[PDF] - P rograms vs P rocesses A pr ogr am is passive 3: P rocesses PDF Document

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3: P rocesses

Last Modif ied: 6/ 1/ 2004 11:53:05 AM

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P rograms vs P rocesses

A pr ogr am is passive

Sequence of commands wait ing t o be r un

A pr ocess is act ive

An inst ance of pr ogr am being execut ed Ther e may be many pr ocesses r unning t he same

pr ogr am

Also called j ob or t ask

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What makes up a process?

Address space Code Data St ack (nest ing of procedure calls made) Regist er values (including t he PC) Resources allocat ed t o t he process

Memor y, open f iles, net wor k connect ions

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Address Space Map

St ack

(Space f or local var iables et c. For each nest ed pr ocedur e call)

Heap

(Space f or memor y dynamically allocat ed e.g. wit h malloc)

St at ically declar ed var iables

(Global var iables)

Code

(Text Segment )

St ack Point er PC Ox0000 Biggest Vir t ual Address Somet imes Reserved f or OS Somet imes Reserved f or Error Cat ching

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What kinds of processes are t here?

Comput e bound/ I O bound Long-running/ short-r unning I nt eract ive/ bat ch Large/ small memory f oot print Cooperat ing wit h ot her processes? … How does t he OS cat egorize processes?

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P rocess St at es

During t heir lif et ime, processes move

bet ween various st at es

Ready – wait ing f or a t ur n t o use t he CPU Running – cur r ent ly execut ing on t he CPU

How many processes can be in t his st at e?

☺

Wait ing – Unable t o use t he CPU because

blocked wait ing f or an event

Ter minat ed/ Zombie – Finished execut ing but

st at e maint ained unt il par ent pr ocess r et r ieves st at e

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St at e Transit ions

New Ready Running Terminat ed Wait ing

Schedule/ unschedule Request Resource

r Service

Gr ant Resour ce

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St at e Queues

OSes of t en maint ain a number of queues of

processes t hat represent t he st at e of t he processes

All t he r unnable pr ocesses ar e linked t oget her

int o one queue

All t he pr ocesses blocked (or per haps blocked

f or a par t icular class of event ) ar e linked t oget her

As a pr ocess changes st at e, it is unlinked f r om

ne queue and linked int o anot her
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Cont ext Swit ch

When a process is running, some of it s

st at e is st ored direct ly in t he CPU (regist er values, et c.)

When t he OS st ops a process, it must save

all of t his hardware st at e somewhere (PCB) so t hat it can be r est or ed again

The act of saving one processes hardware

st at e and rest oring anot her’s is called a cont ext swit ch

100s or 1000s per second!

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Cont ext Swit ch

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Schedulers

Long-t erm scheduler (or j ob scheduler) –

select s which processes should be brought int o t he ready queue.

Short-t erm scheduler (or CPU scheduler) –

select s which process should be execut ed next and allocat es CPU.

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Schedulers (cont )

Shor t-t er m scheduler is invoked ver y f r equent ly

(milliseconds) ⇒ (must be f ast ).

Long-t er m scheduler is invoked ver y inf r equent ly

(seconds, minut es) ⇒ (may be slow).

The long-t er m scheduler cont r ols t he degr ee of

mult ipr ogr amming.

Pr ocesses can be descr ibed as eit her :

I / O- bound process – spends more t ime doing I / O t han

comput at ions, many short CP U burst s.

CPU-bound process – spends more t ime doing

comput at ions; f ew very long CP U burst s.

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Family Tree

Age old quest ions – wher e do new pr ocesses come

f rom?

New pr ocesses ar e cr eat ed when an exist ing

pr ocess r equest s it

Creat ing process called t he parent ; creat ed called t he

child

Children of same parent called siblings

Childr en of t en inher it pr ivileges/ at t r ibut es f r om

t heir parent

Working direct ory, Clone of address space

When child is cr eat ed, par ent may eit her wait f or

it or cont inue in par allel

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pst ree

gt op

- - 8*[nautilus]]

| - 2*[naut ilus- - - naut ilus

- -10*[naut ilus]]

| - 3*[naut ilus- - - naut ilus

- - 9*[nautilus]]

| - naut ilus- - - naut ilus---7*[naut ilus] | - 7*[naut ilus- hist or] | - naut ilus- mozill- - - naut ilus

mozill- - - 4*[naut ilus-

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I nit process

I n last st age of boot pr ocess, ker nel cr eat es a

user level pr ocess, init

I nit is t he par ent (or gr andpar ent …

) of all ot her pr ocesses

I nit does var ious impor t ant housecleaning

act ivit ies

checks and mount s t he f ilesyst ems, set s host name,

t imezones, et c. I nit r eads var ious “r esour ce conf igur at ion f iles”

(/ et c/ rc.conf , et c) and spawns of f pr ocesses t o pr ovide var ious ser vices

I n mult i -user mode, init maint ains processes f or

each t er minal por t ( tty )

Usually runs get t y which execut es t he login program

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How is a process represent ed?

