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CPU Scheduling. Outline. Basic Concepts Scheduling Criteria Scheduling Algorithms Multiple-Processor Scheduling Real-Time Scheduling Algorithm Evaluation. Basic Concepts. multiprogramming has been introduced in order to maximize CPU utilization CPU–I/O Burst Cycle
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Outline • Basic Concepts • Scheduling Criteria • Scheduling Algorithms • Multiple-Processor Scheduling • Real-Time Scheduling • Algorithm Evaluation
Basic Concepts • multiprogramming has been introduced in order to maximize CPU utilization • CPU–I/O Burst Cycle • Process execution consists of a cycle of CPU execution and I/O wait. • CPU and I/O burst distribution • Can be viewed as a random variable • Can be characterized with an empirical and/or theoretical probability distribution
CPU Scheduler • Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them. • CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state. 2. Switches from running to ready state. 3. Switches from waiting to ready. 4. Terminates. • Scheduling under 1 and 4 is nonpreemptive. • Once a process is given the CPU it holds it until it terminates or changes state to waiting • All other scheduling is preemptive.
Dispatcher • The Dispatcher module gives control of the CPU to the process selected by the CPU (short-term) scheduler • The dispatcher’s function involves: • switching context • switching to user mode • jumping to the proper location in the user program to restart that program • Dispatch latency • The time it takes for the dispatcher to stop one process and start running running.
Scheduling Algorithms • Before we look into scheduling algorithms need to set • Criteria (measures) and objectives that will be used in designing and evaluating such algorithms • Scheduling algorithms should optimize some objectives derived from certain criteria • Need to understand various properties of scheduling algorithms so that we can choose an algorithm that is most suited to a particular computing environment
Scheduling Criteria • CPU utilization • Percent of time the CPU is busy executing processes • Throughput • number of processes that complete their execution per time unit • Turnaround time • amount of time to execute a particular process • 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 (program) was submitted until the first response is produced, • Does not include output time
Optimization Objectives • Maximize CPU utilization • Maximize throughput • Minimize the maximum turnaround time • Minimize average waiting time • Minimize average response time • Minimize the variance of response time • Etc • We will consider • Minimizing the average waiting time
First-Come First-Served (FCFS) Scheduling • Process requesting the CPU first gets it first • Simple to implement with a single FIFO queue of ready processes • Link their PCB’s into that queue • Non-preemptive scheduling algorithm
P1 P2 P3 0 24 27 30 FCFS Scheduling Example ProcessBurst Time P1 24 P2 3 P3 3 • Suppose that the processes arrive at time 0 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
P2 P3 P1 0 3 6 30 FCFS Scheduling Example • 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 • Average waiting time depends on order of processes in FIFO queue • Convoy effect • Arises when a number of short (I/O bound) processes wait behind a long (CPU bound) process
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. • Two schemes: • Non-preemptive • once the CPU is given to the process it cannot be preempted until completes its CPU burst. • preemptive • if a new process arrives with CPU burst length less than the remaining time of the current executing process, preempt it. • This scheme is known as the Shortest-Remaining-Time-First (SRTF). • SJF is optimal • It gives the minimum average waiting time for a given set of processes.
P1 P3 P2 P4 0 3 7 8 12 16 Non-Preemptive SJF Example Process Arrival TimeBurst Time P1 0 7 P2 2 4 P3 4 1 P4 5 4 • SJF (non-preemptive) • Average waiting time = (0 + 6 + 3 + 7)/4 =4
P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Preemptive SJF Example Process Arrival TimeBurst Time P1 0 7 P2 2 4 P3 4 1 P4 5 4 • SJF (preemptive) • Average waiting time = (9 + 1 + 0 +2)/4 =3
Difficulties with SJF • Length of next CPU burst is generally not known ahead of time • Can only estimate the length of the next CPU burst • Need appropriate estimating functions • Can be done by using the length of previous CPU bursts, using exponential averaging.
