Context switching in os

A Context switch is a time spent between two processes (i.e., bringing a waiting process into execution and sending an executing process into a waiting-for state). This happens in multitasking. The operating system must bring the state information if waiting for the process into memory and save the state information of the currently running process. In order to solve this problem, we would like to record the timestamps of the first and last instructions of the swapping processes. The context switch time is the difference between the two processes. Let’s take an example: Assume there are only two processes, P1 and P2.P1 is executing and P2 is waiting for execution. At some point, the operating system must swap P1 and P2, let’s assume it happens at the nth instruction of P1. If t(x, k) indicates the timestamp in microseconds of the kth instruction of process x, then the context switch would take t(2, 1) – t(1, n). Another issue is that swapping is governed by the scheduling algorithm of the operating system and there may be many kernel-level threads that are also doing context switches. Other processes could be contending for the CPU or the kernel handling interrupts. The user does not have any control over these extraneous context switches. For instance, if at time t(1, n) the kernel decides to handle an interrupt, then the context switch time would be overstated. In order to avoid these obstacles, we must construct an environment such that after P1 executes, the task sched...

Programs are sets of instructions designed to accomplish specific tasks. Similarly, a process refers to a runtime instance of a computer program. During the execution of a program, several threads may be running in parallel. Single-threaded processes refer to the thread itself as the process. 1. Context switching : An operating system uses this technique to switch a process between states to execute its functions through CPUs. It is a process of saving the context(state) of the old process(suspend) and loading it into the new process(resume). It occurs whenever the CPU switches between one process and another. Basically, the state of CPU’s registers and program counter at any time represent a context. Here, the saved state of the currently executing process means to copy all live registers to PCB(Process Control Block). Moreover, after that, restore the state of the process to run or execute next, which means copying live registers’ values from PCB to registers. 2. Swapping : This is the process by which a process is temporarily swapped (moved) from the main memory (RAM) to the secondary memory (Disk). The main memory is fast but has less space than secondary storage, so the inactive processes are moved to secondary memory, and the system swaps the memory from secondary to the main memory later. During swapping, most of the time is spent transferring information, and the amount of memory swapped is directly proportional to the total time. Swapping has been divided into two...

Many embedded systems reach a level of complexity where having a basic set of scheduling primitives and ability to run different tasks can be helpful. The operation of switching from one task to another is known as a context switch. A Real Time Operating System ( RTOS) will typically provide this functionality. Having a foundational knowledge of how the core of an RTOS works can be a valuable skill set for an embedded engineer to have. In this article we will explore how context switching works on ARM Cortex-M MCUs. We will discuss how the hardware was designed to support this operation, features that impact the context switching implementation such as the Floating Point Unit (FPU), and common pitfalls seen when porting an RTOS to a platform. We will also walk through a practical example of analyzing the FreeRTOS context switcher, xPortPendSVHandler, utilizing gdb to strengthen our understanding. If you’d rather listen to me present this information and see some demos in action, Table of Contents • • • • • • • • • • • • • Cortex-M ARM MCU Features To understand how RTOS context switching works for ARM Cortex-M MCUs, it’s critical to have foundational knowledge about the primitives the architecture provides to make it possible. In this section we go through these building blocks by distilling down the information spread across the ARM Cortex-M reference manuals and the ARM Architecture Procedure Calling Standard ( AAPCS) Application Binary Interface ( ABI) a compiler must a...

September 04, 2018 at 05:35 Tags In this post I want to explore the costs of threads on modern Linux machines, both in terms of time and space. The background context is designing high-load concurrent servers, where Important disclaimer: it's not my goal here to provide an opinion in the threads vs. event-driven models debate. Ultimately, both are tools that work well in some scenarios and less well in others. That said, one of the major criticisms of a thread-based model is the cost - comments like "but context switches are expensive!" or "but a thousand threads will eat up all your RAM!", and I do intend to study the data underlying such claims in more detail here. I'll do this by presenting multiple code samples and programs that make it easy to explore and experiment with these measurements. Linux threads and NPTL In the dark, old ages before version 2.6, the Linux kernel didn't have much specific support for threads, and they were more-or-less hacked on top of process support. Before The Native POSIX Thread Library (NPTL) was proposed by Ulrich Drepper and Ingo Molnar from Red Hat, and integrated into the kernel in version 2.6, circa 2005. I warmly recommend reading its Even though all of this happened 13 years ago, the spirit of NPTL is still easily observable in some system programming code. For example, many thread and synchronization-related paths in glibc have nptl in their name. Threads, processes and the clone system call This was originally meant to be a part ...

Overview Context Switching is the mechanism that allows multiple processes to use a single CPU. Context Switching stores the status of the ongoing process so that the process can be reloaded from the same point from where it was stopped. Scope • This article explains What is Context Switching, Steps involved in Context Switching, Example of Context Switching, and the advantages and disadvantage of Context Switching. • This article does not cover any practical implementation of performing Context Switiching. What is Context Switching? Alice and Bob have been working hard to learn the concepts of an operating system, and they are here to learn context switching in an operating system. Professor: Bob, can you explain what is multitasking system? Bob: Yes, Professor, multiprocessing systems are the type of systems where multiple processes can be executed even if the system contains only one CPU. Professor: Very good, Bob! But have you ever wondered how multiple processes are executed on the system having a single CPU? Bob: No, but I am curious to learn that. Alice: Yes, Professor! Professor: The magic of executing multiple processes on a single CPU takes place with the help of Context Switching. When any process is utilizing CPU, and some other process requires the CPU, Context Switching is carried out before passing the control of CPU from the old process to the new process. Alice: Professor, what does context switching actually do, and why is Context Switching necessary? Pro...

