context switching in os

In context switching, one CPU can be shared among several processes. Remind: Why should XV6 push ESP onto the stack ? 3. This includes updating the process state to running. Context Switching is the characteristic of a multitasking operating system. Now, we should know the whole context switch flow ! 2. Remind: Context is always on the top of some stack. 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). 4. 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. 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. Update the process control block of the selected process. This context switching is a major feature of a multitasking operating system. Advantage of Context Switching The time involved in the context switching of one process by other is called the Context Switching Time. # 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. In this post, summarized the flow of the Context Switch in a small system XV6. The scheduler is the bridge between two processes. The next is how to return back to the user space? What does xchg(volatile uint *addr, uint newval) do? Segment register saves physical address of the segment offset. It is strait-forward to understand it. Think about every process has its own private space for storing data including registers, stacks. Interrupt the current process. The steps involved in context switching are as follows: 1. Then, in trap.c 111, calling yield() to invoke scheduler. Then, the Context Switch is the mechanism to prepare the resources for CPUs when we want to finish the job from different processes. Strategy: Calling Convention ! Ans: As a parameter for void trap(struct trapframe *tf). Context Switch Flow. 5. Let stack pointer points to the context we want to switch. Context switch writes in assembly and links by linker ! 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. Then, switch to cpu->scheduler; Scheduler find a RUNNABLE proc in ptable and context switch from scheduler to another RUNNABLE process; Process -> Scheduler -> Process. 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). Here is the address of the cpu->schedule. Then, we know the context switch to first process to run. xchg did a value exchange with the value store in addr, this uses for lock implementation.

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