Scheduling

1. Operating Systems: Scheduling Scheduling • Processes can be in one of several states • 5 state model : • ‘short-term’ scheduling • organising transitions between states • on page-fault, waiting for or getting semaphores, I/O transfer completions etc. • deciding order in which ready processes should be run • priorities etc. and queue handling

2. Operating Systems: Scheduling • 7 state model with medium and long-term scheduling :

3. Operating Systems: Scheduling • Medium-term scheduling • when main memory is full, processes need to be swapped out to disc • medium-term sched. decides which and when to swap out and back in • level of multiprogramming to achieve desired performance • to have processes ready and waiting to run when running process blocks • separate swap area on disc commonly used • local disc to be effective • networked discs too slow • possible to use file storage sites instead of swap area • for files mapped into virtual space from disc storage site • accessing VM equivalent to accessing file storage site on disc • stacks and heaps etc. can also be mapped files • all VM space can be mapped files • often simpler than separate swap area • each page only has one corresponding disc site instead of possibly two • paging out a dirty page updates the file • clean pages need not be written out (if disc file sites initially cleared to zero)

4. Operating Systems: Scheduling • drawbacks : • cannot page out across a network to a file serve • potential file inconsistency on system crash • EMAS system used mapped files • first version used a swap area on a dedicated drum (fixed head disc) • later versions just paged to and from disc file sites • worked because it was for a stand-alone mainframe with local discs • swap areas are usually fixed partitions on disc • drawbacks : • may not be large enough • may waste disc space if large enough for any eventuality • swap areas probably more efficient overall • all page transfers can be initiated together • probably to a contiguous disc area • disc driver will be more effective in optimising transfers • minimises head movement

5. Operating Systems: Scheduling • Long-term scheduling • whether to allow new processes to enter a system • for a compute server or background job stream : • when to start next job • depends on job priority, CPU-time needs, memory needs etc. • also depends on existing load • for multiple user interactive system • how many users to allow on • each should get acceptable performance • or is it better to let all requestors on and let performance degrade? CPUUtulisation knee No. of Users

6. Operating Systems: Scheduling Short-term scheduling • Define the objectives and criteria to be met; then invent a scheme • Precise scheme will depend on type of system : • Compute server or background job stream processor • overall throughput most important • Single-user workstation • foreground interactive response most important • Multiple-user system • interactive time-sharing • transaction processing • travel agent enquiry and booking systems, banking terminals etc. • interactive response with fairness between users • Real-time systems • meeting hard deadlines • keeping up with processing data streams e.g. comms, audio and video etc. • industrial process control

7. Operating Systems: Scheduling • System Manager objectives : • throughput - to maximise number of jobs completed per unit time • turn-around time for jobs • utilisation - to make best use of expensive resources • CPU - proportion of time spent executing user programs • memory usage • usage of peripherals • overall cost-effectiveness • a mix of CPU-bound and I/O bound tasks might be desirable for balance • to be fair • no favouring or starvation of some processes • ensuring priorities met • performance to degrade gracefully under load • to be reasonably predictable • wide variations in performance can be distracting • to be adaptable to varying circumstances without need for intervention

8. Operating Systems: Scheduling • User objectives : • turn-around time for submitted jobs • adequate response time for interactive working • < 0.1 sec for immediate feedback • e.g. key depressions, menu highlighting etc. • may need special fast path through kernel to achieve • or peripheral processor - keyboard interface or video processor • < 1 sec needed to maintain user attention and interest for long periods • > 1 sec : response can be very distracting - concentration will falter • > 10 secs : intolerable for interaction • even ‘talk’ conversations impossible • time to go for a coffee! • observed phenomenon : • thinking time drops as response time drops • an effect of short-term memory and attention span

9. Operating Systems: Scheduling Scheduling Criteria • Priority of process • basic priority usually decided outwith the scheduler - may be dynamic later • e.g. interactive v. background • CPU boundedness • does process always use its CPU quantum allocation without blocking? • I/O boundedness • does process frequently block for I/O ? • Page-fault frequency • a small PFF usually means the process has all the memory it needs and can make good progress • a large PFF means the process will not use much CPU - always waiting for the page to be brought in from disc

