2.5 Scheduling

1. 2.5 Scheduling • Given a multiprogramming system. • Many times when more than 1 process is waiting for the CPU (in the ready queue). • The scheduler (using a scheduling algorithm) decides which process will run next. • User satisfaction is important.

2. Context switch • When the CPU changes from one process to another. • Expensive operation. • User mode to kernel mode • Save state of current process • Registers, invalidate cache, MMU • Run scheduler • Pick next process to run • Load state of next process • Run next process

3. Process behavior • Types: • I/O bound – spend most time performing I/O • Compute bound – spend most time performing computation • Mixture

4. When do we need to schedule? • creation (before/after parent?) • exit • block (I/O, semaphore, sleep, wait, etc.) • I/O interrupt/completion • clock interrupt • non preemptive (run until you block or “cooperate” • preemptive

5. Scheduling environments • Batch • Interactive • Real time

6. Scheduling algorithm goals

7. Concepts & definitions • Throughput = # of jobs completed per hour • Turnaround time = avg of “start (submit) to completion” times; avg wait time • CPU utilization = avg of CPU busyness • Response time = time between issuing a command and getting the result • Proportionality = user perception that “complex” things take a long time and that is fine but “simple” things must be quick • Predictability = regularity, especially important for audio and video streaming

8. Batch scheduling • First-come first-served • Shortest job first • Shortest remaining time next • Three-level scheduling

9. Batch scheduling • First-come first-served • Simple • Non preemptive • Process runs until it either blocks on I/O or finishes

10. Batch scheduling • Shortest job first • Optimal turnaround time (when all start together) • Requires that we know run time a priori

11. Batch scheduling • Shortest remaining time next • Preemptive version of shortest job first • Requires a priori information • When a new job arrives, its total time is compared to the current process’ remaining time. If the new job needs less time to finish, the new job is started. • New, short jobs get good service

12. Batch scheduling • Three-level scheduling • Admission scheduler – chooses next job begin • Memory scheduler – which jobs are kept in memory and which jobs are swapped to disk • How long swapped in or out? • How much CPU time recently? • How big is the process? • How important is the process? • Degree of multiprogramming – number of processes in memory • CPU scheduler – picks which runs next

13. Interactive scheduling • Round-robin scheduling • Priority scheduling • Multiple queues • Shortest process next • Guaranteed scheduling • Lottery scheduling • Fair-share scheduling

14. Interactive scheduling • Round-robin scheduling • Simple, fair, widely, used, preemptive • Quantum = time interval • Process/context switch is expensive • Too small and we waste time • Too large and interactive system will appear sluggish • ~20-50 msec is good • Every process has equal priority

15. Interactive scheduling • Priority scheduling • Each process is assigned a priority; process with highest priority is next to run. • Types: static or dynamic (ex. I/O bound jobs get a boost) • Unix/Linux nice command

16. Interactive scheduling • Ex. 4 priorities & RR w/in a priority

17. Interactive scheduling • Multiple queues • Different types • Ex. 4 queues for: • Terminal, I/O, short quantum, and long quantum

18. Interactive scheduling • Shortest process next • Shortest job first always produces min avg response time (for batch systems) • How do we estimate this? • From recent behavior • Example of aging (or IIR filter) • alpha near 1 implies little memory • alpha near 0 implies much memory

19. Interactive scheduling • Guaranteed scheduling • Given n processes, each process should get 1/n of the CPU time • Say we keep track of the actual CPU used vs. what we should receive (entitled to/deserved). • K = actual / entitled • K = 1  we got what we deserved • K < 1  we got less than deserved • K > 1  we got more than deserved • Pick process w/ min K to run next

20. Interactive scheduling • Lottery scheduling • Each process gets tickets; # of tickets can vary from process to process. • If you ticket is chosen, you run next. • Highly responsive (new process might run right away) • Processes can cooperate (give each other their tickets)

21. Interactive scheduling • Fair-share scheduling • Consider who (user) owns the process • Ex. • User A has 1 process • User B has 9 processes • Should user A get 10% and user B get 90% or should A get 50% and B get 50% (5.6% for each of the 9 processes)? Latter is fair-share.

22. Real time scheduling • Time plays and essential role • Must react w/in a fixed amount of time • Categories: • Hard • Soft • Event types: • Periodic = occurring at regular intervals • Aperiodic = occurring unpredictably • Algorithm types: • Static • Dynamic

23. Real time scheduling • Given m periodic events. • Event i occurs w/ period Pi and requires Ci seconds of CPU time • Schedulable iff:

24. Schedulable example • Given Pi = 100, 200, and 500 msec • Given Ci = 50, 30, 100 msec • Can we handle another event w/ P4=1 sec? • Yes, as long as C4<=150 msec

25. Thread scheduling • User level threads • No clock interrupts (per thread) • A compute bound thread will dominate its process but not the CPU • A thread can yield to other threads within the same process • Typically round robin or priority + Context switch from thread to thread is simpler + App specific thread scheduler can be used - If a thread blocks on I/O, the entire process (all threads) block

26. Thread scheduling • Kernel level threads - Context switch from thread to thread is expensive (but scheduler can make more informed choices) + A thread, blocking on I/O, doesn’t block all other threads in process