Usually a process or t ask obj ect Process Cont rol Block When not running how does t he OS

remember everyt hing needed t o st art t his j ob r unning again

Regist er s, St at ist ics, Wor king dir ect or y, Open

f iles, User who owns pr ocess, Timer s, Par ent Pr ocess and sibling pr ocess ids I n Linux, t ask_st ruct def ined in

include/ linux/ sched.h

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struct task_struct { /* these are hardcoded- don't touch */ volatile long state; /*

1 unrunnable, 0runnable, >0

stopped */ long counter; long priority; unsigned long signal; unsigned long blocked; /* bitmap of masked signals */ unsigned long flags; /* per process flags, defined below */ int errno; long debugreg[8]; /* Hardware debugging registers */ struct exec_domain *exec_domain; /* various fields */ struct linux_binfmt*binfmt; struct task_struct *next_task, * prev_task; struct task_struct *next_run, * prev_run; unsigned long saved_kernel_stack; unsigned long kernel_stack_page; int exit_code, exit_signal; /* ??? */ unsigned long personality; int dumpable :1; int did_exec:1; /* shouldn't this be pid _t? */ int pid; int pgrp; int tty_old_pgrp; int session; /* boolean value for session group leader */ int leader; int groups[NGROUPS]; /* * pointers to (original) parent process, youngest child, younger sibling, * older sibling,

respectively. (p ->father can be replaced with * p-

>p_pptr ->pid ) */ struct task_struct *p_opptr, *p_ pptr, *p_cptr , *p_ysptr, *p_osptr; struct wait_queue *wait_chldexit; /* for wait4() */ unsigned short uid,euid ,suid,fsuid; unsigned short gid,egid ,sgid,fsgid; unsigned long timeout, policy, rt_priority; unsigned long it_real_value, it_prof_value, it_virt_value; unsigned long it_real_incr , it_prof_incr, it_virt_incr; struct timer_list real_timer; long utime, stime , cutime, cstime , start_time; /* mm fault and swap info: this can arguably be seen as either mm-specific or thread -specific */ unsigned long min_flt, maj_flt, nswap, cmin_flt, cmaj_flt, cnswap; int swappable:1; unsigned long swap_address; unsigned long old_maj_ flt; /* old value of maj_flt */ unsigned long dec_flt; /* page fault count of the last time */ unsigned long swap_cnt; /* number of pages to swap on next pass */ /* limits */ struct rlimit rlim[RLIM_NLIMITS]; unsigned short used_math; char comm[16]; /* file system info */ int link_count; struct tty_struct *tty; /* NULL if no tty */ /* ipc stuff */ struct sem_undo * semundo ; struct sem_queue *semsleeping; /* ldt for this task - used by Wine. If NULL, default_ldt is used */ struct desc_struct *ldt; /* tss for this task */ struct thread_struct tss; /* filesystem information */ struct fs_struct *fs; /* open file information */ struct files_struct *files; /* memory management info */ struct mm_struct *mm; /* signal handlers */ struct signal_struct *sig; #ifdef __SMP__ int processor; int last_processor; int lock_depth; /* Lock depth. We can context switch in and out of holding asyscall kernel lock... */ #endif };

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Management of P CBs

PCBs ar e dat a st r uct ur es (j ust like you ar e used t o

at user level)

Space f or t hem may be dynamically allocat ed as

needed or per haps a f ixed sized ar r ay of PCBs f or t he maximum number of possible pr ocesses is allocat ed at init t ime

As pr ocess is cr eat ed, a PCB is assigned and

init ialized f or it

Of t en process id is an of f set int o an array of P

CBs Af t er pr ocess t er minat es, PCB is f r eed

(somet imes kept ar ound f or par ent t o r et r ieve it s exit st at us)

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St at e Queues

Tail pt r Head pt r Pr ev next Rest of P CB Pr ev next Rest of P CB Pr ev next Rest of P CB Ready queue, queues per device, queue of all pr ocesses, …

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Cont ext Swit ch

When a pr ocess is r unning, some of it s st at e is

st or ed dir ect ly in t he CPU (r egist er values, et c.)

When t he OS st ops a process, it must save all of

t his hardware st at e somewhere (PCB) so t hat it can be r est or ed again

The act of saving one pr ocesses har dwar e st at e

and r est or ing anot her ’s is called a cont ext swit ch

100s or 1000s per second!

