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 The information the

process needs to do the job: code, data, stack, heap. This is known as User

level context.

stack

heap

data

text (code) 0

MAX

Process in memory

User level context (b, *p) - main (a) - foo

heap (p) (char[1000])

data (aa, buf)

text (code) 0

MAX

Process memory

…int aa;char buf[1000];void foo() { int a; … }main() { int b; char *p; p = new char[1000]; foo();}

stack

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

R2 = R0 + 1

R0 = 2

R2 = R0

P1 P2

• R2 in P1 is wrong. How to makeIt 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 ……

Figure 3.3

An example PCB

OS CPU abstraction

Hardware reality: 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; IR = MEM[PC];}

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 onlybe executed within the process’s context to be correct.

OS CPU abstractionProcess 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:Dispatchercode

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:1. Internal events

A process is waiting for I/O A process is waiting for some other process Yield—a process gives up CPU voluntarily

2. 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

1. Running: has the CPU

2. Blocked: waiting for I/O or another thread

3. Ready to run: on the ready list, waiting for the CPU

Figure 3.2