PART II Operating Systems LECTURE 9 PROCESSES MANAGEMENT Ştefan Stăncescu 1

PART IIOperating Systems

LECTURE 9

PROCESSES MANAGEMENT

Ştefan Stăncescu

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blocked - not completely inactive, without time share stored process status, ready to be included in the time share active - active state in time slot share ready - inactive state but ready to receive share time slot


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PROCESSES MANAGEMENT:

Share time slot allocation policy

PROCESSES Dispatcher:

priority process of OS dealing with PROCESSES

MANAGEMENT “applications”

arbitration between processes in the allocation of time slots


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Criteria for assessment of dispatching algorithms:- fair play

any process takes some proper CPU

- Efficiency

CPU busy all the time

- reaction timeinteractive users to be served promptly

- Processing time

users that require outputs to be served quickly

- productivityincreasing the number of work / unit time.


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Specific contradictory processes elements ,

which can not be addressed dispatching standard:

- any evolutionary process is unique and unpredictable (even same evolutionary process is different every resume processing with other data)

- each process uses unpredictable resources quantity (number) and quality (type)

- each process can not be estimated previously running occurrence and duration of locking.


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The main problem: excessive blocking a process.

Simple control : clock 50 Hz (or other ranges)

- every interval - examines state of processes

- decision to continue or end the current process

Variations dispatching processes :

- run to completion - batch processing

one takes it and learn to control their own evolution

(cooperative scheduling)

- preemptive scheduling one process influence the evolution of the other - the dialogue between processes (eg supervisor).

- - RT scheduling – evolution of self-imposed periods previously known

PROCESSES MANAGEMENT – Scheduling algorithms

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Round Robin scheduling

old, simple, fair play, often used

rule: democratic distribution of time resources for CU

- Any process running in the same time period - (quanta common, equal to any process)

- (A) the end of the quantum, or (b) it has been blocked before or if (c) ended before

- Switch in ready tail and

- choose the next process in ready queue


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Round Robin scheduling

Disadvantage: unique quantum size selection:

- 5ms: CU is losing time (1ms) switching each process

- 500ms: large response time in interactive activities

(500ms x 10 trials = 5 s response time console)

Compromise (100ms) is difficult - unacceptable for all


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Priority scheduling any process has a priority level, the high priority runs first => it decrements the priority process

on CLOCK IT; when sufficiently diminishing his priority

choose another process.

Processes assigned priorities are established:

-fixed – grade: general 100, colonel 90, captain 80 etc.

- cost:100lei/h -100;50lei/h -50 ;1leu/h - 1; etc.

-dynamic – priority assigned according to the current situation demand / supply

Increased priority will be assigned to processes that require more resources available processes that require urgent access (mouse window)


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Multiple queuing scheduling

For example: m m tails round robin


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Multiple queuing schedulingThe process requires 500 quanta

Tails: 1,2,4,8,16,32,64,128

Round Robin on 1-100 switching (500ms CU lost)

Run the m m queue slots 2^^n

=>1=>2=>4=>8=>16=>32=>64 =>>> 7 switching

(7ms CU lost)


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Short Job First scheduling Running necessary time in order ABCD:8444

ABCD => 8+(8)+4+(8+4)+4+(8+4+4)+4 =56

Total 56:4=14 for each average.

CDBA=> 4+(4)+4+(4+4)+4+(4+4+4)+4=44

Total 44:4=11 for each average

It requires previous knowledge of the runtime(applications RT – algorithms RT)


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Short Job First scheduling - OKbut => we need run interval DR for each process

solution=collect info from previous runs of the processes

DRestimated for future = a * DRpast + (1-a) * DRpresent

At each run, from precedent and present DR => future DR

a < 1/2 => fast radical change => liberalism,

a => 0 instability,

a > 1/2 => slow change => conservatory,

a => 1 no change al all, inadaptability


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Guaranteed schedulinggood run guaranteed for each process

ex: each from n processes should have DR=1/n of DRtotal

=> A cell memory DRcosummed for each process

Calculate new priority for next run with

1/priority= DRcosummed / DRtotal

Random lottery schedulingEach process receive a set of lottery tickets

Run probability = nr tickets ( may be OS controlled)

Threads in each process may transfer tickets between

From blocked threads to “important” threads


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RT schedulingembedded systems – microcomputers

RT requirements – deadline mark for complete run

Hard RT - absolute requirements to be respected

Soft RT - easy – recommended requirements

Generally DR known for each, start from RT events

For a/periodic events – RT general schedule

For periodic events – Liu Layland schedulability formula∑m

i=1 DRi / Pi ≤ 1

(n processes, DRi= DR of i process, started by event at Pi time interval )


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RT scheduling EXAMPLE periodic RT (rotating shaft w/sensors & actuators)

Pi=50ms

5 processes with DRi=25ms each

` ∑mi=1 DRi / Pi = 5 * 25 / 50 = 2,5 > 1

3 processors are needed (or clock increase > 2,5 times)


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Threads scheduling Scheduling in kernel or user zone

Threads list in user

Application management fast and optimized in the process

Threads list in kernel

Centralized management for all threads in all processes

fairness, fot all processes, but slow in save/restore

Compromise:

Kernel list in supervisor w/parameters by user

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PART II Operating Systems LECTURE 9 PROCESSES MANAGEMENT Ştefan Stăncescu 1