Presentation on theme: "Scheduling in Batch Systems"— Presentation transcript:
1 Scheduling in Batch Systems Three level schedulingAdmission: job mix (long term scheduler)Memory: degree of multiprogramming (medium term)CPU Scheduler: algorithm to choose ready process to run (short term)
2 Basic Concepts Maximum CPU utilization obtained with multiprogramming CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait.CPU burst distribution
4 Histogram of CPU-burst Times Lots of short CPU activitiesFew CPU intensive
5 CPU SchedulerSelects 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.All other scheduling is preemptive.
6 DispatcherDispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:switching contextswitching to user modejumping to the proper location in the user program to restart that programDispatch latency – time it takes for the dispatcher to stop one process and start another running.
7 Scheduling CriteriaCPU utilization – percentage of time the CPU is executing a process (* more on next slide *)Throughput – # of processes that complete their execution per time unitTurnaround time – amount of time to execute a particular processWaiting time – amount of time a process has been waiting in the ready queueResponse time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time-sharing environment)
8 CPU Utilization Keep the CPU as busy as possible Load on system affects level of utilizationHigh level of utilization is easier to reach on heavily loaded systemOn single-user system, CPU utilization is not very importantOn time-shared system, CPU utilization may be primary consideration
10 Optimization Criteria Max CPU utilizationMax throughputMin turnaround timeMin waiting timeMin response time
11 Scheduling Algorithms Non-preemptiveProcess retains control of CPU until process blocks or is terminatedGood for batch jobs when response time is of little concernCommon: FCFS, SJFPreemptiveScheduler may preempt a process before it blocks or terminates, in order to allocate CPU to another processNecessary on interactive systemsCommon: SRT, RR
12 First-Come, First-Served (FCFS) Scheduling Process Burst TimeP1 24P2 3P3 3Suppose 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 = 27Average waiting time: ( )/3 = 17P1P2P3242730
13 FCFS Scheduling (Cont.) Suppose that the processes arrive in the orderP2 , P3 , P1 .The Gantt chart for the schedule is:Waiting time for P1 = 6; P2 = 0; P3 = 3Average waiting time: ( )/3 = 3Much better than previous case.Convoy effect short process behind long processShort jobs sufferFavors CPU bound processesP1P3P26330
14 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. Tie breaker via FCFS.Two schemes:nonpreemptive – once CPU 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 remaining time of current executing process, preempt. This scheme is know as the SJF-preemptive or Shortest-Remaining-Time-First (SRT or SRTF).SJF is optimal – gives minimum average waiting time for a given set of processes.
15 Example of Non-Preemptive SJF Process Arrival Time Burst TimePPPPSJF (non-preemptive)Average waiting time = ( )/4 = 4P1P3P27316P4812
16 Example of Preemptive SJF Process Arrival Time Burst TimePPPPSJF (preemptive)Average waiting time = ( )/4 = 3Consider the context switching overhead costP1P3P24211P45716
17 SJF Favors short jobs over long Constant arrival of small jobs can lead to starvation of long jobs
18 Priority SchedulingA priority number (integer) is associated with each processBase on process characteristic (memory usage, I/O frequency)Base on userBase on usage cost (CPU time for higher priority costs more)User or administrator assigned (static)May be dynamic (e.g., changing with amount of time running)
19 Priority Scheduling (continued) The CPU is allocated to the process with the highest priority (smallest integer highest priority).PreemptivenonpreemptiveSJF is a priority scheduling algorithm where priority is the predicted next CPU burst time.Problem Starvation – low priority processes may never execute.Solution Aging – as time progresses increase the priority of the process.
20 Round Robin (RR)Each process gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue. (Interval timer generates interrupt.)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.Performanceq large FIFOq small good response time; however, q must be large with respect to context switch, otherwise overhead is too high.
21 Ex. of RR with Time Quantum = 20 Process Burst TimeP1 53P2 17P3 68P4 24The Gantt chart is:Typically, higher average turnaround than SJF, but better response.P1P2P3P42037577797117121134154162
23 Treating All Jobs the Same These algorithms basically treat all jobs the sameEach algorithm favors a certain kind of processTo address this deficiency, multilevel feedback queues customize the scheduling of processes based on the process’s performance characteristics by utilizing 2 or more scheduling algorithmsFlexibleComplex
24 Multilevel QueueReady queue is partitioned into separate queues: foreground (interactive) background (batch)Each queue has its own scheduling algorithm, foreground – RR background – FCFSScheduling must be done between the queues.Fixed priority scheduling; (i.e., 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 RR20% to background in FCFS
26 Multilevel Feedback Queue A process can move between the various queues; aging can be implemented this way.Multilevel-feedback-queue scheduler defined by the following parameters:number of queuesscheduling algorithms for each queuemethod used to determine when to upgrade a processmethod used to determine when to demote a processmethod used to determine which queue a process will enter when that process needs service
27 Example of Multilevel Feedback Queue Three queues:Q0 – time quantum 8 millisecondsQ1 – time quantum 16 millisecondsQ2 – FCFSSchedulingA new job enters queue Q0 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 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.