Ceng Operating Systems Chapter 2.2 : Process Scheduling Process concept  Process scheduling Interprocess communication Deadlocks Threads

Ceng Operating Systems Scheduling Select process(es) to run on processor(s) Process state is changed from “ready” to “running” The component of the OS which does the scheduling is called the scheduler

Ceng Operating Systems Types of Scheduling Scheduling is divided into various levels. These levels are defined by the location of the processes A process can be –available to be executed by the processor –partially or fully in main memory –in secondary memory –is not started yet

Ceng Operating Systems Types of Scheduling Long Term Scheduling. –The decision to add to the pool of processes to be executed. Medium Term Scheduling. –The decision to add to the process in main memory. Short Term Scheduling. –The decision as to which process will gain the processor. I/O Scheduling. –The decision as to which process's I/O request shall be handled by a device.

Ceng Operating Systems Scheduling Criteria Fairness : each process should get a fair share of the CPU Efficiency: keep CPU 100% utilized Response time : should be minimized for interactive users Turnaround : minimize batch turnaround times Throughput : maximize number of jobs processed per hour

Ceng Operating Systems User-Oriented, Performance Criteria CriteriaAim Response Timelow response time, maximum number of interactive users Turnaround Timetime between submission and completion Deadlinesmaximise deadlines met

Ceng Operating Systems System-oriented, Performance Criteria CriteriaAim Throughputallow maximum number of jobs to complete Processormaximise percentage of time processor is busy utilisation Overheadminimise time processor busy executing OS

Ceng Operating Systems System oriented, other criteria CriteriaAim Fairnesstreat processes the same avoid starvation Enforcing Prioritiesgive preference to higher priority processes Balancing Resourceskeep the system resources busy

Ceng Operating Systems Important Factors I/O boundedness of a process CPU boundedness of a process Is the process interactive or batch? Process priority Page fault frequency Preemption frequency Execution time received Execution time required to complete

Ceng Operating Systems Types of Scheduling A scheduling algorithm is non-premptive (run to completion) if the CPU cannot be taken away by the OS. A scheduling algorithm is preemptive if the CPU can be taken away by the OS.

Ceng Operating Systems The Interrupting Clock The OS sets the interrupting clock to generate an interrupt at some specified future time. This interrupt time is the process quantum. Provides reasonable response times and prevents the system being held up by processes in infinite loops.

Ceng Operating Systems Scheduling Algorithms FCFS Round Robin Virtual Round Robin Priority Priority Classes Shortest Job First Shortest Remaining Time Highest Response Ratio Next Feedback Queues

Ceng Operating Systems FCFS (First Come First Serve) Implementation: –As each process becomes ready, it joins the ready queue. –When the current process finishes the oldest process is selected next. Characteristics: –Simple to implement –Nonpremptive –Penalises short and I/O-bound processes

Ceng Operating Systems Round Robin (RR) Implementation: –Processes are dispatched FIFO. But are given a fixed time on the CPU (quantum - time slice). Characteristics: –Preemptive –Effective in time sharing environments –Penalises I/O bound processes

Ceng Operating Systems Quantum Size Some Options: –Large or small quantum –Fixed or variable quantum –Same for everyone or different –If quantum is to large RR degenerates into FCFS –If quantum is to small context switching becomes the primary job being executed A good guide is quantum should be slightly larger than the time required for a typical interaction

Ceng Operating Systems Virtual Round Robin (VRR) A modification to the RR algorithm to remove the bias towards CPU bound processes. Implementation: –Two “ready” queues, one called an AUX queue for storing “completed” IO processes –AUX queue has priority over READY queue –IO processes only runs for remaining tim e Characteristics: –Performance studies indicate fairer than RR

Ceng Operating Systems Priority Implementation: –Each process is assigned a priority and the scheduler always selects the highest priority process first Characteristics: –High priority processes may run indefinitely, so decrease the priority of these processes at regular intervals –Assign high priority to system processes with known characteristics such as being I/O bound

Ceng Operating Systems Priority Classes Priority Class 4 Priority Class 2 Priority Class 1 Priority Class 3 Highest Lowest

Ceng Operating Systems Implementation: –Processes are grouped into priority classes –Round Robin is used within a class –When selecting process start with the highest class. If the class is empty, use a lower class Characteristics: –If priorities are not adjusted from time to time, lower classes may starve to death

Ceng Operating Systems Shortest-Job-First (SJF) Sometimes known as Shortest Process Next (SPN) Implementation: –The process with the shortest expected execution time is given priority on the processor

Ceng Operating Systems Characteristics: –Nonpremptive –Reduces average waiting time over FIFO –Always produces the minimum average turnaround time –Must know how long a process will run –Possible user abuse –Suitable for batch environments. Not useful in a timesharing environment

Ceng Operating Systems Shortest Remaining Time (SRT) Preemptive counterpart of SPN Implementation: –Process with the smallest estimated run- time to completion is run next –A running process may be preempted by a new process with a shorter estimate run- time

Ceng Operating Systems Characteristics: –Still requires estimates of the future –Higher overhead than SJF –No additional interrupts are generated as in RR –Elapsed service times must be recorded

Ceng Operating Systems Highest Response Ratio Next (HRRN) How do you get around the problem of Indefinite postponement? Implementation: –Once a job gets the CPU it runs it to completion –The priority of a job is a function of the job's service time and the time it has been waiting for service priority = (time waiting + service time) / service time

Ceng Operating Systems Characteristics: –Nonpremptive –Shorter jobs still get preference over longer jobs –However aging ensures long jobs will eventually gain the processor –Estimation still involved

Ceng Operating Systems Feedback Queues Sometimes called multi-level feedback queues Implementation: –There is a network of ready queues –A new process enters at the top queue –Moves through the queue FIFO

Ceng Operating Systems I/O processes: –If the job requires I/O before quantum expiration it leaves the network and comes back at the same level queue CPU bound processes: –If the quantum expires first, the process is placed on the next lower queue –This continues until it reaches the bottom queue

Ceng Operating Systems Dispatching: –A process is only placed on the CPU if all higher level queues are empty –A running process is preempted by a process arriving in a higher queue –Processes from lower level queues receive a larger quantum Modifications: –In some systems processes can proceed back up the network by becoming I/O bound

Ceng Operating Systems A Scheduling Mechanism Should: Favour short jobs Favour I/O bound jobs to get good I/O device utilisation Determine the nature of a job and schedule accordingly