Lecture 8 Process Management

8/2/2019 Lecture 8 Process Management

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Process management

Information maintained by OS for processmanagement

process context process control block

OS virtualization of CPU for each process. Context switching

Dispatching loop


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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.


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Process context

Contains all statesnecessary to run a

program The information the

process needs to dothe job: code, data,

stack, heap. This is known as User

level context.

stack

heap

data

text (code)

0

MAX

Process in memory


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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


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Process context

Contains all statesnecessary to run aprogram Is the user level

context sufficient? Only if the system runs

through one program ata time

The system typicallyneeds to switch backand forth betweenprograms.

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 switchingfrom P2 to P1.

Registers should be a part of process

context: the register context!


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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.


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Why is process contextimportant?

To run a process correctly, the processinstructions must be executed within

the process context!


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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 otherinformation that the OS needs to manage the

processes. Process status (running, waiting, etc)

Process priority


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Figure 3.3

An example PCB


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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 theprocess.

Each process executeswithin its context.

Load PC;IR = MEM[PC];While (IR != HALT) {

PC ++;

Execute IR;IR = MEM[PC];

}


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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 processs context to be correct.


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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 Xs context], [restore Ys context] y0, y1, y2

OS must do this to keep programs executewithin its context: Context switching


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Dispatching Loop

The hardware view of the systemexecution: dispatching loop

LOOP Run process

Save process states

Choose a new process to run

Load states for the chosen process

Context

Switch:Dispatchercode

Scheduling


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Simple? Not Quite

How does the dispatcher (OS) regaincontrol after a process starts running?

What states should a process save?

How does the dispatcher choose thenext thread?


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How Does the DispatcherRegain Control?

Two ways:

1. Internal events

A process is waiting for I/O

A process is waiting for some other process

Yielda process gives up CPU voluntarily

2.

External events Interruptsa complete disk request

Timerits like an alarm clock


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What States Should a processsave?

Anything that the next process maytrash

Program counter

Registers

Etc.


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How Does the DispatcherChoose the Next process?

The dispatcher keeps a list ofprocesses that are ready to run

If no processes are ready Dispatcher just loops

Otherwise, the dispatcher uses a

scheduling algorithm to find the nextprocess.


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Process States

A process is typically in one of thethree states

1. Running: has the CPU

2. Blocked: waiting for I/O or anotherthread

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


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Figure 3.2

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Lecture 8 Process Management