10. Operating Systems: Scheduling • Urgency of required response • important for user interaction • may be vitally urgent for a real-time system • nearness to a deadline • CPU time already received • may decide to give CPU to a process that has not had much yet • or ‘to him that hath shall be given’ ? • CPU time to completion • average waiting time minimised if process with least time to completion run • need to know how much time still needed - usually unknown • Regularity of requirements • CPU required at fixed time intervals - real-time systems • fairly straightforward to schedule given adequate system performance

11. Operating Systems: Scheduling • Schedulers aim to optimise future performance • need to know future characteristics of processes • easiest to assume processes will continue to behave as previously • must be able to adapt when processes change characteristics • Pre-emptive scheduling • currently running process can be interrupted before finishing • put back onto the Run queue to get another go on the CPU later • typically the scheduler is re-entered on regular clock or timer interrupts • may also get re-entered whenever kernel entered • peripheral interrupts, semaphore operations etc. • Non Pre-emptive scheduling • running process continues until it has finished or blocks • low kernel overheads • scheduler usually needs more control than this • other processes may be seriously adversely affected

12. Operating Systems: Scheduling Priority Queues • Pre-emptive priority • processes in highest priority queue always get run first • those in lower priority queues always wait • starvation likely

13. Operating Systems: Scheduling • Non-Pre-Emptive Priority • higher priority processes still favoured but not exclusively • every so often, take a process from a lower priority queue • a priority ratio table (used in EMAS medium-term scheduling) 1 2 1 3 1 2 1 4 1 2 1 3 1 2 1 • make priorities dynamic • lower a processes priority after each time it has been run for a quantum • used in Windows NT • process given an initial boost in priority, then gradual decay • favours short interactions • boost a processes priority if it has not had a go on the CPU lately • priority purchase ? • a user may wish to pay more to get better service • whether funny money i.e. computing time allocations, or real money • Higher and lower priority bands • kernel processes v. background job stream • real-time processes (NT)

14. Operating Systems: Scheduling • First-Come-First-Served • processes queued on run queue in order of arrival • oldest process on queue always run next to completion - no pre-emption • simple to implement • primarily used for background and batch streams • performs much better for long jobs than for short ones • example: measure turnaround time normalised by service time : Tq/Ts • favours CPU- bound processes over I/O bound processes • unacceptable for interactive systems - might be OK if combined with priority queue system for known long run-time processes having lower priority

15. Operating Systems: Scheduling

16. Operating Systems: Scheduling

17. Operating Systems: Scheduling • Round-Robin • each process gets a quantum of time (a time-slice) in turn • good for processes of equal priority • widely used for multi-user interactive systems • and single-user systems with multiple activities in progress • a pre-emptive policy • processes pre-empted by regular clock interrupts to kernel scheduler • main issue is length of time-slice • to optimise interactive response, make quantum just slightly longer than a typical interaction i.e. should complete within first time-slice :

18. Operating Systems: Scheduling • if quantum not large enough : • more than one quantum means additional round-robin delay • short time-slices : • all processes in round-robin queue get a go on the CPU quickly • short processes complete in one go • overheads will increase due to more frequent context changing

19. Operating Systems: Scheduling • large time-slices : • round-robin time longer • some processes will always use their full time-slice • overheads lower • CPU-bound processes favoured over I/O bound or page-faulting processes • CPU-bound processes take full time-slice • I/O bound processes blocked before completing a time-slice • put blocked processes in a special queue • more equitable to give them higher priority when they unblock • or put them on front of round-robin queue to get CPU next

20. Operating Systems: Scheduling • Shortest Process Next • process with shortest expected processing time run next - non-pre-emptive • intended to reduce bias towards long processes • achieves much better turnaround time than FCFS on average • some risk of starvation for longer processes (if new processes admitted on fly) • variability of turnaround time greater than FCFS • need a good estimate of expected processing time (extra overhead) • possible for batch and background streams, if user knows • can be estimated from previous behaviour for interactive processes : Keep a running average of what was used in previous bursts of CPU use: Sn+1 = (  Ti ) / n or Sn+1 = Tn/n + Sn*(n-1)/n Better to use an exponential average function: Sn+1 = Tn + (1-)*Sn

