This article may be incomprehensible or very hard to understand.(October 2017)
In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point. This allows multiple processes to share a single central processing unit (CPU), and is an essential feature of a multitasking operating system.
The precise meaning of the phrase “context switch” varies. In a multitasking context, it refers to the process of storing the system state for one task, so that task can be paused and another task resumed. A context switch can also occur as the result of an interrupt, such as when a task needs to access disk storage, freeing up CPU time for other tasks. Some operating systems also require a context switch to move between user mode and kernel mode tasks. The process of context switching can have a negative impact on system performance.: 28
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 depends on the architectures, operating systems, and the number of resources shared (threads that belong to the same process share many resources compared to unrelated non-cooperating processes). For example, in the Linux kernel, context switching involves switching registers, stack pointer (it's typical stack-pointer register), program counter, flushing the translation lookaside buffer (TLB) and loading the page table of the next process to run (unless the old process shares the memory with the new). Furthermore, analogous context switching happens between user threads, notably green threads, and is often very lightweight, 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.
There are three potential triggers for a context switch:
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 that are still runnable. To prevent other processes from being starved of CPU time, pre-emptive 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.
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 (to the I/O device) and continue with some other task. When the read is over, the CPU can be interrupted (by a hardware in this case, which sends interrupt request to PIC) 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 the system transitions between user mode and kernel mode, 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.
In a switch, the state of the process currently executing must be saved somehow, so that when it is rescheduled, this state can be restored.
The process state 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 is usually stored in a data structure called a process control block (PCB) or switchframe.
The PCB might be stored on 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. A handle to the PCB is added to a queue of processes that are ready to run, often called the ready queue.
Since the operating system has effectively suspended the execution of one process, it can then switch context by choosing a process from the ready queue and restoring its PCB. In doing so, the program counter from the PCB is loaded, and thus execution can continue in the chosen process. Process and thread priority can influence which process is chosen from the ready queue (i.e., it may be a priority queue).
Considering a general arithmetic addition operation A = B+1. The instruction is stored in the instruction register and the program counter is incremented. A and B are read from memory and are stored in registers R1, R2 respectively. In this case, B+1 is calculated and written in R1 as the final answer. This operation as there are sequential reads and writes and there's no waits for function calls used, hence no context switch/wait takes place in this case.
However, certain special instructions require system calls that require context switch to wait/sleep processes. A system call handler is used for context switch to kernel mode. A display(data x) function may require data x from the Disk and a device driver in kernel mode, hence the display() function goes to sleep and waits on the READ operation to get the value of x from the disk, causing the program to wait and a wait for function call to the released setting the current statement to go to sleep and wait for the syscall to wake it up. To maintain concurrency however the program needs to re-execute the new value and the sleeping process together again.
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. 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.
The time to switch between two separate processes is called the process switching latency. The time to switch between two threads of the same process is called the thread switching latency. The time from when a hardware interrupt is generated to when the interrupt is serviced is called the interrupt latency.
Hardware vs. software
Context switching can be performed primarily by software or hardware. Some processors, like the Intel 80386 and its successors, have hardware support for context switches, by making use of a special data segment designated the task state segment (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 (IDT). 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, 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
TSbit is automatically turned on in the
CR0control 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.
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