A Context switch is a time spent between two processes (i.e., bringing a waiting process into execution and sending an executing process into a waiting-for state). This happens in multitasking. The operating system must bring the state information if waiting for the process into memory and save the state information of the currently running process. In order to solve this problem, we would like to record the timestamps of the first and last instructions of the swapping processes. The context switch time is the difference between the two processes. Let’s take an example: Assume there are only two processes, P1 and P2.P1 is executing and P2 is waiting for execution. At some point, the operating system must swap P1 and P2, let’s assume it happens at the nth instruction of P1. If t(x, k) indicates the timestamp in microseconds of the kth instruction of process x, then the context switch would take t(2, 1) – t(1, n). Another issue is that swapping is governed by the scheduling algorithm of the operating system and there may be many kernel-level threads that are also doing context switches. Other processes could be contending for the CPU or the kernel handling interrupts. The user does not have any control over these extraneous context switches. For instance, if at time t(1, n) the kernel decides to handle an interrupt, then the context switch time would be overstated. In order to avoid these obstacles, we must construct an environment such that after P1 executes, the task sched...

Many embedded systems reach a level of complexity where having a basic set of scheduling primitives and ability to run different tasks can be helpful. The operation of switching from one task to another is known as a context switch. A Real Time Operating System ( RTOS) will typically provide this functionality. Having a foundational knowledge of how the core of an RTOS works can be a valuable skill set for an embedded engineer to have. In this article we will explore how context switching works on ARM Cortex-M MCUs. We will discuss how the hardware was designed to support this operation, features that impact the context switching implementation such as the Floating Point Unit (FPU), and common pitfalls seen when porting an RTOS to a platform. We will also walk through a practical example of analyzing the FreeRTOS context switcher, xPortPendSVHandler, utilizing gdb to strengthen our understanding. If you’d rather listen to me present this information and see some demos in action, Table of Contents • • • • • • • • • • • • • Cortex-M ARM MCU Features To understand how RTOS context switching works for ARM Cortex-M MCUs, it’s critical to have foundational knowledge about the primitives the architecture provides to make it possible. In this section we go through these building blocks by distilling down the information spread across the ARM Cortex-M reference manuals and the ARM Architecture Procedure Calling Standard ( AAPCS) Application Binary Interface ( ABI) a compiler must a...

September 04, 2018 at 05:35 Tags In this post I want to explore the costs of threads on modern Linux machines, both in terms of time and space. The background context is designing high-load concurrent servers, where Important disclaimer: it's not my goal here to provide an opinion in the threads vs. event-driven models debate. Ultimately, both are tools that work well in some scenarios and less well in others. That said, one of the major criticisms of a thread-based model is the cost - comments like "but context switches are expensive!" or "but a thousand threads will eat up all your RAM!", and I do intend to study the data underlying such claims in more detail here. I'll do this by presenting multiple code samples and programs that make it easy to explore and experiment with these measurements. Linux threads and NPTL In the dark, old ages before version 2.6, the Linux kernel didn't have much specific support for threads, and they were more-or-less hacked on top of process support. Before The Native POSIX Thread Library (NPTL) was proposed by Ulrich Drepper and Ingo Molnar from Red Hat, and integrated into the kernel in version 2.6, circa 2005. I warmly recommend reading its Even though all of this happened 13 years ago, the spirit of NPTL is still easily observable in some system programming code. For example, many thread and synchronization-related paths in glibc have nptl in their name. Threads, processes and the clone system call This was originally meant to be a part ...

Overview Context Switching is the mechanism that allows multiple processes to use a single CPU. Context Switching stores the status of the ongoing process so that the process can be reloaded from the same point from where it was stopped. Scope • This article explains What is Context Switching, Steps involved in Context Switching, Example of Context Switching, and the advantages and disadvantage of Context Switching. • This article does not cover any practical implementation of performing Context Switiching. What is Context Switching? Alice and Bob have been working hard to learn the concepts of an operating system, and they are here to learn context switching in an operating system. Professor: Bob, can you explain what is multitasking system? Bob: Yes, Professor, multiprocessing systems are the type of systems where multiple processes can be executed even if the system contains only one CPU. Professor: Very good, Bob! But have you ever wondered how multiple processes are executed on the system having a single CPU? Bob: No, but I am curious to learn that. Alice: Yes, Professor! Professor: The magic of executing multiple processes on a single CPU takes place with the help of Context Switching. When any process is utilizing CPU, and some other process requires the CPU, Context Switching is carried out before passing the control of CPU from the old process to the new process. Alice: Professor, what does context switching actually do, and why is Context Switching necessary? Pro...

Programs are sets of instructions designed to accomplish specific tasks. Similarly, a process refers to a runtime instance of a computer program. During the execution of a program, several threads may be running in parallel. Single-threaded processes refer to the thread itself as the process. 1. Context switching : An operating system uses this technique to switch a process between states to execute its functions through CPUs. It is a process of saving the context(state) of the old process(suspend) and loading it into the new process(resume). It occurs whenever the CPU switches between one process and another. Basically, the state of CPU’s registers and program counter at any time represent a context. Here, the saved state of the currently executing process means to copy all live registers to PCB(Process Control Block). Moreover, after that, restore the state of the process to run or execute next, which means copying live registers’ values from PCB to registers. 2. Swapping : This is the process by which a process is temporarily swapped (moved) from the main memory (RAM) to the secondary memory (Disk). The main memory is fast but has less space than secondary storage, so the inactive processes are moved to secondary memory, and the system swaps the memory from secondary to the main memory later. During swapping, most of the time is spent transferring information, and the amount of memory swapped is directly proportional to the total time. Swapping has been divided into two...