Context switch writes in assembly and links by linker ! Think about every process has its own private space for storing data including registers, stacks. 4. It is strait-forward to understand it. 5. Strategy: Calling Convention ! Context switching means storing the process state so that we can reload the process when needed, and the execution of the process can be resumed from the same point later. 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. 2. This context switching is a major feature of a multitasking operating system. Here is the address of the cpu->schedule. Update the process control block and other important fields. Remind: Context is always on the top of some stack. Use EIP to get the return address, switch.S -> sched(proc.c) -> yield(proc.c) -> trap(trap.c) -> trapret(trapasm.S, but set up in allocproc() ) -> iret (trapasm.S). The scheduler is the bridge between two processes Then, the Context Switch is the mechanism to prepare the resources for CPUs when we want to finish the job from different processes. 3. Also, the scheduler is the bridge to help processes to do so. Save the context of the process that is currently running on the CPU. Back to our topic, focus on function scheduler(): What it does is after initializing all requirements we need for XV6, it launches one proc from current context(cpu->scheduler). Segment register saves physical address of the segment offset. Interrupt the current process. 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. The time involved in the context switching of one process by other is called the Context Switching Time. The scheduler is the bridge between two processes. 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. What does xchg(volatile uint *addr, uint newval) do? Whenever the scheduler switches the CPU from a process to execute another process in this case the context switcher saves the contents of all the processors registers for the process which is being removed from the CPU in it process descriptor. This includes updating the process state to running. However, how does XV6 prepare context for process? Context Switch Flow. Then, we know the context switch to first process to run. Advantage of Context Switching The next is how to return back to the user space? Now, we should know the whole context switch flow ! Context Switching is the characteristic of a multitasking operating system. xchg did a value exchange with the value store in addr, this uses for lock implementation. # Call trap(tf), where tf=%esp by calling convention, movl %esp, (%eax) # (%eax) means *old (points to address of old), for(p = ptable.proc; p < &ptable.proc[NPROC]; p++) {, Improve Your Productivity Using Git and Bash Aliases, Hello World in IntelliJ — the 2020.1 Edition, Introducing runqstat — New Linux Run Queue & Load Average Tool, How to Profile a Golang gRPC Server Using pprof, Step by Step Guide to Developing a Custom Maven Plugin, Invoke the interrupt (ex: Timer Interrupt (trap.c 109) ), Invoke the scheduler ( yield (proc.c 390) ), Start the context switch(swtch (proc.c 387) ), local variables | EBP(frame pointer) | return address | parameters, free other CPU’s activities to access address of addr, Unfreeze other CPU’s activities toward address of addr, Interrupt the current process. Update the process control block of the selected process. Remind: Why should XV6 push ESP onto the stack ? Ans: As a parameter for void trap(struct trapframe *tf). Well, when first user process created in userinit() or doing the process fork(), both of them invoked allocproc to find an UNUSED proc in ptable, static struct proc* allocproc(void) (proc.c 75). Then, switch to cpu->scheduler, Scheduler find a RUNNABLE proc in ptable and context switch from scheduler to another RUNNABLE process. Context Switching or Process Switching in OS is one of the simplest topic and concept but when we try to make it more simple some time we forget to mention the most important and intresting things which happens in the Context switching. Now, we should know the whole context switch flow ! Then, switch to cpu->scheduler; Scheduler find a RUNNABLE proc in ptable and context switch from scheduler to another RUNNABLE process; Process -> Scheduler -> Process. 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. In this post, summarized the flow of the Context Switch in a small system XV6. Also, after invoking scheduler, the system starts context switch said before in swtch(struct context **old, struct context *new). In context switching, one CPU can be shared among several processes. Let stack pointer points to the context we want to switch. The steps involved in context switching are as follows: 1. Then, in trap.c 111, calling yield() to invoke scheduler. Discuss about switch from current process to scheduler when time interrupt happens !

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