Process Descript ion and Cont rol 1 Process:t he concept Pr ocess - - PowerPoint PPT Presentation

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Process Descript ion and Cont rol

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Process:t he concept

Pr ocess = a pr ogr am in execut ion

• Example pr ocesses:

– OS ker nel – OS shell – Pr ogr am execut ing af t er compilat ion – www-br owser

Pr ocess management by OS :

– Allocat e r esour ces – Schedule: int er leave t heir execut ion (wat ch f or pr ocessor ut ilizat ion, r esponse t ime) – I nt er pr ocess communicat ion, synchr onizat ion (wat ch f or deadlocks; int er leaving, nondet er minism imply incr eased dif f icult ies!)

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Two-St at e Process Model

• Pr ocess may be in one of t wo st at es

– Running, Not -r unning – Not -Running Pr ocess in a Queue

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What are not -running processes doing?

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Act ually t here are more queues …

Obser ve: I / O’s much slower t han CPU What if all pr ocesses ar e wait ing f or I O and all memor y is allocat ed? Or if not enough memor y f or all pr ocesses t o execut e? Or if ...

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Suspended Processes

I dea: Swap t hese processes t o disk t o f ree up more memory (t o admit new processes)

• Blocked st at e becomes suspend st at e when

swapped t o disk

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Two Suspend St at es

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Reasons f or Process Suspension

e.g. t o prevent deadlock

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OS Cont rol St ruct ures

What a pr ocess needs in or der t o execut e (pr ocess’ image):

• Pr ogr am
• Dat a
• St ack
• Pr ocess Cont r ol Block

(cont ext ; f or mult ipr ogr amming) The OS must keep:

• I nf or mat ion about t he

cur r ent st at us of each pr ocess and r esour ce

• Tables ar e const r uct ed f or

each ent it y t he oper at ing syst em manages

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Process Table (and ot her OS t ables)

Each ent r y:

• Wher e pr ocess is

locat ed

• At t r ibut es necessar y

f or it s management

– Process I D – Process st at e – Locat ion in memory – Et c: process cont rol block

• Ot her t ables hold

r esour ce-specif ic inf o (zoom int o lat er ):

– How main/ secondary memory is allocat ed, – I / O device st at us, buf f er in memory,… – File st at us, locat ion, at t ribut es, …

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Process Cont rol Block

PCB cont ains:

• I dent if ier s (pr ocess, par ent

pr ocess, user , … )

• Pr ocessor St at e I nf or mat ion

(r egist er values: must be copied and r est or ed in st at e t r ansit ions: r unningr eady,… )

• Ot her Pr ocess Cont r ol I nf or mat ion:

– Scheduling and St at e I nf ormat ion (priorit y, event await ed, … ) – Process memory t ables – Resources (open f iles, ownership, … ) – Links (t o ot her process in a queue, … ) – Privileges – …

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Process Creat ion

Examples:

• Execut ion of a compiled user program
• User logs on (shell st art s execut ing)
• Process creat ion t o provide a service

(e.g. print ing: user execut es lpr)

• A process creat es anot her process (e.g.

shell creat es lpr; user programs can creat e processes, as well)

OS must :

• Assign a unique process ident if ier
• Allocat e space f or t he process
• I nit ialize process cont rol block
• Set up appropriat e linkages

– “I nclude” t he process in t he syst em (in some queue(s), … )

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When t o Swit ch a Running Process

• Clock int er r upt

– pr ocess has execut ed f or t he maximum allowable t ime slice

• I / O int er r upt
• Memor y f ault

– memor y addr ess is in vir t ual memor y so it must be br ought int o main memor y

• Tr ap

– er r or occur r ed – may cause pr ocess t o be moved t o Exit st at e

• Super visor call (or syst em call)

– such as f ile open

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Change of Process St at e (cont ext swit ching)

r unningot her

• Save cont ext of pr ocessor including pr ogr am count er and
• t her r egist er s (in PCB)
• Move PCB t o appr opr iat e queue (r eady, blocked, …

)

• Select anot her pr ocess f or execut ion
• Updat e t he PCB of t he pr ocess select ed
• Updat e memor y-management dat a st r uct ur es
• Rest or e cont ext (in pr ocessor ) of t he select ed pr ocess

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Modes of Execut ion

• User mode

– Less-pr ivileged mode – User pr ogr ams t ypically execut e in t his mode

• Syst em mode, cont r ol mode, or ker nel mode

– Mor e-pr ivileged mode – Ker nel of t he oper at ing syst em

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Execut ion of t he Operat ing Syst em

• Non-pr ocess Ker nel

– oper at ing syst em code is execut ed as a separ at e ent it y t hat oper at es in pr ivileged mode

• Pr ocess-Based Oper at ing

Syst em

– maj or ker nel f unct ions ar e separ at e pr ocesses – Usef ul in mult i-pr ocessor or mult i-comput er envir onment

