1. OS Spring ’ 04 Processes Operating Systems Spring 2004

2. OS Spring ’ 04 What is a process?  An instance of an application execution  Process is the most basic abstractions provided by OS An isolated computation context for each application  Computation context CPU state + address space + environment

3. OS Spring ’ 04 CPU state=Register contents  Process Status Word (PSW) exec. mode, last op. outcome, interrupt level  Instruction Register (IR) Current instruction being executed  Program counter (PC)  Stack pointer (SP)  General purpose registers

4. OS Spring ’ 04 Address space  Text Program code  Data Predefined data (known in compile time)  Heap Dynamically allocated data  Stack Supporting function calls

5. OS Spring ’ 04 Environment  External entities Terminal Open files Communication channels  Local  With other machines

6. OS Spring ’ 04 Process control block (PCB) state memory files accounting priority user CPU registers storage text data heap stack PSW IR PC SP general purpose registers PCB CPU kernel user

7. OS Spring ’ 04 Process States running ready blocked created schedule preempt event done wait for event terminated

8. OS Spring ’ 04 UNIX Process States running user running kernel ready user ready kernel blocked zombie sys. call interrupt schedule created return terminated wait for event event done schedule preempt interrupt

9. OS Spring ’ 04 Multiprocesses mechanisms  Context switch  Create process and dispatch (in Unix, fork()/exec())  End process (in Unix, exit() and wait())

10. OS Spring ’ 04 Threads  Thread: an execution within a process  A multithreaded process consists of many co-existing executions  Separate: CPU state, stack  Shared: Everything else  Text, data, heap, environment

11. OS Spring ’ 04 Thread Support  Operating system Advantage: thread scheduling done by OS  Better CPU utilization Disadvantage: overhead if many threads  User-level Advantage: low overhead Disadvantage: not known to OS  E.g., a thread blocked on I/O blocks all the other threads within the same process

12. OS Spring ’ 04 Multiprogramming  Multiprogramming: having multiple jobs (processes) in the system Interleaved (time sliced) on a single CPU Concurrently executed on multiple CPUs Both of the above  Why multiprogramming? Responsiveness, utilization, concurrency  Why not? Overhead, complexity

13. OS Spring ’ 04 Responsiveness Job 1 arrives Job 1 terminates Job1 Job2 Job3 Job 2 terminates Job 3 terminates Job 2 arrives Job 3 arrives Job1 Job3 Job2 Job 1 terminates Job 3 terminates Job 2 terminates

14. OS Spring ’ 04 Workload matters! ? Would CPU sharing improve responsiveness if all jobs were taking the same time? No. It makes it worse!  For a given workload, the answer depends on the value of coefficient of variation (CV) of the distribution of job runtimes CV=stand. dev. / mean CV CPU sharing does not help CV > 1 => CPU sharing does help

15. OS Spring ’ 04 Real workloads  Exp. Dist: CV=1;Heavy Tailed Dist: CV>1  Dist. of job runtimes in real systems is heavy tailed CV ranges from 3 to 70  Conclusion: CPU sharing does improve responsiveness  CPU sharing is approximated by Time slicing: interleaved execution

16. OS Spring ’ 04 Utilization idle 1 st I/O operation I/O ends 2 nd I/O operation I/O ends 3 rd I/O operation CPU Disk CPU Disk idle Job1 Job2

17. OS Spring ’ 04 Workload matters! ? Does it really matter? Yes, of course: If all jobs are CPU bound (I/O bound), multiprogramming does not help to improve utilization  A suitable job mix is created by a long- term scheduling Jobs are classified on-line to be CPU (I/O) bound according to the job ’ s history

18. OS Spring ’ 04 Concurrency  Concurrent programming Several process interact to work on the same problem  ls – l | more Simultaneous execution of related applications  Word + Excel + PowerPoint Background execution  Polling/receiving Email while working on smth else

19. OS Spring ’ 04 The cost of multiprogramming  Switching overhead Saving/restoring context wastes CPU cycles  Degrades performance Resource contention Cache misses  Complexity Synchronization, concurrency control, deadlock avoidance/prevention

20. OS Spring ’ 04 Short-Term Scheduling running ready blocked created schedule preempt event done wait for event terminated

21. OS Spring ’ 04 Short-Term scheduling  Process execution pattern consists of alternating CPU cycle and I/O wait  CPU burst – I/O burst – CPU burst – I/O burst...  Processes ready for execution are hold in a ready (run) queue  STS schedules process from the ready queue once CPU becomes idle

22. OS Spring ’ 04 Metrics: Response time Job arrives/ becomes ready to run Starts running Job terminates/ blocks waiting for I/O T wait T run T resp T resp = T wait + T run  Response time (turnaround time) is the average over the jobs ’ T resp

23. OS Spring ’ 04 Other Metrics  Wait time: average of T wait This parameter is under the system control  Response ratio or slowdown slowdown=T resp / T run  Throughput, utilization depend on user imposed workload=> Less useful

24. OS Spring ’ 04 Note about running time (T run )  Length of the CPU burst When a process requests I/O it is still “ running ” in the system But it is not a part of the STS workload  STS view: I/O bound processes are short processes Although text editor session may last hours!

25. OS Spring ’ 04 Off-line vs. On-line scheduling  Off-line algorithms Get all the information about all the jobs to schedule as their input Outputs the scheduling sequence Preemption is never needed  On-line algorithms Jobs arrive at unpredictable times Very little info is available in advance Preemption compensates for lack of knowledge

26. OS Spring ’ 04 First-Come-First-Serve (FCFS)  Schedules the jobs in the order in which they arrive Off-line FCFS schedules in the order the jobs appear in the input  Runs each job to completion  Both on-line and off-line  Simple, a base case for analysis  Poor response time

27. OS Spring ’ 04 Shortest Job First (SJF)  Best response time Short Long job Short  Inherently off-line All the jobs and their run-times must be available in advance

28. OS Spring ’ 04 Preemption  Preemption is the action of stopping a running job and scheduling another in its place  Context switch: Switching from one job to another

29. OS Spring ’ 04 Using preemption  On-line short-term scheduling algorithms Adapting to changing conditions  e.g., new jobs arrive Compensating for lack of knowledge  e.g., job run-time  Periodic preemption keeps system in control  Improves fairness Gives I/O bound processes chance to run

30. OS Spring ’ 04 Shortest Remaining Time first (SRT)  Job run-times are known  Job arrival times are not known  When a new job arrives: if its run-time is shorter than the remaining time of the currently executing job: preempt the currently executing job and schedule the newly arrived job else, continue the current job and insert the new job into a sorted queue  When a job terminates, select the job at the queue head for execution

31. OS Spring ’ 04 Round Robin (RR)  Both job arrival times and job run-times are not known  Run each job cyclically for a short time quantum Approximates CPU sharing Job 1 arrives Job13 Job 2 arrives Job 3 arrives 2 1 2 31 2 31 2 1 Job2 Job 3 terminates Job 1 terminates Job 2 terminates

32. OS Spring ’ 04 Responsiveness Job 1 arrives Job 1 terminates Job1 Job2 Job3 Job 2 terminates Job 3 terminates Job 2 arrives Job 3 arrives Job1 Job3 Job2 Job 1 terminates Job 3 terminates Job 2 terminates

33. OS Spring ’ 04 Priority Scheduling  RR is oblivious to the process past I/O bound processes are treated equally with the CPU bound processes  Solution: prioritize processes according to their past CPU usage  T n is the duration of the n-th CPU burst  E n+1 is the estimate of the next CPU burst

34. OS Spring ’ 04 Multilevel feedback queues quantum=10 quantum=20 quantum=40 FCFS new jobs terminated

35. OS Spring ’ 04 Multilevel feedback queues  Priorities are implicit in this scheme  Very flexible  Starvation is possible Short jobs keep arriving => long jobs get starved  Solutions: Let it be Aging

36. OS Spring ’ 04 Priority scheduling in UNIX  Multilevel feedback queues The same quantum at each queue A queue per priority  Priority is based on past CPU usage pri=cpu_use+base+nice  cpu_use is dynamically adjusted Incremented each clock interrupt: 100 sec -1 Halved for all processes: 1 sec -1

37. OS Spring ’ 04 Fair Share scheduling  Given a set of processes with associated weights, a fair share scheduler should allocate CPU to each process in proportion to its respective weight  Achieving pre-defined goals Administrative considerations  Paying for machine usage, importance of the project, personal importance, etc. Quality-of-service, soft real-time  Video, audio

38. OS Spring ’ 04 Perfect Fairness A fair share scheduling algorithm achieves perfect fairness in time interval (t 1,t 2 ) if

39. OS Spring ’ 04 Wellness criterion for FSS An ideal fair share scheduling algorithm achieves perfect fairness for all time intervals  The goal of a FSS algorithm is to receive CPU allocations as close as possible to a perfect FSS algorithm

40. OS Spring ’ 04 Fair Share scheduling: algorithms  Weighted Round Robin Shares are not uniformly spread in time  Lottery scheduling: Each process gets a number of lottery tickets proportional to its CPU allocation The scheduler picks a ticket at random and gives it to the winning client Only statistically fair, high complexity

41. OS Spring ’ 04 Fair Share scheduling: VTRR  Virtual Time Round Robin (VTRR) Order ready queue in the order of decreasing shares (the highest share at the head) Run Round Robin as usual Once a process that exhausted its share is encountered: Go back to the head of the queue

42. OS Spring ’ 04 Multiprocessor Scheduling  Homogeneous vs. heterogeneous  Homogeneity allows for load sharing Separate ready queue for each processor or common ready queue?  Scheduling Symmetric Master/slave

43. OS Spring ’ 04 A Bank or a Supermarket? departing jobs departing jobs departing jobs departing jobs arriving jobs shared queue CPU1 CPU2 CPU3 CPU4 CPU1 CPU2 CPU3 CPU4 arriving jobs M/M/44 x M/M/1

44. OS Spring ’ 04 It is a Bank! M/M/4 4 x M/M/1