Operating Systems II. Processessoc.eurecom.fr/OS/docs/slideProcesses.pdf · I Process = unit of work of a system ... Ludovic Apvrille Operating Systems - II. Processes - Fall 2011

Operating Systems

II. Processes

Ludovic Apvrille

Telecom ParisTechEurecom, Office 223

[email protected]

Fall 2011

ConceptsBoot sequence of an OS

Managing processes

Definitions and basic conceptsProcess management by OSIssues

Outline

ConceptsDefinitions and basic conceptsProcess management by OSIssues

Boot sequence of an OS

Managing processes

Ludovic Apvrille Operating Systems - II. Processes - Fall 2011 2 of 35


Managing processes


Process Definition

Programs and processes

I One execution of a sequential program = one process

I Two executions of the same program = two processes

Computer system = set of processes

I Process = unit of work of a system

I Operating System processes

I User processes



Managing processes


Running a Set of Processes

I MonoprocessorI Pseudo-parallelism: 1 process running at a time

I So, either the OS or a user process is runningI Multiprocessor

I A process can be running on each processorI The OS scheduler dispatches processes on processors

I Scheduling policy



Managing processes


States of Processes



Managing processes


Process data

I Data of processes are stored in memory!

Various data of a process

I Program code = text section (static)I Current Activity

I Program counter = Processor’s registerI Next instruction to execute

I Stack: function calls are stored in a LIFO mannerI Function parametersI Return addressI Local variables

I Heap: Data section



Managing processes


Memory Layout of a C Program



Managing processes


Organization of Processes

UNIXI Process hierarchy

I A parent can possibly have many childrenI A child has exactly one parent

I Group of a process = this process + children + furtherdescendants

I All processes belong to the group of the init process (root)

WindowsI No process hierarchyI When a parent creates a child, it is given a special token

(called a handle)I A handle can be passed to other processesI 6= UNIX: processes cannot disinherit their children



Managing processes


UNIX: Hierarchy of Processes



Managing processes


Process Control Block



Managing processes


Switching Between Processes



Managing processes


Processes Data Structure



Managing processes


Four Important Issues Regarding Processes

1. How is it possible to launchthe first user process?

Boot sequence

2. How to manage processesfrom a programmer’s point ofview?

APIs

3. How to schedule processes(efficiently)?

Scheduling policies

4. How can processescommunicate with each others?

I Signals, shared memory,message passing, etc.

I Synchronization betweenprocesses

Attend next courses to knowmore!



Managing processes

Outline

Concepts


Managing processes



Managing processes

Steps of Boot Sequence

1. Reset of the hardwareI All logic gates are reset to a known state

2. Diagnostic tests are run from the PROM Monitor

3. Bootstrap execution

4. Secondary boot loader execution

5. Starting of the OS kernel

6. User processes can execute!



Managing processes

Step 2: The PROM Monitor

Power On Self TestI System hardware is initialized

I Miniature diagnostic program: Ensures minimal operationalbase for the OS to run (memory, keyboard)

I ⇒ Does not guarantee that the hardware is fully functional!

Scan of busses

To detect hardware devices connected to the system

Loading and starting of the bootstrap

The boot block is located on a fixed position on the boot device



Managing processes

Step 3: Executing the Bootstrap

I Initialization of some peripherals and annex memories

I Loading secondary boot loader



Managing processes

Example: PC BIOS

I POST loads in RAM a program (BIOS) contained in ROM(Read Only Memory)

I When POST completes, it raises an interrupt (]19h)I Interrupt routine

I Loads into RAM a boot sector (location 7c00h)I The boot sector is searched on various devices

I CDROM, Hard disk, Network, Floppy disk, etc.

I Transfer control of the computer to the secondary boot loader

I Note that the BIOS program can be configured before theboot sequence starts

I Interrupt of devicesI Boot devices (e.g., CDROM, etc.)



Managing processes

Step 4: Secondary Bootloader

I Offers interaction with users

I Selection of the kernel to useI Modification of boot parameters

I Maintenance mode, multi-user mode, etc.

I Loads the chosen kernel, and starts itI Example of secondary boot loader

I LiloI GrubI Windows multi-boot loader



Managing processes

Step 5: Starting the OS Kernel

I Allocation of memoryI Kernel, data storage areas, I/O buffers

I Probe of devicesI The kernel builds a device tree and loads corresponding devices

I Creation of spontaneous system processesI Swapper (sched), init, pagedaemon

I Start-up scripts are executed according to run-levelI Start system services (e.g., rlogin daemon)I Windows runlevels

I Multi-user, Safe mode, Safe mode with network



Managing processes

Overviewfork(), vfork(), exec()And one last question!

Outline

Concepts


Managing processesOverviewfork(), vfork(), exec()And one last question!



