PROCESS MANAGEMENT Information maintained by OS for process management Process context Process control block OS virtualization of CPU for each process. Context switching Dispatching loop

PROCESS A program in execution We should know about processes by now. How does the OS correctly run multiple processes concurrently? What kind of information to be kept? What does the OS have to do in order to run processes correctly.

PROCESS CONTEXT Contains all states necessary to run a program MAX stack Contains all states necessary to run a program The information the process needs to do the job: code, data, stack, heap. This is known as User level context. heap data text (code) Process in memory

USER LEVEL CONTEXT … int aa; char buf[1000]; void foo() { int a; } MAX (b, *p) - main (a) - foo … int aa; char buf[1000]; void foo() { int a; } main() { int b; char *p; p = new char[1000]; foo(); stack heap (p) (char[1000]) data (aa, buf) text (code) Process memory

PROCESS CONTEXT Contains all states necessary to run a program Is the user level context sufficient? Only if the system runs through one program at a time The system typically needs to switch back and forth between programs. R0 = 1 R0 = 2 R2 = R0 + 1 R2 = R0 R2 in P1 is wrong. How to make It correct? Save R0 in P1 before switching Restore R0 in P1 when switching from P2 to P1. Registers should be a part of process context: the register context!

Process context: User level context Code, data, stack, heap Register context (R0, R1,…, PC, stack pointer, PSW, etc). What else? OS resources. E.g open files, signal related data structures, etc.

WHY IS PROCESS CONTEXT IMPORTANT? To run a process correctly, the process instructions must be executed within the process context!

Where is the process context stored? User level context is in memory. Other context information is stored in a data structure called process control block. The OS has a process control block table. For each process, there is one entry in the table. Process control block also contains other information that the OS needs to manage the processes. Process status (running, waiting, etc) Process priority ……

An example PCB Figure 3.3

OS CPU ABSTRACTION Hardware reality: OS abstraction: One CPU runs the fetch-execute algorithm OS abstraction: Each process has one CPU, running the fetch-execute algorithm for the process. Each process executes within its context. Load PC; IR = MEM[PC]; While (IR != HALT) { PC ++; Execute IR; }

OS CPU ABSTRACTION What does the OS have to do? Embed the process instruction sequence into hardware instruction sequence. Process X instructions: x0, x1, x2, …. Process Y instructions: y0, y1, y2, … Process Z instructions: z0, z1, z2, … Hardware instructions? x0, x1, x2, y0, y1, y2, z0, z1, z2, x3, x4, x5, … Does this embedding work? No!! Instructions in a process should only be executed within the process’s context to be correct.

OS CPU ABSTRACTION Process X instructions: x0, x1, x2, …. Process Y instructions: y0, y1, y2, … Process Z instructions: z0, z1, z2, … x0, x1, x2, [store X’s context], [restore Y’s context] y0, y1, y2… OS must do this to keep programs execute within its context: Context switching

DISPATCHING LOOP The hardware view of the system execution: dispatching loop LOOP Run process Save process states Choose a new process to run Load states for the chosen process Context Switch: Dispatcher code Scheduling

SIMPLE? NOT QUITE… How does the dispatcher (OS) regain control after a process starts running? What states should a process save? How does the dispatcher choose the next thread?

HOW DOES THE DISPATCHER REGAIN CONTROL? Two ways: Internal events A process is waiting for I/O A process is waiting for some other process Yield—a process gives up CPU voluntarily External events Interrupts—a complete disk request Timer—it’s like an alarm clock

WHAT STATES SHOULD A PROCESS SAVE? Anything that the next process may trash Program counter Registers Etc.

HOW DOES THE DISPATCHER CHOOSE THE NEXT PROCESS? The dispatcher keeps a list of processes that are ready to run If no processes are ready Dispatcher just loops Otherwise, the dispatcher uses a scheduling algorithm to find the next process.

PROCESS STATES A process is typically in one of the three states Running: has the CPU Blocked: waiting for I/O or another thread Ready to run: on the ready list, waiting for the CPU

Figure 3.2