Context switch

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",^[1] 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.

Cost

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.^[2][3] 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.

When to switch?

There are three potential triggers for a context switch:

Multitasking

Most commonly, within some scheduling scheme, one process must be switched out of the CPU so another process can run. This context switch can be triggered by the process making itself unrunnable, such as by waiting for an I/O or synchronization operation to complete. On a pre-emptive multitasking system, the scheduler may also switch out processes which are still runnable. To prevent other processes from being starved of CPU time, preemptive schedulers often configure a timer interrupt to fire when a process exceeds its time slice. This interrupt ensures that the scheduler will gain control to perform a context switch.

Interrupt handling

Modern architectures are interrupt driven. This means that if the CPU requests data from a disk, for example, it does not need to busy-wait until the read is over; it can issue the request and continue with some other execution. When the read is over, the CPU can be interrupted and presented with the read. For interrupts, a program called an interrupt handler is installed, and it is the interrupt handler that handles the interrupt from the disk.

When an interrupt occurs, the hardware automatically switches a part of the context (at least enough to allow the handler to return to the interrupted code). The handler may save additional context, depending on details of the particular hardware and software designs. Often only a minimal part of the context is changed in order to minimize the amount of time spent handling the interrupt. The kernel does not spawn or schedule a special process to handle interrupts, but instead the handler executes in the (often partial) context established at the beginning of interrupt handling. Once interrupt servicing is complete, the context in effect before the interrupt occurred is restored so that the interrupted process can resume execution in its proper state.

User and kernel mode switching

When a transition between user mode and kernel mode is required in an operating system, a context switch is not necessary; a mode transition is not by itself a context switch. However, depending on the operating system, a context switch may also take place at this time.

Steps

In a switch, the state of the first process (assuming that the first process is the process in execution and is to be switched) must be saved somehow, so that, when the scheduler gets back to the execution of the first process, it can restore this state and continue.

The state of the process includes all the registers that the process may be using, especially the program counter, plus any other operating system specific data that may be necessary. This data is usually stored in a data structure called a process control block (PCB), or switchframe.

In order to switch processes, the PCB for the first process must be created and saved. The PCBs are sometimes stored upon a per-process stack in kernel memory (as opposed to the user-mode call stack), or there may be some specific operating system defined data structure for this information.

Since the operating system has effectively suspended the execution of the first process, it can now load the PCB and context of the second process. In doing so, the program counter from the PCB is loaded, and thus execution can continue in the new process. New processes are chosen from a queue or queues (often referred as ready queue). Process and thread priority can influence which process continues execution, with processes of the highest priority checked first for ready threads to execute.

Performance

Context switching itself has a cost in performance, due to running the task scheduler, TLB flushes, and indirectly due to sharing the CPU cache between multiple tasks.^[4] Switching between threads of a single process can be faster than between two separate processes, because threads share the same virtual memory maps, so a TLB flush is not necessary.^[5]

Hardware vs. software

Context switching can be performed primarily by software or hardware. Some processors, like the Intel 80386 and its successors,^[6] have hardware support for context switches, by making use of a special data segment designated the task state segment or TSS. A task switch can be explicitly triggered with a CALL or JMP instruction targeted at a TSS descriptor in the global descriptor table. It can occur implicitly when an interrupt or exception is triggered if there's a task gate in the interrupt descriptor table. When a task switch occurs the CPU can automatically load the new state from the TSS.

As with other tasks performed in hardware, one would expect this to be rather fast; however, mainstream operating systems, including Windows and Linux,^[7] do not use this feature. This is mainly due to two reasons:

• Hardware context switching does not save all the registers (only general purpose registers, not floating point registers — although the TS bit is automatically turned on in the CR0 control register, resulting in a fault when executing floating point instructions and giving the OS the opportunity to save and restore the floating point state as needed).
• Associated performance issues, e.g., software context switching can be selective and store only those registers that need storing, whereas hardware context switching stores nearly all registers whether they are required or not.

References

External links

• Context Switching - at OSDev.org
• Context Switch Definition - by The Linux Information Project (LINFO)
• Context Switches - from the Microsoft Developer Network (MSDN)
• General Architecture and Design -Interrupt Handling at FreeBSD.org