Managing processes


Controlling processes

I Creation of new processes

I Management of processes

I Termination of processes



Managing processes


Starting and Terminating a Process

A C program starts its execution with a call to:

i n t main ( i n t argc , char ∗ a rgv [ ] )

Termination

I Normal terminationI Return from mainI Call to exit() (or exit())

I Abnormal terminationI Call to abort()I Allocated resources exceededI Cascading terminationI . . .

#inc l u d e < s t d l i b . h>vo id e x i t ( i n t s t a t u s )

#inc l u d e <un i s t d . h>vo id e x i t ( i n t s t a t u s )



Managing processes


Life of a C Program



Managing processes


Call to exit()

1. Process resources are freedI File descriptors

I Shared memory segments

I Semaphores

I Messages queues

I ...

2. The process becomes a zombie process

I Entry in the process allocation table is conserved

I Children’s parent is set to the init process

I All children executing in foreground are terminated



Managing processes


UNIX: Creation of a New Process with fork()

I A running process calls fork()

fork()

#inc l u d e <s y s / t yp e s . h>#inc l u d e <un i s t d . h>

p i d t f o r k ( vo id ) ;

I The new process is a child processI The function is called once but returns twice:

I 0 is returned to the child processI The pid of the child is return to the parent process

I For more information on fork()

$man f o r k



Managing processes


fork(): Code Example

Code#include <sys/types.h>

int glob = 6;

int main(void) {

pid_t pid;

int var = 66;

pid = fork();

if (pid == 0) { /* Child */

glob ++; var ++;

} else {

sleep(1);

}

printf("process pid = %d glob=%d var=%d\n", pid, glob, var);

exit(0);

}

Guess what happens when executing this!



Managing processes


fork(): Drawback and Solution

Drawback: Created child is a clone of its parent

Recopy of data space, heap, stack, etc.⇒ Not very efficient

A few solutions...I fork() under Linux: Copy-On-Write technique

I Memory pages shared by parent and childI Memory pages set to read-only for bothI Copy of the memory when a write operation is performed

I vfork()I Parent process is suspended until the child process makes a

call to exec() or to exit() (Linux)

I exec()I Replaces the current process image with a new process image



Managing processes


vfork() and exec(): Code Example


#include <unistd.h>

int glob = 6;

int main(void) {

int var = 66;

pid_t pid = fork();

if (pid == 0) { /* Child */

if (execl("/bin/sh", "sh", "-c", "/bin/ls", NULL) <0) {

exit(127); /* error */

}

} else {

sleep(1);

}


exit(0);

}

Guess what happens when executing this!Ludovic Apvrille Operating Systems - II. Processes - Fall 2011 29 of 35


Managing processes


Linux: Excerpt of The Manual of fork()

fork()

fork creates a child process that differs from the parent processonly in its PID and PPID, and in the fact that resource utilizationsare set to 0. File locks and pending signals are not inherited.Under Linux, fork is implemented using copy-on-write pages, so theonly penalty incurred by fork is the time and memory required toduplicate the parent’s page tables, and to create a unique taskstructure for the child.



Managing processes


Linux: Excerpt of The Manual of vfork()

vfork()

vfork, just like fork(2), creates a child process of the callingprocess.vfork() is a special case of clone(2). It is used to create newprocesses without copying the page tables of the parent process. Itmay be useful in performance sensitive applications where a childwill be created which then immediately issues an execve.vfork() differs from fork in that the parent is suspended until thechild makes a call to execve(2) or exit(2). The child shares allmemory with its parent, including the stack, until execve is issuedby the child. The child must not return from the current functionor call exit, but may call exit().



Managing processes


Solaris: Excerpt of The Manual of fork() and vfork()

fork()

The fork() and fork1() functions create a new process. The newprocess (child process) is an exact copy of the calling process(parent process).

vfork()

The vfork() function creates new processes without fully copyingthe address space of the old process. This function is useful ininstances where the purpose of a fork(2) operation would be tocreate a new system context for an execve() operation (seeexec(2)).



Managing processes


POSIX: Excerpt of The Manual of vfork()

vfork()

The vfork() function has the same effect as fork(), except that thebehavior is undefined if the process created by vfork() eithermodifies any data other than a variable of type pid t used to storethe return value from vfork(), or returns from the function in whichvfork() was called, or calls any other function before successfullycalling exit() or one of the exec family of functions.



Managing processes


vfork(): Another Code example


int glob = 6;

int main(void) {

pid_t pid;

int var = 66;

pid = vfork(); // previously was fork

if (pid == 0) { /* Child */

glob ++; var ++;

} else { sleep(1);}


exit(0);

}

Executionprocess pid = 0 glob=7 var=67

process pid = 7262 glob=7 var=67



Managing processes


Questions

When I need some information on a function provided by anOperating System, what should I do?

I Use google? wikipedia? Other sites?

I Use local manual pages?

Why?

Local manual pages

$man −s2 f o r k$ i n f o f o r k


Documents

Operating Systems II. Processessoc.eurecom.fr/OS/docs/slideProcesses.pdf · I Process = unit of work of a system ... Ludovic Apvrille Operating Systems - II. Processes - Fall 2011