Context switch
Encyclopedia
A context switch is the computing
process of storing and restoring the state
(context) of a CPU
so that execution can be resumed from the same point at a later time. This enables multiple process
es to share a single CPU
. The context switch is an essential feature of a multitasking
operating system
. Context switches are usually computationally intensive and much of the design of operating system
s is to optimize the use
of context switches. A context switch can mean a register
context switch, a task context switch, a thread
context switch, or a process context switch. What constitutes the context is determined by the processor and the operating system. 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 list etc.
scheme, one process needs to be switched out of the CPU so another process can run. Within a preemptive multitasking operating system, the scheduler allows every task to run for some certain amount of time, called its time slice.
If a process does not voluntarily yield the CPU (for example, by performing an I/O
operation), a timer interrupt fires, and the operating system schedules another process for execution instead. This ensures that the CPU cannot be monopolized by any one processor-intensive application.
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 cause the interrupt handler to start running). 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.
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 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. Process and thread priority can influence which process continues execution, with processes of the highest priority checked first for ready threads to execute.
and more advanced CPUs, 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
or the local 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
, do not use this feature. This is due mainly to two reasons: that 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 do saving and restoring of the floating point state), and associated performance issues.
The extreme case is the barrel processor
architecture, which switches between threads of execution on every cycle.
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...
process of storing and restoring the state
State (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
(context) of a CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
so that execution can be resumed from the same point at a later time. This enables multiple process
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
es to share a single CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
. The context switch is an essential feature of a multitasking
Computer multitasking
In computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
. Context switches are usually computationally intensive and much of the design of operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s is to optimize the use
of context switches. A context switch can mean a register
Processor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
context switch, a task context switch, a thread
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
context switch, or a process context switch. What constitutes the context is determined by the processor and the operating system. 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 list etc.
Multitasking
Most commonly, within some schedulingScheduling (computing)
In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...
scheme, one process needs to be switched out of the CPU so another process can run. Within a preemptive multitasking operating system, the scheduler allows every task to run for some certain amount of time, called its time slice.
If a process does not voluntarily yield the CPU (for example, by performing an I/O
Input/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
operation), a timer interrupt fires, and the operating system schedules another process for execution instead. This ensures that the CPU cannot be monopolized by any one processor-intensive application.
Interrupt handling
Modern architectures are interruptInterrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
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
Interrupt handler
An interrupt handler, also known as an interrupt service routine , is a callback subroutine in microcontroller firmware, operating system or device driver whose execution is triggered by the reception of an interrupt...
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 cause the interrupt handler to start running). 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
Kernel (computing)
In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...
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.Context switch: steps
In a switch, the state of the first process 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
Program counter
The program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...
, 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
Process control block
Process Control Block is a data structure in the operating system kernel containing the information needed to manage a particular process...
(PCB), or switchframe
Switchframe
A Switchframe is a data structure used to store context switch information about a process. They are used by the scheduler to implement multitasking....
.
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 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. Process and thread priority can influence which process continues execution, with processes of the highest priority checked first for ready threads to execute.
Software vs hardware context switching
Context switching can be performed primarily by software or hardware. Some processors, like the Intel 80386Intel 80386
The Intel 80386, also known as the i386, or just 386, was a 32-bit microprocessor introduced by Intel in 1985. The first versions had 275,000 transistors and were used as the central processing unit of many workstations and high-end personal computers of the time...
and more advanced CPUs, have hardware support for context switches, by making use of a special data segment designated the Task State Segment
Task State Segment
The task state segment is a special structure on x86-based computers which holds information about a task. It is used by the operating system kernel for task management...
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
Global Descriptor Table
The Global Descriptor Table or GDT is a data structure used by Intel x86-family processors starting with the 80286 in order to define the characteristics of the various memory areas used during program execution, including the base address, the size and access privileges like executability and...
or the local descriptor table
Local Descriptor Table
The Local Descriptor Table is a memory table used in the x86 architecture in protected mode and containing memory segment descriptors: start in linear memory, size, executability, writability, access privilege, actual presence in memory, etc....
. It can occur implicitly when an interrupt or exception is triggered if there's a task gate in the interrupt descriptor table
Interrupt descriptor table
The Interrupt Descriptor Table is a data structure used by the x86 architecture to implement an interrupt vector table. The IDT is used by the processor to determine the correct response to interrupts and exceptions....
. When a task switch occurs the CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
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
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
and Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, do not use this feature. This is due mainly to two reasons: that hardware context switching does not save all the registers (only general purpose registers, not floating point
Floating point
In computing, floating point describes a method of representing real numbers in a way that can support a wide range of values. Numbers are, in general, represented approximately to a fixed number of significant digits and scaled using an exponent. The base for the scaling is normally 2, 10 or 16...
registers—although the TS bit is automatically turned on in the CR0 control register
Control register
A control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and coprocessor control.-CR0:The CR0 register is 32 bits...
, resulting in a fault when executing floating point instructions and giving the OS the opportunity to do saving and restoring of the floating point state), and associated performance issues.
The extreme case is the barrel processor
Barrel processor
A barrel processor is a CPU that switches between threads of execution on every cycle. This CPU design technique is also known as "interleaved" or "fine-grained" temporal multithreading...
architecture, which switches between threads of execution on every cycle.
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
- Measuring Basic Linux Operations