21. Operating Systems: Scheduling For 0 <  < 1, all previous observations carry decreasing weight: Sn+1 = Tn + (1-)Tn-1 + ... + (1-)..(1-)Tn-i + (1-)..(1-)S0 For  = 0.8, virtually all weight given to previous four observations : Sn+1 = 0.8Tn + 0.16Tn-1 + 0.032Tn-2 + 0.0064Tn-3 + . . . For  = 0.5, all weight given to previous eight observations. Higher values of  reflect changes more quickly - but jerkily

22. Operating Systems: Scheduling • Shortest Time Remaining • process with least expected run-time to completion dispatched next • in effect a pre-emptive version of SPN • a new process entering the queue may pre-empt the running process • need estimate of remaining run-time for each process • record previous elapsed times and use weighted average as in SPN • can use regular clock interrupts to re-evaluate best next process • better turn-around time than SPN • no bias in favour of long processes • risk of starvation for longer processes • in the example, three shortest processes all receive immediate service

23. Operating Systems: Scheduling • Highest Response Ratio Next • aim to minimise Tq/Ts for each process • can approximate an a priori measure : • Response ratio = (w + s)/s • where w = waiting time s = expected service time • expected service time must be estimated again • waiting time measured as time progresses • process with highest RR dispatched next • longer the wait, the higher the priority • short processes favoured but long processes not starved • good balance on the whole • non-pre-emptive as defined but could be made pre-emptive as STR • overheads in recomputing RR

24. Operating Systems: Scheduling Threads and Scheduling • Threads are each scheduled separately • Threaded applications may wish to assign relative priorities to threads • a background screen update thread - low priority • foreground interactive thread - high priority • check-point thread - low priority, but must not be starved • In a multi-user environment, CPU must be allocated equitably • spawning lots of threads must not gain the process unfair advantage • need to add up CPU used in all threads belonging to a process when re-evaluating priorities or otherwise control allocation fairly • In a single-user environment • equitable CPU allocation less important • Windows NT introduced fibers to give users more precise control of scheduling than threads • important for database and transaction processing systems

25. Operating Systems: Scheduling Scheduling Scheme Evaluation • Analytic methods v. Simulation • Queuing network models : • arrival rates and service times • multiple servers and queues • expected performance can be analysed mathematically for simple systems • Simulation : • model of scheduling scheme programmed • process characteristics, rate of interaction, service time needs, can be modelled to match experience • traces of real sessions can be kept and used in simulations • On EMAS multi-access system, sessions were recorded on a PDP-11 • re-run in simulation later • ERTE - Edinburgh Remote Terminal Emulator • produced useful data for tailoring scheduling schemes

26. Operating Systems: Scheduling Real-time System Scheduling • Soft and Hard deadlines for tasks • soft deadlines are desirable aims but not obligatory • hard deadlines must be met • may need to reserve resources ahead of time • schedule hard deadlines first, then fit soft deadlines in later • Continuous data streams or regular data packets a feature • much easier if timing characteristics and processing needs known in advance • Need to avoid Priority Inversion : • where a low priority task has a resource that is blocking a high priority task • possible solution : priority inheritance • low priority task inherits high priority of task needing the resource it has

27. Operating Systems: Scheduling • Cyclic Static scheduling : • for processing data appearing continuously or at regular times • pre-allocate a fixed amount of CPU at regular intervals • use timer interrupts to regain scheduling control from other tasks • can interlace multiple processing needs : • Dynamic scheduling : Most Urgent First • dispatch process with earliest deadline • Maximum Lateness First • lateness = processing time left for task - time left until deadline • task with largest lateness selected next • Rate Monotonic • assigns a priority to each task a priori proportional to the frequency of occurrence of its triggering event. A B A A A A B B

28. Operating Systems: Scheduling • task A : period 3ms, processing time 1.5ms • task B : period 2ms, processing time 0,2ms • task C : period 1ms, processing time 0.2ms • A Priori Analysis • mathematical and simulation techniques for predicting whether scheduling objectives can be met and a schedule to meet them

29. Operating Systems: Scheduling • Round Robin and other schemes also used in real-time systems • Hierarchical Scheduling : • form groups of tasks and apply different scheduling schemes to each group • Important that user able to select an appropriate scheduling scheme Priority Cyclic Static Round Robin T1 T3 T6 T2 T4 T5