Operating Systems: Introduction

1. Operating Systems: Introduction • 1. Historical Development • 2. The OS as a Resource Manager • 3. Definitions • 4. The Process

2. The OS as a Resource Manager • Resources • 4 functions • Keep track of resource • Enforce policy - • Allocate • Reclaim

3. The OS as a Resource Manager • Memory management • Keep track • If multiprogramming: • Allocate • Reclaim (pi relinquishes it or terminates)

4. The OS as a Resource Manager • Processor management • Keep track of • Scheduling: • Allocate : • Reclaim : pi relinquishes P, terminates, exceeds TS

5. The OS as a Resource Manager • Device management • Keep track of devices, channels, control units • Efficient allocation policy • Allocate : • Reclaim : Normally automatic termination

6. The OS as a Resource Manager • Information management • Keep track : Location, use, status ( ) • Who gets use of resources, enforce protection, provide accessing routines • Allocate (“Open file”) • Deallocate (“Close file”)

7. Processor management lower module (P,V), Process Scheduler Level 5 Kernel Level 4 Level 3 Memory management Level 2 Level 1 Processor management upper module (messages, create-destroy process) Bare machine Device management (I/O traffic control) Info management (File system)

8. Jobs Supervisor process (Job scheduler) Hierarchical OS Structure Process 1 Process 2 SPOOLing Process 3 User-created process I/O process I/O process working with User process 3 Layer 1 Layer 2 Kernel (Layer 0)

9. Definitions • Operating System (OS) : • Software governing control of resources • ( ) • Interface • Processor : Hardware which interprets and executes instructions • Process (task) : • Job : Set of modules req’d to perform some task

10. Definitions • Multiprogramming : • Privileged instruction : • Instruction available only to • Executed in supervisor (executive) state as opposed to problem (user) state. • Interrupt : • Mechanism forcing processor to take notice of event

11. I.4 The Process • Process (pi) : A program in a state of execution • Life : (Higher level view) • 1. Run : pi<-P • 2. Wait : For event • 3. Ready : RUN Wait for I/O completion pi P READY WAIT I/O complete

12. Life of a Process • Lower level view : COMPLETE RUN HOLD READY WAIT SUBMIT

13. Processor Management and Scheduling • 1. Introduction • 2. Basic Concepts • 3. Processor Scheduling Algorithms

14. Introduction • Given : 2 jobs, A and B. • Each executes for 1 sec, waits for 1 sec • Repeat this for 60 cycles [ 2 min ] • 1. Let’s run A, then B A: B: 2 min 2 min Elapsed Time : 4 min Compute Time : 2 min CPU Utilization : 2/4 = 50%

15. Introduction • 2. Now multi-program A and B A: B: • Elapsed Time : 2+ min • Compute Time : 2 min CPU Utilization : 2/2+ = ~100% • Compare to 1 : A finishes at same time, • B in half the time.

16. Introduction • So now we have multiprogramming: • N jobs in memory • Job i P • When i waits for I/O, j P • Overlap CPU and I/O to keep P busy • Benefits : Increased CPU utilization and higher throughput. • Throughput : Work done in given period. • E.g. 10 jobs per hour.

17. Basic Concepts • Process : a program in a state of execution. • E.g. Batch job, transaction (in T-S system) • Also called : job, program, task, activity. • Process behavior : pi alternate between execution(CPU burst) & I/O(I/O burst). • Generally begins and ends with CPU bursts.

18. Basic Concepts • pi change state as they execute • READY, RUN, WAIT, HOLD. • Process Control Block (PCB) • Process is represented internally by its PCB • Active representation of passive entity, the PGM • “The only tangible part of a process”

19. PCB • Process id • Current State • Priority • Other CPU Scheduling info • State info : PC (address of NI to be executed) register, cc contents • Memory management info : base-bounds registers, page tables • Accounting info : amount of CPU time, account # • I/O status info : I/O requests pending, I/O devices allocated, list of open files • Pointer to list of all pi in same state • etc.

