Context switching

In computing, a context switch is the process of storing and restoring the state (more specifically, the execution context) of a process or thread so that execution can be resumed from the same point at a later time. This enables multiple processes to share a single CPU and is an essential feature of a multitasking operating system.

The precise meaning of "context switch" varies significantly in usage, most often to mean "thread switch or process switch" or "process switch only", either of which may be referred to as a "task switch". More finely, one can distinguish thread switch (switching between two threads within a given process), process switch (switching between two processes), mode switch (domain crossing: switching between user mode and kernel mode within a given thread), register switch, a stack frame switch, and address space switch (memory map switch: changing virtual memory to physical memory map). The computational cost of context switches varies significantly depending on what precisely it entails, from little more than a subroutine call for light-weight user processes, to very expensive, though typically much less than that of saving or restoring a process image.

Context switches are usually computationally intensive, and much of the design of operating systems is to optimize the use of context switches. Switching from one process to another requires a certain amount of time for doing the administration – saving and loading registers and memory maps, updating various tables and lists, etc. What is actually involved in a context switch varies between these senses and between processors and operating systems. For example, in the Linux kernel, context switching involves switching registers, stack pointer, and program counter, but is independent of address space switching, though in a process switch an address space switch also happens. Further still, analogous context switching happens between user threads, notably green threads, and is often very light-weight, saving and restoring minimal context. In extreme cases, such as switching between goroutines in Go, a context switch is equivalent to a coroutine yield, which is only marginally more expensive than a subroutine call.

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