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UNI X process creat ion

Fork() syst em call

Cr eat es a new PCB and a new addr ess space I nit ializes t he new addr ess space wit h a * copy*

f t he par ent ’s addr ess space

I nit ializes many ot her r esour ces t o copies of

t he par ent s (e.g. same open f iles)

Places new pr ocess on t he queue of r unnable

pr ocesses Fork() ret urns t wice: t o parent and child

Ret ur ns child’s pr ocess I D t o t he par ent Ret ur ns 0 t o t he child

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Example Code Snippet

int main (int argc, char **argv) { int childPid; childPid = fork(); if (childPid == 0){ printf(“Child running\n”); } else { printf(“Parent running: my child is %d\n”, childPid); } }

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Out put

%./tryfork Parent running: my child is 707 Child running %

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Experiment s

Tr y put t ing an inf init e loop in t he child’s por t ion (

do you r et ur n t o t he command shell?) and t hen looking f or it in t he ps out put

Tr y put t ing an inf init e loop in t he par ent ’s por t ion

(do you r et ur n t o t he command shell?)

Put an inf init e loop in bot h

t ry killing t he child (look in t he ps out put f or t he child

and t he parent )

Try killing t he parent – what happens t o t he child?

WARNI NG: DO NOT PUT THE FORK COMMAND

I TSELF I N AN I NFI NI TE LOOP!!! YOU WI LL CRASH THE SYSTEM!

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Fork and Exec

How do we get a br and new pr ocess not j ust a copy

f t he parent ?

Exec () syst em call int exec (char * prog, char ** argv)

Exec:

St ops t he current process Loads t he program, prog, int o t he address space P

asses t he argument s specif ied in argv

P

laces t he P CB back on t he ready queue Exec “t akes over ” t he pr ocess

There is no going back t o it when it ret urns Try t o exec somet hing in your shell (example: exec ls) –

when ls is done your shell is gone because ls replaced it !

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UNI X Shell

int main (int argc, char **argv) { while (1){ int childPid; char * cmdLine = readCommandLine(); if (userChooseExit(cmdLine)){ wait for all background jobs } childPid = fork(); if (childPid == 0){ setSTDOUT _STDIN_STDERR(cmdLine); exec ( getCommand (cmdLine )); } else { if (runInForeground(cmdLine)){ wait ( childPid); } } }

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Windows P rocess Creat ion

BOOL CreateProcess( LPCTSTR lpApplicationName, // name of executable module LPTSTR lpCommandLine, // command line string LPSECURITY_ATTRIBUTES lpProcessAttributes, // SD LPSECURITY_ATTRIBUTES lpThreadAttributes, // SD BOOL bInheritHandles, // handle inheritance option DWORD dwCreationFlags, // creation flags LPVOID lpEnvironment, // new environment block LPCTSTR lpCurrentDirectory, // current directory name LPSTARTUPINFO lpStartupInfo, // startup information LPPROCESS_INFORMATION lpProcessInformation // process information );

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Windows vs Unix

Windows doesn’t maint ain t he same

relat ionship bet ween parent and child

Lat er ver sions of Windows have concept of

“j ob” t o mir r or UNI X not ion of par ent and childr en (pr ocess gr oups) Wait ing f or a process t o complet e?

Wait f or SingleObj ect t o wait f or complet ion Get Exit CodePr ocess ( will r et ur n STI LL_ALI VE

unt il pr ocess has t er minat ed)

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Cooperat ing P rocesses

Pr ocesses can r un independent ly of each ot her or

pr ocesses can coor dinat e t heir act ivit ies wit h

t her pr ocesses

To cooper at e, pr ocesses must use OS f acilit ies t o

communicat e

One example: parent process wait s f or child Many ot hers

Shared Memory
Files
Socket s
Pipes
Signals
Event s
Remot e Pr ocedur e Call
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Socket s

A socket is an end-point f or communicat ion

ver t he net work

Creat e a socket

int socket(int domain, int type, int

protocol)

Type = SOCK_STREAM f or TCP

Read and wr it e socket j ust like f iles Can be used f or communicat ion bet ween

t wo processes on same machine or over t he net work

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P ipes

Bi-direct ional dat a channel bet ween t wo

processes on t he same machine

Creat ed wit h:

int pipe (int fildes[2])

Read and wr it e like f iles

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Signals

Pr ocesses can r egist er t o handle signals wit h t he

signal f unct ion

void signal (int signum, void (*proc) (int))

Pr ocesses can send signals wit h t he kill f unct ion

kill (pid, signum)

Syst em def ined signals like SI GHUP (0), SI GKI LL

(9), SI GSEGV(11)

I n UNI X shell, t ry: