Schedulers. Operating Systems. CPU-IO Burst Cycle. When does OS do Scheduling? Scheduling Criteria. Question. Short-Term.
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1 Schedulers Operating Systems Scheduling (h 4., ) Short-Term Which process gets the PU? Fast, since once per 00 ms Long-Term (batch) Which process gets the Ready Queue? Medium-Term Which Ready Queue process to memory? Swapping We ll be talking about Short-Term, unless otherwise noted PU-IO urst ycle When does OS do Scheduling? add read (I/O Wait) store increment write (I/O Wait) Frequency urst Duration Four times to re-schedule Running to Waiting (I/O wait) Running to Ready (time slice) Waiting to Ready (I/O completion) 4 Termination # and # optional ==> Preemptive Timing may cause unexpected results updating shared variable kernel saving state Question What performance criteria related to processes should the scheduler seek to optimize? Ex: PU minimize time spent in queue Others? Scheduling riteria PU utilization (typically, 40% to 90%) Throughput (processes/time, higher is better) Waiting time (in queue, lower is better), or... Turn-around time (in queue plus run time) Maximize #, # Minimize # Note, response time often OS metric but is beyond short-term scheduler Self-regulated by users (go home) ounded ==> Variance!
2 General FFS Scheduling lgorithms SFJ Priority Round-Robin Specific NT Linux Gantt hart First-ome, First-Served urst Time vg Wait Time ( ) / = 5.7 Shortest Job First 0 0 vg Wait Time (0 + + ) / = Optimal vg Wait urst Time Prediction tough Ideas? Priority Scheduling Special case of SJF urst Time Priority vg Wait Time ( ) / =. Priority Scheduling riteria? Priority Scheduling riteria Internal open files memory requirements PU time used - time slice expired (RR) process age - I/O wait completed External $ department sponsoring work process importance super-user (root) - nice
3 Round Robin Fixed time-slice and Preemption urst Time 5 vg = ( ) / = 9. FFS? SJF? 9 SOS: Dispatcher How is the next process chosen? Line 79 has an infinite loop. Why? There is no return from the Dispatcher() function call. Why not? See TimerInterruptHandler() Linux: /usr/src/linux/kernel/sched.c /usr/src/linux/include/linux/sched.h linux-pcb.h Round Robin Fun Turn-around time? q = 0 q = q --> 0 urst Time Rule: 0% within one quantum More Round Robin Fun vg. Turn-around Time D urst Time Time Quantum Fun with Scheduling urst Time 0 Priority More Fun with Scheduling rrival Time urst Time 4 Gantt harts: FFS SJF Priority RR (q=) Performance: Throughput Waiting time Turnaround time Turn around time: FFS SJF q= PU idle q=0.5 PU idle
4 Multi-Level Queues ategories of processes, each at a priority level Priority System Priority Interactive Priority atch Run all in first, then Starvation! Divide between queues: 70%, 5% Multi-Level Feedback Queues llow processes to move between prio levels Ex: time slice expensive but want interactive Priority Priority Priority Queue Queue Queue Quantum Quanta 4 Quanta onsider process needing 00 quanta, 4,, 6,, 64 = 7 swaps! Favor interactive users Most general. Used in WinNT and Linux es Outline P Interrupt Handlers Scheduling lgorithms WinNT Linux Windows NT Scheduling asic scheduling unit is a thread For now, just think of a thread as a process Priority based scheduling per thread Preemptive operating system No shortest job first, no quotas Priority ssignment NT kernel uses priority levels is the highest; 0 is system idle thread Realtime priorities: 6 - Dynamic priorities: - 5 Users specify a priority class: realtime (4), high (), normal () and idle (4) and a relative priority: highest (+), above normal (+), normal (0), below normal (-), and lowest (-) to establish the starting priority Threads also have a current priority Quantum Determines how long a thread runs once selected Varies based on: NT Workstation or NT Server Intel or lpha hardware Foreground/ackground application threads NOTE: NT 4.0 increases quantum for foreground threads while NT.5 increased priorities. Why? 4
5 Dispatcher Ready List Selecting the Ready Thread Dispatcher Ready List Ready Threads Keeps track of all threads in the ready state Queue of threads assigned to each level (Multi-level feedback queue) Locates the highest priority thread that is ready to execute Scans dispatcher ready list Picks front thread in highest priority nonempty queue When is this like round robin? oosting and Decay When does the feedback occur? oost priority Event that wakes blocked thread oosts never exceed priority 5 for dynamic Realtime priorities are not boosted Decay priority by one for each quantum decays only to starting priority (no lower) Starvation Prevention Low priority threads may never execute nti-pu starvation policy thread that has not executed for seconds boost priority to 5 double quantum Decay is swift not gradual after this boost Linux Scheduling Two classes of processes: Real-Time Normal Real-Time: lways run Real-Time above Normal Round-Robin or FIFO Soft not Hard Linux Scheduling Normal: redit-ased process with most credits is selected time-slice then lose a credit (0, then suspend) no runnable process (all suspended), add to every process: credits = credits/ + priority utomatically favors I/O bound processes 5
6 Questions True or False: FFS is optimal in terms of avg waiting time Most processes are PU bound The shorter the time quantum, the better What is the idle thread? Where did we see it? 6
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