Examples of Exponential Averaging • =0 • n+1 = n • Recent history does not count. • =1 • n+1 = tn • Only the actual last CPU burst counts. • If we expand the formula, we get: n+1 = tn+(1 - ) tn -1 + … +(1 - )j tn -1 + … +(1 - )n=1 tn 0 • Since both and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor.
Priority Scheduling • A priority number (integer) is associated with each process • The CPU is allocated to the process with the highest priority • Here, we use the convention that smallest integer highest priority. • It can be • Preemptive • Non-preemptive • SJF is a priority scheduling where priority is determined by the predicted next CPU burst time. • Can cause Starvation • A low priority processes may never execute • Aging is a technique that can be used to avoid starvation • as time progresses increase the priority of processes waiting for the CPU
Round Robin (RR) Scheduling • Each process gets a small unit of CPU time (time quantum or slice), usually 10-100 milliseconds. • After this time slice has elapsed, the process is preempted and added to the end of the ready queue. • 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. • When deciding on the time slice need to consider the overhead due to the dispatcher • Performance characteristics • q large FIFO • q small q must be large with respect to context switch, otherwise overhead is too high.
P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 RR Scheduling Example with Time Quantum = 20 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround time than SJF, but better response time.
Multilevel Queue Scheduling • Ready queue is partitioned into separate queues • foreground (interactive) • background (batch) • Each queue has its own scheduling algorithm • foreground – RR • background – FCFS
Multilevel Queue Scheduling • Scheduling must be done between the queues. • Fixed priority scheduling • serve all from foreground then from background • Possibility of starvation. • Time slice • each queue gets a certain amount of CPU time which it can schedule amongst its processes; • i.e., 80% to foreground in RR, 20% to background in FCFS
Multilevel Feedback Queue Scheduling • A process can move between the various queues • aging can be implemented this way. • Multilevel-feedback-queue scheduler is defined by the following parameters • number of queues • scheduling algorithms for each queue • method used to determine when to promote 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
Multilevel Feedback Queue Scheduling Example • Three queues • Q0 – time quantum 8 milliseconds • Q1 – time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0which is servedFCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. • At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
Multiple-Processor Scheduling • CPU scheduling is more complex when multiple CPUs are available. • Homogeneous processors • All processors are identical within the multiprocessor system • Load sharing • Idle processors share the load of busy processors • Maintain a single ready queue shared among all the processors • Symmetric multiprocessing • Each processor schedules a process autonomously from the shared ready queue • Asymmetric multiprocessing • only one processor accesses the system data structures, alleviating the need for data sharing.
Real-Time Scheduling • Hard real-time systems • When it is required to complete a critical task within a guaranteed amount of time • Soft real-time computing • When it is required that critical processes receive priority over less fortunate ones. • For soft real-time scheduling • System must have priority scheduling • The dispatch latency must be small • Problem caused by the fact that many OSs wait for a context switch until either a system call completes or an I/O blocks
Keeping the Dispatch Latency Small • Introduce preemption points within the kernel • Where it is safe to preempt a system call by a higher priority process • Trouble is only few such points can be practically added • Make the entire kernel preemptive • Need to ensure that it is safe for a higher priority process to access shared kernel data structures • Complex method that is widely used • What happens when a high priority process waits for lower priority processes to finish using a shared kernel data structure (priority inversion)? • Priority inheritance
Evaluating Scheduling Algorithms • Deterministic modeling • take a particular predetermined workload and determine the performance of each algorithm for that workload • Queueing Models • Use mathematical formulas to analyze the performance of algorithms under some (simple and possible unrealistic) workloads • Simulation Models • Use probabilistic models of workloads • Use workloads captured in a running system (traces) • Implementation
Queueing Models • Assuming that • Process inter-arrival times and CPU burst times are independent identical exponentially distributed random variables • FCFS scheduling
Linux Scheduling • Two algorithms: time-sharing and real-time • Time-sharing • Prioritized credit-based – process with most credits is scheduled next • Credit subtracted when timer interrupt occurs • When credit = 0, another process chosen • When all processes have credit = 0, recrediting occurs • Based on factors including priority and history • Real-time • Soft real-time • Posix.1b compliant – two classes • FCFS and RR • Highest priority process always runs first
