Operating Systems (CS 340 D) Princess Nora University Faculty of Computer & Information Systems Computer science Department Operating Systems (CS 340 D) Dr. Abeer Mahmoud

(Chapter-6) CPU Scheduling

Chapter 5: CPU Scheduling Basic Concepts Scheduling Criteria Scheduling Algorithms Dr. Abeer Mahmoud

To describe various CPU-scheduling algorithms OBJECTIVES: To introduce CPU scheduling, which is the basis for multiprogrammed operating systems To describe various CPU-scheduling algorithms Dr. Abeer Mahmoud

Basic Concepts Dr. Abeer Mahmoud

Basic Concepts Maximum CPU utilization (keep the CPU as busy as possible ) obtained with multiprogramming CPU–I/O Burst Cycle Process execution consists of a cycle of CPU execution and I/O wait Process execution begins with a CPU burst…That is followed by an I/O burst, which is followed by another CPU burst, then another I/O burst, and so on. Eventually, the final CPU burst ends with a system request to terminate execution Dr. Abeer Mahmoud

Histogram of CPU-burst Times The durations of CPU bursts vary greatly from process to process. There is large number of short CPU bursts and a small number of long CPU bursts. An I/O-bound program >>>> has many short CPU bursts. A CPU-bound program >>>> has a few long CPU bursts Dr. Abeer Mahmoud

CPU Scheduler CPU Scheduler (/ short-term scheduler): Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them The ready queue is not necessarily a first-in, first-out (FIFO) queue. It can be implemented as a FIFO queue, a priority queue, a tree, or an unordered linked list. Dr. Abeer Mahmoud

Preemptive Scheduling CPU scheduling decisions may take place when a process: Switches from running to waiting state Switches from running to ready state Switches from waiting to ready Terminates Scheduling under 1 and 4 is non-preemptive (/cooperative). scheduling under 2 and 3 is preemptive Dr. Abeer Mahmoud

Preemptive Scheduling (cont..) Under non-preemptive scheduling, once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or by switching to the waiting state Dr. Abeer Mahmoud

Dispatcher Dispatcher : is the module gives control of the CPU to the process selected by the short-term scheduler. The dispatcher should be as fast as possible, since it is invoked during every process switch. Dispatch latency – time it takes for the dispatcher to stop one process and start another running Dr. Abeer Mahmoud

Scheduling Criteria Dr. Abeer Mahmoud

Scheduling Criteria Throughput – # of processes that complete their execution per time unit (e.g. 10 processes /sec) Turnaround time – amount of time to execute a particular process (the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O.) Dr. Abeer Mahmoud

Scheduling Criteria (cont) Waiting time – amount of time a process has been waiting in the ready queue Response time – amount of time it takes from when a request was submitted until the first response is produced (for time-sharing environment) Dr. Abeer Mahmoud

Scheduling Algorithm Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time Dr. Abeer Mahmoud

Scheduling Algorithms Dr. Abeer Mahmoud

Scheduling Algorithms First-Come, First-Served Scheduling Shortest-Job-First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue scheduling Multilevel Feedback Queue Dr. Abeer Mahmoud

(1) First-Come, First-Served (FCFS) The simplest CPU-scheduling algorithm The process that requests the CPU first is allocated first. Can be implemented using FIFO queue: When a process enters the ready queue, its PCB is linked onto the tail of the queue. When the CPU is free, it is allocated to the process at the head of the queue. The running process is then removed from the queue. FCFS algorithm is non-preemptive Dr. Abeer Mahmoud

(1) First-Come, First-Served (FCFS) -cont.. Gantt chart: is a bar chart that illustrates a particular schedule, including the start and finish times of each of the processes. Example(1): Consider the following set of processes that arrive at time 0,with the length of the CPU burst given in milliseconds Process Burst Time(ms) P1 24 P2 3 P3 3 Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 P1 P2 P3 24 27 30 Dr. Abeer Mahmoud

