What is Context Switching in Operating System?

Context Switching involves storing the context or state of a process so that it can be reloaded when required and execution can be resumed from the same point as earlier. This is a feature of a multitasking operating system and allows a single CPU to be shared by multiple processes.

A diagram that demonstrates context switching is as follows −

In the above diagram, initially Process 1 is running. Process 1 is switched out and Process 2 is switched in because of an interrupt or a system call. Context switching involves saving the state of Process 1 into PCB1 and loading the state of process 2 from PCB2. After some time again a context switch occurs and Process 2 is switched out and Process 1 is switched in again. This involves saving the state of Process 2 into PCB2 and loading the state of process 1 from PCB1.

Context Switching Triggers

There are three major triggers for context switching. These are given as follows −

Multitasking: In a multitasking environment, a process is switched out of the CPU so another process can be run. The state of the old process is saved and the state of the new process is loaded. On a pre-emptive system, processes may be switched out by the scheduler.

Interrupt Handling: The hardware switches a part of the context when an interrupt occurs. This happens automatically. Only some of the context is changed to minimize the time required to handle the interrupt.

User and Kernel Mode Switching: A context switch may take place when a transition between the user mode and kernel mode is required in the operating system.

Context Switching Steps

The steps involved in context switching are as follows −

• Save the context of the process that is currently running on the CPU. Update the process control block and other important fields.
• Move the process control block of the above process into the relevant queue such as the ready queue, I/O queue etc.
• Select a new process for execution.
• Update the process control block of the selected process. This includes updating the process state to running.
• Update the memory management data structures as required.
• Restore the context of the process that was previously running when it is loaded again on the processor. This is done by loading the previous values of the process control block and registers.

Context Switching Cost

Context Switching leads to an overhead cost because of TLB flushes, sharing the cache between multiple tasks, running the task scheduler etc. Context switching between two threads of the same process is faster than between two different processes as threads have the same virtual memory maps. Because of this TLB flushing is not required.

Forcing context switch in Windows

Is there a way to force a context switch in C++ to a specific thread, assuming I have the thread handle or thread ID?

6 Answers 6

No, you won’t be able to force operating system to run the thread you want. You can use yield to force a context switch though.

yield in Win32 API is function SwitchToThread . If there is no other thread available for running, then a ZERO value will be returned and current thread will keep running anyway.

You can only encourage the Windows thread scheduler to pick a certain thread, you can’t force it. You do so first by making the thread block on a synchronization object and signaling it. Secondary by bumping up its priority.

Explicit context switching is supported, you’ll have to use fibers. Review SwitchToFiber(). A fiber is not a thread by a long shot, it is similar to a co-routine of old. Fibers’ heyday has come and gone, they are not competitive with threads anymore. They have very crappy cpu cache locality and cannot take advantage of multiple cores.

The only way to force a particular thread to run is by using process/thread affinity, but I can’t imagine ever having a problem for which this was a reasonable solution.

The only way to force a context switch is to force a thread onto a different processor using affinity.

In other words, what you are trying to do isn’t really viable.

Calling SwitchToThread() will result in a context switch if there is another thread ready to run that are eligible to run on this processor. The documentation states it as follows:

If calling the SwitchToThread function causes the operating system to switch execution to another thread, the return value is nonzero.

If there are no other threads ready to execute, the operating system does not switch execution to another thread, and the return value is zero.

What is context switching in windows

In the Operating System, there are cases when you have to bring back the process that is in the running state to some other state like ready state or wait/block state. If the running process wants to perform some I/O operation, then you have to remove the process from the running state and then put the process in the I/O queue. Sometimes, the process might be using a round-robin scheduling algorithm where after every fixed time quantum, the process has to come back to the ready state from the running state. So, these process switchings are done with the help of Context Switching. In this blog, we will learn about the concept of Context Switching in the Operating System and we will also learn about the advantages and disadvantages of Context Switching. So, let’s get started.

What is Context Switching?

A context switching is a process that involves switching of the CPU from one process or task to another. In this phenomenon, the execution of the process that is present in the running state is suspended by the kernel and another process that is present in the ready state is executed by the CPU.

It is one of the essential features of the multitasking operating system. The processes are switched so fastly that it gives an illusion to the user that all the processes are being executed at the same time.

But the context switching process involved a number of steps that need to be followed. You can’t directly switch a process from the running state to the ready state. You have to save the context of that process. If you are not saving the context of any process P then after some time, when the process P comes in the CPU for execution again, then the process will start executing from starting. But in reality, it should continue from that point where it left the CPU in its previous execution. So, the context of the process should be saved before putting any other process in the running state.

A context is the contents of a CPU’s registers and program counter at any point in time. Context switching can happen due to the following reasons:

• When a process of high priority comes in the ready state. In this case, the execution of the running process should be stopped and the higher priority process should be given the CPU for execution.
• When an interruption occurs then the process in the running state should be stopped and the CPU should handle the interrupt before doing something else.
• When a transition between the user mode and kernel mode is required then you have to perform the context switching.

Steps involved in Context Switching

The process of context switching involves a number of steps. The following diagram depicts the process of context switching between the two processes P1 and P2.

