operating systems

Operating Systems

Silberschatz, Galvin and Gagne 20024.2

Agenda of Today

Review PCB Contents of PCB Context Switching Schedulers Types of Schedulers Operations on Processes

Creation of a process Termination of a process

Operating System Concepts

Silberschatz, Galvin and Gagne 20024.3Operating System Concepts

Process Control Block (PCB)

Information associated with each process.

Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information


CPU Switch From Process to Process

Silberschatz, Galvin and Gagne 20024.5CMP320 PUCIT Arif Butt

Context Switch

Steps involved in a full Process switch are:

Save context of currently running process (including PC and

other registers)

Move this PCB to an appropriate Queue

Select another process for execution (Kernel Schedules)

Update PCB of selected process

Update memory management data structures

Restore the context of the process

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Schedulers

“Scheduling is a matter of managing queues to minimize queuing delay and to optimize performance in a queuing environment”.

Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue.

Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU.


Addition of Medium Term Scheduling


Schedulers (Cont.) Long-term scheduler (or job scheduler) – selects processes from the

job pool (processes spooled on hard disk) to be brought into the ready queue (inside main memory)

Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow).

The long-term scheduler controls the degree of multiprogramming. Must select a good mix of I/O bound and CPU bound processes.

Short-term scheduler is invoked very frequently (milliseconds) (must be fast). Select processes from ready queue and execute them.

Medium-term scheduler handles swapping. Swap-out: When cpu is idle: all processes are blocked and another new job

is waiting while memory is full. Then CPU suspend a process from blocked queue and swap it out to the disk. Then swap-in another process. This adds another state in the state diagram.


Process Creation

Parent process create children processes, which, in turn create other processes, forming a tree of processes.

Resource sharing Parent and children share all resources. Children share subset of parent’s resources. Parent and child share no resources.

Execution Parent and children execute concurrently. Parent waits until children terminate.


Process Creation (Cont.) Address space

Child duplicate of parent. Child has a program loaded into it. Child same as parent: Exp. HTTP server processing clients through

multiple clones

UNIX examples fork system call creates new process exec system call used after a fork to replace the process’ memory space

with a new program.

Important process-related UNIX/Linux system calls

fork()

exit()

wait()

exec()


Processes Tree on a UNIX System

Daemon: System processes running in background.


Process Termination

Process executes last statement and asks the operating system to decide it (exit). Output data from child to parent Process’ resources are de-allocated by operating system. Exit (0): normal or (1) for less space etc.

Parent may terminate execution of children processes (abort). Child has exceeded allocated resources. Task assigned to child is no longer required. Parent is exiting.

Operating system does not allow child to continue if its parent terminates.

Cascading termination.


Process Termination

• A process may terminate due to following reasons:

• Normal completion• Memory unavailable• Protection error• Mathematical error• I/O failure• Cascading termination (by OS)• Operator intervention



System Call - fork() When the fork system call is executed, a new process is

created which consists of a copy of the address space of the

parent

An exact copy of the parent program is created

This mechanism allows the parent process to communicate

easily with the child process.

SYNOPSIS

#include <sys/types.h>

#include <unistd.h>pid_t fork(void);

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System Call - fork() ... On success: (Child process is created)

The return code for fork is zero for the child process

The child process ID is returned to the parent process

Both processes continue execution at the instruction after

the fork call

On failure: (No child process is created)

A -1 is returned to the parent process

Variable errno is set appropriately to indicate the reason of

failure

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PID: 597

16

1. //fork1.c2. int main()3. {4. int i = 54, cpid = -1;5. cpid = fork();6. if (cpid == -1)7. {8. printf (“\nFork failed\n”);9. exit (1);10. }11. if (cpid == 0) //child code12. printf (“\n Hello I am child \n”);13. else //parent code14. printf (“\n Hello I am parent \

n”);15. }

•Parent forks

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Parent

i = 54cpid = -1

DATA

Using fork() & exit() system call


PID: 597

17

1. //fork1.c2. int main()3. {4. int i = 54, cpid = -1;5. cpid = fork();6. if (cpid == -1)7. {8. printf (“\nFork failed\n”);9. exit (1);10. }11. if (cpid == 0) //child code12. printf (“\n Hello I am child \

n”);13. else //parent code14. printf (“\n Hello I am parent \

n”);15. }

PID: 6321. //fork1.c2. int main()3. {4. int i = 54, cpid = -1;5. cpid = fork();6. if (cpid == -1)7. {8. printf (“\nFork failed\n”);9. exit (1);10. }11. if (cpid == 0) //child code12. printf (“\n Hello I am child \

n”);13. else //parent code14. printf (“\n Hello I am parent \

n”);15. }

Using fork() & exit() system call

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Parent Child

i = 54cpid = 632

DATAi = 54

cpid = 0

DATA

• After fork parent and child are identical except for the return value of fork (and of course the PIDs).

• Because data are different therefore program execution differs.

• When both will execute line 11, parent will now execute line 12 while child will execute line 14.


fork() – Child inherits from the Parent

The child process inherits the following attributes

form the parent:

Environment

Open File Descriptor table

Signal handling settings

Current working directory

Root directory

File mode creation mask (umask)

18CMP320 PUCIT Arif Butt

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fork() – Child Differs from the Parent

The child process differs from the parent

process:

Different process ID (PID).

Different parent process ID (PPID).


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fork() – Reasons for Failure

Maximum number of processes allowed to

execute under one user has exceeded

Maximum number of processes allowed on

the system has exceeded

Not enough swap space


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Unix Commands related to Processes


Command Description# ps [-aelu] Display status of processes

# top Display information about top 20 processes

# vim file1.txt & & cause Running a command in back ground

CTRL + Z Suspend a foreground process to go back to the shell

# jobs Display status of suspended and background processes

# bg %job_id Put a process in background

# fg %job_id Move background process to the foreground

CTRL + C Kill foreground process

# kill [-signal_no] proc_list Send the signal for signal_no to processes whose PIDs or jobIDs are specified in proc_list. jobIDs must start with %.

operating systems

Documents

cpu scheduler

job scheduler

children processes

process switch

shortterm scheduler

cpu bound processes

tree of processes

job pool processes