Scheduling & Dispatching © 2004, D. J. Foreman

What is "Scheduling"? Managing resources and demands for them Determine next "run-user" Determine resources required Add new "run-user" to "ready" list All above usually done in kernel mode "Run-user" Current thread/process that is executing © 2004, D. J. Foreman

What is "Dispatching"? Determine user at head of "ready list" Preempt/wait for current run-user to yield Save state of current run-user Load state data for new run-user Mode switch to new run-user © 2004, D. J. Foreman

Questions How would you design a scheduler that runs in user-mode? What problems would you have to handle? Why would you want to run it in user-mode? © 2004, D. J. Foreman

What is a “job”? Fixed set of programs (NOT processes) Sort employee records Compute wages for each employee Print checks for all employees Resources pre-specified by “job control” statements Employee DB Payroll DB High-speed printer Programs run in strict sequence © 2004, D. J. Foreman

How is the scheduler invoked? Voluntary Process/thread blocks itself (“yield”s the CPU) Calls scheduler Processes can yield to each other Problems-> greed, errors Involuntary Pre-emption by interrupt (device/timer) Kernel calls scheduler before/after interrupt is processed, before return to pre-empted user CIMPAC is a voluntary system © 2004, D. J. Foreman

Policies Ideally Practically Additionally Selectable Changeable Built into OS Can be changed with versions of OS Additionally Scheduler can BE the Resource Manager © 2004, D. J. Foreman

Control Interval timer Multiple policies possible in one system Quantum or slice Fixed or variable Multiple policies possible in one system Interactive applications May become compute-bound Batch jobs Deadline production Real-time – process-control systems © 2004, D. J. Foreman

Policy implimentation Mechanism – fixed Varies by requirements CPU usage Wait time Job completion time Controls: Fair share Favor long/short jobs Priorities Deadlines © 2004, D. J. Foreman

Scheduling Variables Let P = {pi | 0  i < n}, be a set of processes Let {Pij} = set of threads, j, in Pi Let S(pi)  {running, ready, blocked} Let t(pi) = required runtime or service time Let W(pi) = initial wait time Let TTRnd(pi) = wall clock: endtime – start time (turnaround time) Batch Throughput rate = 1/(avg TTRnd) Timesharing response time = W(pi) © 2004, D. J. Foreman

Optimizing Schedules Criteria Methods CPU usage Wait time Deadlines Restrict # of processes pi Pre-determine service time τ(pi) Compute all schedules and choose best © 2004, D. J. Foreman

Optimization problems τ(pi) are estimates Schedule-compute time is O(n2) Only an approximation of optimum New jobs arrive during processing © 2004, D. J. Foreman

Estimating CPU Utilization l = average rate at which processes are placed in the Ready List= arrival rate (arrivals/sec) m = the average service rate  1/ m = the average service time, t(pi), per process r = expected CPU busy time, computed as: r = arrival rate * avg CPU time each r = l * 1/ m = l / m Notice: must have l < m (i.e., r < 1) What if r approaches 1? © 2004, D. J. Foreman

Non-preemptive Schedulers Using the simplified scheduling model: new->ready-> scheduled-> running->done Only considers running and ready states Ignores time in blocked state: New process created when it becomes ready Process is destroyed when it is blocked Only looking at “small phases” of a process © 2004, D. J. Foreman

Non-preemptive Schedulers First Come First Served Shortest Job Next Priority Deadline © 2004, D. J. Foreman

First Come, First Served Simple Ignores service time Average wait time=simple avg of all W(p) Predicted W(p)=Wavg Wavg=Lw/m + .5/m = (L/m) +1/(2m) © 2004, D. J. Foreman

Shortest Job Next Minimizes wait time “Bad” jobs may take excessive time Service times must be known in advance Known for batch systems © 2004, D. J. Foreman

Priority Pre-determined rules required Low priority jobs may get poor service Addressable via Age Re-assesment Other resource requirements © 2004, D. J. Foreman

Deadline Based on required finish time Requires known runtime © 2004, D. J. Foreman

Preemptive Scheduling Almost always by priority Most common form today Complex analysis Requires interval timer support Examples: Round Robin (RR) Round Robin with Overhead (RRO) Multi-level queues © 2004, D. J. Foreman

Round Robin Each process gets a fixed time slice to run (Time in Queue) Fair share Most common preemptive scheduler today Optional placement of new processes: queue vs. ring For n processes, q CPU units, C context Total real time available=n*(q+C) Or q=(T/n)-C C is usually ignored, but should not be © 2004, D. J. Foreman

RR w/Overhead included Fixed overhead for C added between slices Changes average performance time © 2004, D. J. Foreman

Multi-level Queues More than 1 ready list Interactive Small batch Large batch Compute bound Top-level queue must clear before next level runs Within a level use RR, etc © 2004, D. J. Foreman

Current systems Linux BSD 4.4 Win 2K/NT/XP/7/10 See book for descriptions © 2004, D. J. Foreman