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Last Time:OS & Computer Architecture

Modern OS Functionality (brief review)

Architecture Basics Hardware Support for OS

Features

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Last Time: Services & Hardware Support

OS Service Hardware Support

Protection

Interrupts

Traps

I/O

Synchronization

Virtual Memory

Scheduling

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This Time:OS Organizations & Processes

OS Organizations (kernels) Processes

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Monolithic Kernel

Classic UNIX approach, Linux

Everything in kernel+ Fast- Risky

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Layered OS Design

+ Modular, simple, portable, easy to design/debug

- Communication overhead, copying, bookkeeping

“THE” operating system Dijkstra

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The Microkernel Approach

Goal: Minimize contents of kernel Why?

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Microkernel: Motivation

Minimize contents of kernel: Reduces risk of crashing OS

Put as much functionality as possible in user-level processes

Simplifies extension, modification & customization

First μ-kernel: Hydra (CMU) Current systems: Mach (also CMU),

by Rick Rashid et al. (now head of MSR)

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μ-kernels vs. Monolithic Kernels

Past conventional wisdom: (1990’s) Mach – beautiful research idea but “failed”

in practice Too slow!

Linux – ugly, monolithic, but fast

Today: much faster computers Mach: fast enough (Mac OS X)

Reliability, simplicity, robustness now more important than performance

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OS Structures & Processes

OS Organizations (kernels) Processes

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Process

OS manages variety of activities: User programs, batch jobs, command

scripts Daemons: print spoolers, name

servers, file servers, etc.

Each activity encapsulated in process: Context (PC, registers, etc.) required

for activity to run

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Process != program

Process is a running program One program may comprise

many processes Many processes may run same

program

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OS and Processes

OS manages & schedules processes Creation, deletion Resource allocation (e.g., CPU,

memory) Suspension & resumption Inter-process communication Synchronization

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Processes

Process Concept Process States Process Scheduling Process Management Interprocess Communication

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Process Concept

Process – program in execution Process includes:

text section: program code current activity: program counter,

contents in registers stack data section heap

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Processes

Process Concept Process States Process Scheduling Process Management Interprocess Communication

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Process States

New Process being created

Running Instructions being executed

Waiting Process waiting for some event to occur

Ready Process waiting to be assigned to a

processor Terminated

(duh)

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Process State Diagram Transitions:

Program actions (system calls)

OS actions (scheduling)

External events (interrupts)

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Process Execution Example

void main() { printf (“Hello world\n”);}

1. New2. Ready3. Running4. Waiting for I/O5. Ready6. Running7. Terminated

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Process Data Structures Process Control Block:

Tracks state OS allocates, places on

queue OS deallocates on

termination

Lots of info: Process state Program counter CPU registers Memory-management

information Accounting information I/O status information

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Processes

Process Concept Process States Process Scheduling Process Management Interprocess Communication

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Process Scheduling Queues

Job queue Set of all processes in system

Ready queue Set of processes residing in main memory

ready & waiting to execute Device queues

Set of processes waiting for I/O device One per device

Process migration between the various queues.

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Process Queues Example

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CPU Scheduling

time

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PCBs and Hardware State

Switching processes: context switch Relatively expensive

Time between switches (quantum) must be long enough to amortize this cost

Start: OS picks ready process Loads register values from PCB

Stop: OS saves registers into PCB

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Process Scheduling

Queuing-diagram

representation:

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Processes

Process Concept Process States Process Scheduling Process Management Interprocess Communication

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Process Creation

One process can create other processes Creator = parent, new processes = children Parent can wait for child to complete,

or continue in parallel

UNIX: fork() used to create child processes Copies variables & registers from parent to

child Only difference between parent & child: return

value Parent: returns process id of child Child: returns 0

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Process Creation Example

Logging into UNIX creates shell process Every command typed into shell:

Child of shell process (spawned by fork) Executes command via exec

Example: Type “emacs” OS forks new process exec executes emacs If followed by “&”, runs in parallel;

otherwise, waits until done

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Example UNIX Program: Fork

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Fork Example: What happened?

#include <unistd.h>#include <sys/wait.h>#include <stdio.h>int main(){

int parentID = getpid();char prgname[1024];gets(prgname);int cid = fork();if(cid == 0){

execlp(prgname, prgname, 0); printf(“I did not find program %s\n“, prgname);}else{ sleep(1);

waitpid(cid, 0, 0); printf("Program %s is finished.

\n");}return 0; }

Parent process:#include <unistd.h>#include <sys/wait.h>#include <stdio.h>int main(){

int parentID = getpid();char prgname[1024];gets(prgname);int cid = fork();if(cid == 0){

execlp(prgname, prgname, 0); printf(“I did not find program %s\n“, prgname);}else{ sleep(1);

waitpid(cid, 0, 0); printf("Program %s is finished.

\n");}return 0; }

Child process:

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Parent & Child Process#include <unistd.h>#include <stdio.h>

int main(){int pid;pid = fork();if(pid == 0){

printf("Child: My PID = %d\n", getpid());printf("Child: Running...\n");sleep(20);printf("Child: Done sleeping, returning.\n");

}else{

printf("Parent: My PID = %d\n", getpid());printf("Parent: Running...\n");sleep(10);printf("Parent: Done sleeping, returning.\n");

}return 0;

}

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Process Termination

On termination, OS reclaims all resources assigned to process

UNIX processes: Can terminate self via exit system

call Can terminate child via kill system

call

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Example: Process Termination

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Processes

Process Concept Process States Process Scheduling Process Management Interprocess Communication

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Cooperating Processes

Cooperating processes: one process can affect or be affected by other processes

Advantages: Can improve performance by overlapping

activities or performing work in parallel Can enable simpler program design Simplifies distribution

Processes can live on different machines

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Interprocess Communication

Processes communicate in one of two ways:

Message passing: Send and receive information Numerous means: sockets, pipes, etc.

Shared memory: Establish mapping to named memory

object Use mmap Fork processes that share this structure

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Process Summary

Process = unit of execution Represented by Process Control Blocks

Contain process state, registers, etc. Process state:

New, Ready, Waiting, Running, or Terminated One running process at a time (on a

uniprocessor) Context switch when changing process

executing on CPU Communicate by message passing or shared

memory