In the above figure, you can see that initially, the process P1 is in the running state and the process P2 is in the ready state. Now, when some interruption occurs then you have to switch the process P1 from running to the ready state after saving the context and the process P2 from ready to running state. The following steps will be performed:

1. Firstly, the context of the process P1 i.e. the process present in the running state will be saved in the Process Control Block of process P1 i.e. PCB1.
2. Now, you have to move the PCB1 to the relevant queue i.e. ready queue, I/O queue, waiting queue, etc.
3. From the ready state, select the new process that is to be executed i.e. the process P2.
4. Now, update the Process Control Block of process P2 i.e. PCB2 by setting the process state to running. If the process P2 was earlier executed by the CPU, then you can get the position of last executed instruction so that you can resume the execution of P2.
5. Similarly, if you want to execute the process P1 again, then you have to follow the same steps as mentioned above(from step 1 to 4).

For context switching to happen, two processes are at least required in general, and in the case of the round-robin algorithm, you can perform context switching with the help of one process only.

The time involved in the context switching of one process by other is called the Context Switching Time.

Advantage of Context Switching

Context switching is used to achieve multitasking i.e. multiprogramming with time-sharing(learn more about multitasking from here). Multitasking gives an illusion to the users that more than one process are being executed at the same time. But in reality, only one task is being executed at a particular instant of time by a processor. Here, the context switching is so fast that the user feels that the CPU is executing more than one task at the same time.

The disadvantage of Context Switching

The disadvantage of context switching is that it requires some time for context switching i.e. the context switching time. Time is required to save the context of one process that is in the running state and then getting the context of another process that is about to come in the running state. During that time, there is no useful work done by the CPU from the user perspective. So, context switching is pure overhead in this condition.

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What is saved in a context switch?

What is exactly saved and restored in a context switch between two threads

• in the same process
• between two processes

4 Answers 4

This is rather a complex question since the answer(s) are dependent on many things:

1. The CPU in question
   • It can vary significantly even within the same family for example the additional registers added for SSE/MMX operations.
2. The operating system, since it controls the handlers which trigger on a context switch and decide whether the CPU’s hardware (if any) to assist in a context switch is used or not.
   • For example Windows does not use the Intel hardware that can do much of the context switch storage for you since it does not store floating point registers.
3. Any optimizations enabled by a program aware of it’s own requirements and capable of informing the OS of this
   • Perhaps to indicate that it isn’t using FP registers so don’t bother with them
   • In architectures with sizeable register files like most RISC designs there is considerable benefit to knowing you need only a smaller subset of these registers

At a minimum the in use general purpose registers and program counter register will need to be saved (assuming the common design of most current CISC/RISC style general purpose CPUs).

Note that attempting to do only the minimal amount of effort in relation to a context switch is a topic of some academic interest

Linux obviously has more info available on this in the public domain though my references may be a little out of date.

There is a ‘task_struct’ which contains a large number of fields relating to the task state as well as the process that the task is for.

One of these is the ‘thread_struct’

/* CPU-specific state of this task */
— struct thread_struct thread;
holds information about cache TLS descriptors, debugging registers,
fault info, floating point, virtual 86 mode or IO permissions.

Each architecture defines it’s own thread_struct which identifies the registers and other values saved on a switch.

This is further complicated by the presence of rename registers which allow multiple in flight instructions (either via superscalar or pipeline related architectural designs). The restore phase of a context swicth will likely rely on the CPU’s pipeline being restored in a initially empty state such the the instructions which had not yet been retired in the pipeline have no effect and thus can be ignored. This makes the design of the CPU that much harder.

The difference between a process and a thread is that the process switch (which always means a thread switch in all main stream operating systems) will need to update memory translation information, IO related information and permission related structures.

These will mainly be pointers to the more rich data structures so will not be a significant cost in relation to the thread context switch.

What is a context switch?

I was reading about the debuggerstepperboundary attribute and a site says it is is useful in a context switch.

What exactly is a context switch? I’m assuming it is a switch from one thread to another, or in execution or security context? However, these are not particularly educated guesses so I’m asking here.

5 Answers 5

Context switch is the switching of the CPU from one process/thread to another process/thread.

People sometimes use the term context switch outside of the specific computer world to reflect what they are doing in their own lives. «If I am going to answer that question, I need to context switch from thinking about A to thinking about B».

A context switch (also sometimes referred to as a process switch or a task switch) is the switching of the CPU (central processing unit) from one process or thread to another.

Context switching can be described in slightly more detail as the kernel (i.e., the core of the operating system) performing the following activities with regard to processes (including threads) on the CPU: (1) suspending the progression of one process and storing the CPU’s state (i.e., the context) for that process somewhere in memory, (2) retrieving the context of the next process from memory and restoring it in the CPU’s registers and (3) returning to the location indicated by the program counter (i.e., returning to the line of code at which the process was interrupted) in order to resume the process.

A context switch is sometimes described as the kernel suspending execution of one process on the CPU and resuming execution of some other process that had previously been suspended. Although this wording can help clarify the concept, it can be confusing in itself because a process is, by definition, an executing instance of a program. Thus the wording suspending progression of a process might be preferable.