called) can be viewed as the code that divides the finite resource of processor time between the runnable processes on a system. The scheduler is the basis of a multitasking operating system such as Linux. By deciding what process can run, the scheduler is responsible for best utilizing the system and giving the impression that multiple processes are simultaneously executing.
The idea behind the scheduler is simple. To best utilize processor time, assuming there are runnable processes, a process should always be running. If there are more processes than processors in a system, some processes will not always be running. These processes are waiting to run. Deciding what process runs next, given a set of runnable processes, is a fundamental decision the scheduler must make.
Multitasking operating systems come in two flavors: cooperative multitasking and preemptive multitasking. Linux, like all Unix variants and most modern operating systems, provides preemptive multitasking. In preemptive multitasking, the scheduler decides when a process is to cease running and a new process is to resume running. The act of involuntarily suspending a running process is called preemption. The time a process runs before it is preempted is predetermined, and is called the timeslice of the process. The timeslice, in effect, gives each process a slice of the processor's time. Managing the timeslice enables the scheduler to make global scheduling decisions for the system. It also prevents any one process from monopolizing the system. As we will see, this timeslice is dynamically calculated in the Linux scheduler to provide some interesting benefits.
Conversely, in cooperative multitasking, a process does not stop running until it voluntary decides to do so. The act of a process voluntarily suspending itself is called yielding. The shortcomings of this approach are numerous: The scheduler cannot make global decisions regarding how long processes run, processes can monopolize the processor for longer than the user desires, and a hung process that never yields can potentially bring down the entire system. Thankfully, most operating systems designed in the last decade have provided preemptive multitasking, with Mac OS 9 and earlier being the most notable exceptions. Of course, Unix has been preemptively multitasked since the beginning.
During the 2.5 kernel series, the Linux kernel received a scheduler overhaul. A new scheduler, commonly called the O(1) scheduler because of its algorithmic behavior1, solved the shortcomings of the previous Linux scheduler and introduced powerful new features and performance characteristics. In this section, we will discuss the fundamentals of scheduler design and how they apply to the new O(1) scheduler and its goals, design, implementation, algorithms, and related system calls.