20. Basic Concepts • Scheduling queues • Ready queue : List of processes (PCBs) in ready state (awaiting assignment of P) • Device queue : List of pi waiting on this device • I/O queue : pi waiting for I/O (once served, pi moves to Ready queue). . . . Head Tail PCB i PCB j PCB n Registers Registers Registers Queue . . . . . . . . . header

21. Basic Concepts • Schedulers : • Job (long-term) scheduler : • Selects job from spool queue to enter system, loads it into memory • Processor (CPU, short-term) scheduler : • Selects ready pi and dispatches (assigns P to) it

22. Difference : How often they execute • Job scheduler • Infrequently once steady state is reached • Controls degree of multiprogramming (number of pi in memory) • Processor scheduler • Must select new pi very frequently (every 10 ms) • FAST or much of the processor time is spent in scheduling !

23. Basic Concepts • Dispatcher • Assigns P to pi selected by Processor scheduler • Loads pi’s registers • Changes to user mode • Jumps to proper address [ to (re)start it ]

24. Processor Scheduling Algorithms • Problem : Which pi in Ready queue gets P? • Performance Criteria (Comparing algorithms): • Throughput : Work done in given period. • CPU utilization : P busy time / Total elapsed time • Want it as busy as possible (40-90%). • Turnaround time : Interval between submission to completion of job (Batch OS). • Wait time : Time spent by pi in Ready queue. • Response time : Interval from submission until response produced (Interactive system).

25. Processor Scheduling Algorithms • Let’s optimize as follows : • Maximize CPU utilization, throughput • Minimize turnaround time (TT), wait, response time. • Operationally : • Optimize average or max or min • e.g. Minimize the max response time Minimize variance in response time.

26. Common Scheduling Algorithms • (a) First Come First Served (FCFS) • (b) Shortest Job First (SJF) • (c) Priority • Preemptive vs Non-preemptive • (d) Round Robin • (e) Multilevel Queues

27. Scheduling Algorithms • First Come First Served (FCFS) • Implementation : FIFO queue • pj enters ready q : • pi dispatched : • Consider : Job CPU burst A 24 B 3 C 3 pi pk ... pj RQ pk ... pj RQ

28. FCFS • Suppose jobs arrive in order A, B, C and are served FCFS. • TT : For A = 24, For B = 27, For C = 30 • Avg TT : (24 + 27 + 30) / 3 = 27 • Now, suppose they arrive in order B, C, A • Avg TT : (3 + 6 + 30) / 3 = 13 A B C 0 24 27 30 B C A 0 3 6 30

29. Scheduling Algorithms • Shortest Job First (SJF) • Associates with job the length of its next CPU burst • Example of priority scheduling (job with shortest next CPU burst gets highest priority) • Consider : Job CPU burst A 6 B 3 C 8 D 7

30. B[3] A[6] D[7] C[8] • SJF • Avg TT : (3 + 9 + 16 + 24) / 4 = 13 • This algorithm can be proved optimal • Gives min avg wait time for given set of jobs • Problem : Length of next CPU burst • Batch jobs : Time limit supplied by user • Processes : Can’t know [but can predict] 0 3 9 16 24

31. Scheduling Algorithms • Priority Scheduling • Can be assigned : who is paying, type of work • Can be computed : • Time limits, memory req., ratio avg I/O burst to CPU burst, number of open files • Major problem : Starvation • Low priority Ready pi can wait indefinitely for CPU • Eventually, load lightens and job gets run, or system crashes and job is lost • Remedy • Aging : Increase priority of low priority job over time

32. Scheduling algorithms • Round Robin (RR) • Designed for TS systems • Given defined TS (10-100 ms). • CPU scheduler traverses Ready queue, selects pi to be dispatched for interval  TS • Implementation : • Ready queue = FIFO queue • CPU scheduler picks 1st job in Ready queue • Sets timer to interrupt after 1 TS • Dispatches process

33. Scheduling algorithms • Multilevel Queue • Good when jobs easily classified into groups • e.g. foreground (FG - interactive) and background (BG - batch) • In a multilevel scheduling algorithm : • Ready queue partitioned into separate queues • Each pi assigned permanently to one queue • Due to memory size, job type, … • Each queue has its own scheduling algorithm • FG : RR, BG : FCFS • Must have scheduling between the queues • e.g. Fixed priority preemptive (such as FG > BG)