Chapter 6: CPU Scheduling. Previous Lectures. Basic Concepts. Histogram of CPU-burst Times. CPU Scheduler. Alternating Sequence of CPU And I/O Bursts

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1 Multithreadig Memory Layout Kerel vs User threads Represetatio i OS Previous Lectures Differece betwee thread ad process Thread schedulig Mappig betwee user ad kerel threads Multithreadig i Java Thread creatio Thread lifecycle Thread collaboratio/sychroizatio Hads-o experiece after this: Process system calls, Java threads Chapter 6: CPU Schedulig Basic Cocepts Schedulig Criteria Schedulig Algorithms Multiple-Processor Schedulig Real-Time Schedulig Algorithm Evaluatio Basic Cocepts Alteratig Sequece of CPU Ad I/O Bursts Maximum CPU utilizatio is obtaied with multiprogrammig Multiple programs executed at the same time CPU I/O Burst Cycle Process executio cosists of periods of CPU executio ad I/O wait. CPU burst distributio a metric that ca be used Histogram of CPU-burst Times CPU Scheduler What this shows is that short bursts are more frequet Selects from amog the processes i memory that are ready to execute, ad allocates the CPU to oe of them. CPU schedulig decisios may take place whe a process: 1. Switches from ruig to waitig state. 2. Switches from ruig to ready state. 3. Switches from waitig to ready. 4. Termiates. Schedulig uder 1 ad 4 is opreemptive. All other schedulig is preemptive. 1

2 Schedulig Criteria Optimizatio Criteria CPU utilizatio keep the CPU as busy as possible Throughput # of processes that complete their executio per time uit Turaroud time amout of time to execute a particular process Waitig time amout of time a process has bee waitig i the ready queue Respose time amout of time it takes from whe a request was submitted util the first respose is produced, ot output (for time-sharig eviromet) Examples: Max CPU utilizatio Max throughput Mi waitig time Mi respose time First-Come, First-Served (FCFS) Schedulig FCFS Schedulig (Cot.) Process Burst Time P 1 24 P 2 3 P 3 3 Suppose that the processes arrive i the order: P 1, P 2, P 3 The Gatt Chart for the schedule is: P 1 P 2 P Waitig time for P 1 = ; P 2 = 24; P 3 = 27 Average waitig time: ( )/3 = 17 Suppose that the processes arrive i the order P 2, P 3, P 1. The Gatt chart for the schedule is: P 2 P Waitig time for P 1 = 6; P 2 = ; P 3 = 3 Average waitig time: ( )/3 = 3 Much better tha previous case. Covoy effect: short process behid log process; show i previous example. P 1 Shortest-Job-First (SJR) Schedulig Example of No-Preemptive SJF Associate with each process the legth of its ext CPU burst. Use these legths to schedule the process with the shortest time. Two schemes: opreemptive oce CPU give to the process it caot be preempted util completes its CPU burst. preemptive if a ew process arrives with CPU burst legth less tha remaiig time of curret executig process, preempt. This scheme is kow as the Shortest-Remaiig-Time-First (SRTF). SJF is optimal gives miimum average waitig time for a give set of processes. Process Arrival Time Burst Time P 1. 7 P P P SJF (o-preemptive) P 1 P 3 P Average waitig time = ( )/4-4 P 4 2

3 Example of Preemptive SJF(SRTF) Determiig Legth of Next CPU Burst Process Arrival Time Burst Time P 1. 7 P P P SJF (preemptive) P 1 P 2 P 3 P 2 P 4 P We ca oly estimate the legth. Ca be approximated by usig the legth of previous CPU bursts (past behavior), usig expoetial averagig. th 1. t = actual leght of CPUburst 2. τ + 1 = predicted value for the ext CPU burst 3. α, α 1 4. Defie: ( 1 α ). τ = α t + τ + 1 Average waitig time = ( )/4-3 Predictio of the Legth of the Next CPU Burst Examples of Expoetial Averagig τ ( 1 α ). + 1 = α t + τ α = τ +1 = τ Recet history does ot cout. α =1 τ +1 = t Oly the actual last CPU burst couts. Note: if we expad formula above we ca see that, sice both α ad (1 - α) are less tha or equal to 1, each successive term has less weight tha its predecessor. Most importat compoet is the last widow before the estimated oe The value of alpha determies how much weight is give to the previous estimated widow legth Priority Schedulig Example of Widows 2 priorities How it works: A priority umber (iteger) is associated with each process The CPU is allocated to the process with the highest priority (smallest iteger highest priority). Ca be: Preemptive opreemptive Problem: Starvatio low priority processes may ever execute. Solutio Agig as time progresses icrease the priority of the process. Notes: SJF is a priority schedulig where priority is the predicted ext CPU burst time. The Java thread schedulig i the VM is similarly fixed priority based 3

4 Roud Robi (RR) Example of RR with Time Quatum = 2 Preemptive versio of FIFO Each process gets a small uit of CPU time (time quatum q), usually 1-1 millisecods. After this time has elapsed, the process is preempted ad added to the ed of the ready queue. Some isights: If there are processes i the ready queue ad the time quatum is q, the each process gets 1/ of the CPU time i chuks of at most q time uits at oce. No process waits more tha (-1)q time uits. Performace: q large FIFO q small q must be large with respect to cotext switch, otherwise overhead is too high. Process Burst Time P 1 53 P 2 17 P 3 68 P 4 24 The Gatt chart is: P 1 P 2 P 3 P 4 P 1 P 3 P 4 P 1 P 3 P Typically, higher average turaroud tha SJF, but better respose. Time Quatum ad Cotext Switch Time Turaroud Time Varies With The Time Quatum Multilevel Queue Multilevel Queue Schedulig How it works: Ready queue is partitioed ito separate queues: foregroud (iteractive) ad backgroud (batch) Each queue has its ow schedulig algorithm, e.g., foregroud RR backgroud FCFS Schedulig must be doe betwee the queues. Example implemetatios: Fixed priority schedulig; serve all from foregroud the from backgroud). Possibility of starvatio. Time slice each queue gets a certai amout of CPU time which it ca schedule amogst its processes; i.e., 8% to foregroud i RR ad 2% to backgroud i FCFS 4

5 Multilevel Feedback Queue Example of Multilevel Feedback Queue A process ca move betwee the various queues; agig ca be implemeted this way. Multilevel-feedback-queue scheduler defied by the followig parameters: umber of queues schedulig algorithms for each queue method used to determie whe to upgrade a process method used to determie whe to demote a process method used to determie which queue a process will eter whe that process eeds service Three queues: Q time quatum 8 millisecods Q 1 time quatum 16 millisecods Q 2 FCFS Schedulig A ew job eters queue Q which is served FCFS. Whe it gais CPU, job receives 8 millisecods. If it does ot fiish i 8 millisecods, job is moved to queue Q 1. At Q 1 job is agai served FCFS ad receives 16 additioal millisecods. If it still does ot complete, it is preempted ad moved to queue Q 2. Multilevel Feedback Queues Multiple-Processor Schedulig CPU schedulig more complex whe multiple CPUs are available. Issues: Homogeeous processors withi a multiprocessor? Each processor treated equal Asymmetric multiprocessig? oly oe processor accesses the system data structures, alleviatig the eed for data sharig. Real-Time Schedulig Hard real-time systems required to complete a critical task withi a guarateed amout of time. Soft real-time computig requires that critical processes receive priority over less fortuate oes. 5