(1) First-Come, First-Served (FCFS) -cont.. Example(2): Consider the same previous set of processes arrive at time 0,with the length of the CPU burst in milliseconds Process Burst Time(ms) P1 24 P2 3 P3 3 Suppose that the processes arrive in the order: P2 , P3 , P1 The Gantt chart for the schedule is: Waiting time for P1 = 6; P2 = 0; P3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 >>>>Much better than example (1) P1 P3 P2 6 3 30 Dr. Abeer Mahmoud

(1) First-Come, First-Served (FCFS) -cont.. Convoy effect - short processes wait for the one big process to get off the CPU. -This effect results in lower CPU and device utilization than might be possible if the shorter processes were allowed to go first. FCFS Pros. (++): Simplest algorithm FCFS Cons. (--): The average waiting time is generally not minimal and affected by processes’ order. Lower CPU and device utilization because of convoy effect Not suitable for time-shared systems Dr. Abeer Mahmoud

(2) Shortest-Job-First (SJF) Scheduling Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time If the next CPU bursts of two processes are the same, FCFS scheduling is used to select the next process Two schemes: Non-preemptive –Preemptive once CPU given to the process it cannot be preempted until completes its CPU burst if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF) Dr. Abeer Mahmoud

(2) Shortest-Job-First (SJF) Scheduling Example(3): Consider the following set of processes with the length of the CPU burst given in milliseconds Process Burst Time(ms) P1 6 P2 8 P3 7 P4 3 SJF scheduling chart Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 P4 P3 P1 3 16 9 P2 24 Dr. Abeer Mahmoud

(2) Shortest-Job-First (SJF) Scheduling Example(4): Consider the following set of processes with the length of the CPU burst given in milliseconds Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 Non-preemptive SJF Average waiting time = (0 + 6 + 3 + 7)/4 = 4 P1 P3 P2 7 3 16 P4 8 12 Dr. Abeer Mahmoud

(2) Shortest-Job-First (SJF) Scheduling Example(5): Consider the following set of processes with the length of the CPU burst given in milliseconds Process Arrival Time Burst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 Preemptive SJF Average waiting time = (9 + 1 + 0 +2)/4 = 3 P1 P3 P2 4 2 11 P4 5 7 16 Dr. Abeer Mahmoud

(2) Shortest-Job-First (SJF) Scheduling SJF Pros. (++): SJF is optimal – gives minimum average waiting time for a given set of processes SJF Cons. (--): The difficulty is knowing the length of the next CPU request ( some times this time is predicted) Dr. Abeer Mahmoud

(3) Priority Scheduling (cont..) A priority number (integer) is associated with each process. The CPU is allocated to the process with the highest priority Equal-priority processes are scheduled in FCFS order. Text book assumes (smallest integer  highest priority) Two schemes: Non-preemptive –Preemptive once CPU given to the process it cannot be preempted until completes its CPU burst if a new process arrives with priorty higher of current executing process, preempt Dr. Abeer Mahmoud

(3) Priority Scheduling (cont..) Example(6): Consider the following set of processes with the length of the CPU burst given in milliseconds Process Burst Time Priority P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 Priority scheduling chart Average waiting time = (6 + 0 + 16 +18 + 1)/5 = 8.2 ms Dr. Abeer Mahmoud

(3) Priority Scheduling (cont..) Priority scheduling Pros. (++): Simple algorithm Priority scheduling Cons. (--): Main Problem - Starvation ( /indefinite blocking) low priority processes may never execute Solution >> Aging >>as time progresses increase the priority of the process Dr. Abeer Mahmoud

(4) Round Robin Scheduling (RR) (RR) algorithm is designed especially for timesharing systems. Each process gets a small unit of CPU time (time quantum ). After this time has elapsed, the process is preempted and added to the end of the ready queue. Time quantum (/ time slice ) (q)>> usually 10-100 ms. The ready queue is treated as a circular queue and implemented as FIFO queue Dr. Abeer Mahmoud

(4) Round Robin Scheduling (RR) Example(6): Consider the following set of processes with the length of the CPU burst given in milliseconds time quantum = 4 ms Process Burst Time P1 24 P2 3 P3 3 The Gantt chart is: Average waiting time = (6 + 4 + 7 )/3 = 5.66 ms P1 P2 P3 4 7 10 14 18 22 26 30 Dr. Abeer Mahmoud

