Module 2 1 CPU Scheduling Scheduling Types Scheduling

Module 2. 1: CPU Scheduling • • Scheduling Types Scheduling Criteria Scheduling Algorithms Performance Evaluation K. Salah 1 Operating Systems

CPU SCHEDULING – The basic problem is as follows: How can OS schedule the allocation of CPU cycles to processes in system, to achieve “good performance”? – Components of CPU scheduling subsystem of OS: T Dispatcher – gives control of the CPU to the new process T Scheduler - selects next process from those in main memory (short-term scheduler) T Swapper - manages transfer of processes between main memory and secondary storage (medium-term scheduler) T Long-Term Scheduler - in a batch system, determines which and how many jobs to admit into system. K. Salah 2 Operating Systems

Types of Scheduling Algorithms – Preemptive: process may have CPU taken away before completion of current CPU burst (e. g. end of CPU quantum. ) – Non-preemptive: processes always run until CPU burst completes – Static Priority – Dynamic Priority K. Salah 3 Operating Systems

Performance Criteria for Scheduling • Scheduling (as an Optimization task): How to best order the ready queue for efficiency purposes. • • CPU utilization: % of time CPU in use Throughput: # of jobs completed per time unit Turnaround Time: wall clock time required to complete a job Waiting Time: amount of time process is ready but waiting to run Response Time: in interactive systems, time until system responds to a command Response Ratio: (Turnaround Time)/(Execution Time) -- long jobs should wait longer The overhead of a scheduling algorithm (e. g. , data kept about execution activity, queue management, context switches) should also be taken into account. Kleinrock's Conservation Law: • No matter what scheduling algorithm is used, you cannot help one class of jobs without hurting the other ones. • Example: A minor improvement for short jobs (say, on waiting time) causes a disproportionate degradation for long jobs. K. Salah 4 Operating Systems

Optimization Criteria • • • Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time K. Salah 5 Operating Systems

Basic Scheduling Algorithm • FCFS - First-Come, First-Served – Non-preemptive – Ready queue is a FIFO queue – Jobs arriving are placed at the end of queue – first job in queue runs to completion of CPU burst • • Advantages: simple, low overhead Disadvantages: long waiting time, inappropriate for interactive systems, large fluctuations in average turnaround time are possible. K. Salah 6 Operating Systems

FCFS Example • Pid Arr CPU Start Finish Turna Wait Ratio ---+---+-----+----+----A 0 3 3 0 1. 0 B 1 5 3 8 7 2 1. 4 C 3 2 8 10 7 5 3. 5 D 9 5 10 15 6 1 1. 2 E 12 5 15 20 8 3 1. 6 ---+---+-----+----+----A 0 1 1 0 1. 00 B 0 100 1 101 1 1. 01 C 0 1 102 101 102. 00 D 0 102 202 102 2. 02 K. Salah 7 Operating Systems

RR - Round Robin • • Preemptive version FCFS Treat ready queue as circular – arriving jobs placed at end – first job in queue runs until completion of CPU burst, or until time quantum expires – if quantum expires, job again placed at end K. Salah 8 Operating Systems

Properties of RR Advantages: simple, low overhead, works for interactive systems Disadvantages: if quantum too small, too much time wasted in context switching; if too large, approaches FCFS. Typical value: 10 - 100 msec Rule of thumb: choose quantum so that large majority (8090%) of jobs finish CPU burst in one quantum K. Salah 9 Operating Systems

SJF - Shortest Job First – non-preemptive – ready queue treated as a priority queue based on smallest CPU-time requirement T arriving jobs inserted at proper position in queue T shortest job (1 st in queue) runs to completion Advantages: provably optimal w. r. t. average turnaround time Disadvantages: in general, unimplementable. Also, starvation possible! Can do it approximately: use exponential averaging to predict length of next CPU burst ==> pick shortest predicted burst next! K. Salah 10 Operating Systems

Exponential Averaging t n+1 = a tn + (1 - a) t n tn+1 : predicted length of next CPU burst tn : actual length of last CPU burst tn : previous prediction a = 0 implies make no use of recent history (t n+1 = t n) a = 1 implies tn+1 = tn (past prediction not used). a = 1/2 implies weighted (older bursts get less and less weight). K. Salah 11 Operating Systems

Prediction of the Length of the Next CPU Burst K. Salah 12 Operating Systems

SRTF - Shortest Remaining Time First – Preemptive version of SJF – Ready queue ordered on length of time till completion (shortest first) – Arriving jobs inserted at proper position – shortest job runs to completion (i. e. CPU burst finishes) or until a job with a shorter remaining time arrives (i. e. placed in the ready queue. ) K. Salah 13 Operating Systems

Performance Evaluation • Deterministic Modeling (vs. Probabilistic) Look at behavior of algorithm on a particular workload, and compute various performance criteria Example: workload - • Job 1: 24 units Job 2: 3 units Job 3: 3 units Gantt chart for FCFS: | Job 1 | Job 2 0 24 | Job 3 27 | 30 Total waiting time: 0 + 24 + 27 = 51 Average waiting time: 51/3 = 17 Total turnaround time: 24 + 27 + 30 = 81 Average turnaround time: 81/3 = 27 K. Salah 14 Operating Systems

RR and SJF • Chart for RR with quantum of 3: | Job 1 | Job 2 | Job 3 | 0 3 6 9 Job 1 | 30 Total waiting time: 6 + 3 = 15 Avg. waiting time: 15 / 3 = 5 • Chart for SJF: | Job 2 | Job 3 | 0 3 6 Job 1 | 30 Total waiting time: 6 + 0 + 3 = 9 Avg. waiting time: 9 / 3 = 3 • Can see that SJF gives minimum waiting time. RR is intermediate. (This can be proved in general. ) K. Salah 15 Operating Systems

HPF - Highest Priority First – general class of algorithms – each job assigned a priority which may change dynamically – may be preemptive or non-preemptive • Problem: how to compute priorities? – SJF is a special case of priority; the longer the CPU burst, the lower the priority. – Priority can be internally computed, e. g. , CPU burst vs. I/O burst. Or it can be externally defined depending on the importance of the process, (e. g. using nice command in Unix). – Effective Priority = System (Internal) + User defined T System: type of process + age (dynamically changes) K. Salah 16 Operating Systems

Windows XP Priorities 6 priority classes (shown with Task Manager) 7 default relative priorities/values (shown with Process Explorer) • 16 -31 (time critical) • 1 -16 (others) K. Salah 17 Operating Systems

Multilevel Queue Scheduling K. Salah 18 Operating Systems

Multilevel Feedback Queue • • • A process is admitted to one class of queues • Multilevel-feedback-queue scheduler defined by the following parameters: – number of queues – scheduling algorithms for each queue – method used to determine when to upgrade a process – method used to determine when to demote a process – method used to determine which queue a process will enter when that process needs service Schedule top queue processes first A process can move between the various queues; aging can be implemented this way K. Salah 19 Operating Systems

Example of Multilevel Feedback Queue • • Attractive scheme for I/O bound jobs • Scheduling – A new job enters queue Q 0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. – At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2. Three queues: – Q 0 – RR with time quantum 8 milliseconds – Q 1 – RR time quantum 16 milliseconds – Q 2 – FCFS K. Salah 20 Operating Systems

Multilevel Feedback Queues K. Salah 21 Operating Systems

Further Readings • • When is processor affinity used? In Windows XP, what is the default base priority for a process when it gets created? K. Salah 22 Operating Systems