• Execut ion Wit hin User

Pr ocesses

– oper at ing syst em sof t war e wit hin cont ext of a user pr ocess – pr ocess execut es in pr ivileged mode when execut ing oper at ing syst em code

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Threads

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Processes and Threads

mult it hr eading: mor e t han one ent it ies can possibly execut e in t he same r esour ce- (i.e. pr ocess-) envir onment (and collabor at e bet t er ) Unit of …

• …

dispat ching is r ef er red t o as a t hr ead

• …

r esour ce

• wner ship is

r ef er red t o as a pr ocess or t ask

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A process has … A t hread has …

• a vir t ual addr ess space which

holds t he pr ocess image

• global var iables, f iles, child

pr ocesses, signals and signal handler s

• an execut ion st at e,

st ack and cont ext (saved when not r unning)

• access t o t he memor y

and r esour ces of it s pr ocess

– all t hreads of a process share t his

• some per -t hr ead

st at ic st or age f or local var iables

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Suspension and t erminat ion

• Suspending a process involves suspending all

t hreads of t he process since all t hreads share t he same address space

• Terminat ion of a process, t erminat es all

t hreads wit hin t he process

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Benef it s of Threads

• Since t hreads wit hin t he same process share

memory and f iles, t hey can communicat e wit h each ot her wit hout invoking t he kernel

• May allow parallelizat ion wit hin a process:

– I / O and comput at ion t o over lap (r emember t he

hist or ical st ep f r om unipr ogr amming t o mult ipr ogr amming?)

– concur r ent execut ion in mult ipr ocessor s

• Takes less t ime t o

– cr eat e/ t er minat e a t hr ead t han a pr ocess – swit ch bet ween t wo t hr eads wit hin t he same pr ocess

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Uses of Threads

• Overlap f oreground (int eract ive) wit h

background (processing) work

• Asynchronous processing (e.g. backup while

edit ing)

• Speed execut ion (parallelize independent

act ions)

• Modular program st ruct ure (must be caref ul

here, not t o int roduce t oo much ext ra

• verhead)

Q: can one achieve parallelism wit hin a process but wit hout using t hreads?

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I mplement ing Threads in User Space

The kernel is not aware of t he exist ence of t hreads;

Fig: Tanenbaum, Modern OS, 2/ e

• Run-t ime syst em (t hr ead-

libr ar y in execut ion) is r esponsible f or bookkeeping, scheduling of t hr eads

• allows f or cust omised

scheduling

• can r un on any OS
• But : pr oblem wit h blocking

syst em calls (when a t hr ead blocks, t he whole pr ocess blocks; i.e ot her t hr eads of t he same pr ocess cannot r un)

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I mplement ing Threads in t he Kernel

Kernel maint ains cont ext inf ormat ion f or t he process and t he t hreads

Fig: Tanenbaum, Modern OS, 2/ e

• Scheduling is done
• n a t hr ead basis
• Does not suf f er

f r om “blocking pr oblem”

• Less ef f icient t han

user -level t hr eads (ker nel is invoked f or t hr ead cr eat ion, t er minat ion, swit ching)

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Hybrid I mplement at ions

Mult iplexing user-level t hreads ont o kernel- level t hreads

Fig: Tanenbaum, Modern OS, 2/ e

To combine t he advant ages of t he ot her t wo appr oaches

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Examples:

• Posix Pt hr eads: (I EEE) st andar d:

– Specif ies int er f ace – I mplement at ion (using user / ker nel level t hr eads) is up t o t he developer (s) – Mor e common in UNI X syst ems

• Win32 t hr ead libr ar y:

– Ker nel-level libr ar y, windows syst ems

• J ava t hr eads:

– Suppor t ed by t he J VM (VM: a r un-t ime syst em, a gener al concept , wit h deeper r oot s and pot ent ial f ut ur e in t he OS wor ld) – I mplement at ion is up t o t he developer s –e.g. can use Pt hr eads API or Win32 API , et c

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Examples (cont )

• Solaris: hybrid model

– User -level t hr eads – Light weight pr ocesses – Ker nel t hr eads

• W2K, XP: hybr id

model

– Thr ead: ker nel (use win32 API )

• Fiber : user -level

t hr ead (libr ar y)

• Linux: Only ker nel-

level – clone sys-call: uses par amet er s t o descr ibe amount of r esour ce shar ing – Clone can somet imes equal a f or k, i.e. new t hr ead can be ”new pr ocess”

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Scheduler Act ivat ions

• Goal – mimic f unct ionalit y of kernel t hreads

– gain per f or mance of user space t hr eads

• Avoids unnecessary user/ kernel t ransit ions

– Ker nel assigns vir t ual pr ocessor s t o each pr ocess – ”signals” (make upcalls) t o r un-t ime syst em upon blocking and unblocking of (user -level) t hr eads – let s r unt ime syst em allocat e t hr eads t o pr ocessor s

• Concern:

Fundament al reliance on kernel (lower layer) calling procedures in user space (higher layer)