(4) Round Robin Scheduling (RR) If there are n processes in the ready queue and the time quantum is q, then : Each process gets 1/n of the CPU time in chunks of at most (q) time units at once. No process waits more than (n-1) *q time units. Performance (depends on the size of the time quantum) If q is very large  RR is same as FCFS If q is very small  decrease the performance because of context switch time and increase system overhead Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process. Dr. Abeer Mahmoud

(4) Round Robin Scheduling (RR) Turnaround time depends on the size of the time quantum. The average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum. Dr. Abeer Mahmoud

(4) Round Robin Scheduling (RR) RR Scheduling Pros. (++): Suitable to time-shared system (better response time) RR Scheduling Cons. (--): The average waiting time under the RR policy is often long Context switch overhead is higher Dr. Abeer Mahmoud

(5) Multilevel Queue Scheduling Processes are easily classified into different groups, Such as: Foreground (interactive) processes (may have higher priority) Background (batch) processes Ready queue is partitioned into separate queues The processes are permanently assigned to one queue, Each queue has its own scheduling algorithm. E.g.: foreground processes >> scheduled by RR & background process >> scheduled by FCFS Scheduling must be done between the queues. Dr. Abeer Mahmoud

(5) Multilevel Queue Scheduling Example (8): A multilevel queue scheduling algorithm with five queues, listed in order of priority Each queue has absolute priority over lower-priority queues. E.g. No process in the batch queue could run unless the queues for system processes, interactive processes, and interactive editing processes were all empty. If an interactive editing process entered the ready queue while a batch process was running, the batch process would be preempted. Dr. Abeer Mahmoud

(5) Multilevel Queue Scheduling Multilevel Queue Scheduling Pros. (++): Low scheduling overhead Consider different process prosperities & requirements Multilevel Queue Scheduling Cons. (--): Inflexible: a process can’t change it’s queue Starvation possibility Dr. Abeer Mahmoud

(6) Multilevel Feedback Queue A process can move between the various queues The idea is to separate processes according to the characteristics of their CPU bursts. If a process uses too much CPU time, it will be move to a lower- priority queue. This scheme leaves I/O-bound and interactive processes in the higher-priority queues. A process that waits too long in a lower-priority queue may be moved to a higher-priority queue. This form of aging prevents starvation. Dr. Abeer Mahmoud

(6) Multilevel Feedback Queue Example(9): consider a multilevel feedback queue scheduler with three queues: Q0 – RR with time quantum 8 milliseconds (higher priority) Q1 – RR time quantum 16 milliseconds Q2 – FCFS Scheduling Processes in lower priority queue is selected if the higher queues are empty A new job enters queue Q0 which is served RR. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. If Q0 is empty, process at Q1 job is again served RR and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. Q0 ( highest priority) Q1 Q2 (lowest priority) Dr. Abeer Mahmoud

(6) Multilevel Feedback Queue 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 Dr. Abeer Mahmoud

(6) Multilevel Feedback Queue Multilevel Feedback Queue Scheduling Pros. (++): Very flexible>>>it is the most general CPU-scheduling algorithm. Can be configured to prevent starvation. Multilevel Feedback Queue Scheduling Cons. (--): Most complex algorithm Dr. Abeer Mahmoud

Multiple-Processor Scheduling Dr. Abeer Mahmoud

Multiple-Processor Scheduling CPU scheduling is more complex when multiple CPUs are available. load sharing becomes possible Homogeneous processors -processors are identical in functionality (i.e. any processor can run any process in the ready queue) Dr. Abeer Mahmoud

Multiple-Processor Scheduling Approaches to Multiple-Processor Scheduling Asymmetric multiprocessing Symmetric multiprocessing (SMP) Master processor executes system code & slave processors execute user code Only the master processor has all scheduling decisions, I/O processing, and other system activities Simple & reduce the need for data sharing Each processor is self-scheduling, All processes in common ready queue, or each has its own private queue of ready processes OS must ensure that two processors do not choose the same process and that processes are not lost from the queue. Dr. Abeer Mahmoud

Thank you End of Chapter 5 Dr. Abeer Mahmoud