PART II Operating Systems

1. Ştefan Stăncescu PART IIOperating Systems LECTURE 9 PROCESSES MANAGEMENT

2. PROCESSES MANAGEMENT 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

3. PROCESSES MANAGEMENT • 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

4. PROCESSES MANAGEMENT • Criteria for assessment of dispatching algorithms: • fair play • any process takes some proper CPU • Efficiency • CPU busy all the time • -reaction time • interactive users to be served promptly • Processing time • users that require outputs to be served quickly • productivity • increasing the number of work / unit time.

5. PROCESSES MANAGEMENT • 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.

6. PROCESSES MANAGEMENT • 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

7. PROCESSES MANAGEMENT – Scheduling algorithms • 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

8. PROCESSES MANAGEMENT – Scheduling algorithms • 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

9. PROCESSES MANAGEMENT – Scheduling algorithms • 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)

10. PROCESSES MANAGEMENT – Scheduling algorithms • Multiple queuing scheduling • For example: m m tails round robin

11. PROCESSES MANAGEMENT – Scheduling algorithms • Multiple queuing scheduling • The 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)

12. PROCESSES MANAGEMENT – Scheduling algorithms • 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)

13. PROCESSES MANAGEMENT – Scheduling algorithms Short Job First scheduling - OK but => 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 13

14. PROCESSES MANAGEMENT – Scheduling algorithms Guaranteed scheduling good 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 scheduling Each 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 14

15. PROCESSES MANAGEMENT – Scheduling algorithms RT scheduling embedded 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 ∑mi=1 DRi / Pi ≤ 1 (n processes, DRi= DR of i process, started by event at Pi time interval ) 15

16. PROCESSES MANAGEMENT – Scheduling algorithms RT scheduling EXAMPLE periodicRT (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) 16

17. PROCESSES MANAGEMENT – Scheduling algorithms 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 17