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Chapter6

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Chapter 6: CPU Scheduling

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Chapter6

  1. 1. Preemptive vs. Non-preemptive Chapter 6: CPU Scheduling Scheduling • Always want to have CPU working • Non-preemptive scheduling • Usually many processes in ready queue – A new process is selected to run either – Ready to run on CPU • when a process terminates or – Consider a single CPU here • when an explicit system request causes a wait state (e.g., I/O or wait for child) • Need strategies • Preemptive scheduling – Selecting next process to run – For allocating CPU time – New process selected to run also when – What happens after a process does a system call? • An interrupt occurs • Short-term scheduling • When new processes become ready – Must not take much CPU time Performance Criteria Scheduling Algorithms • CPU utilization • First-come, First-Served (FCFS) – Percentage of time that CPU is busy (and not idle), over – Complete the jobs in order of arrival some period of time • Shortest Job First (SJF) • Throughput – Complete the job with shortest next CPU requirement – Number of jobs completed per unit time (e.g., burst) • Turnaround time – Provably optimal w.r.t. average waiting time – Time interval from submission of a process until • Priority completion of the process – Processes have a priority number • Waiting time – Allocate CPU to process with highest priority – Sum of the time periods spent in the ready queue • Round-Robin (RR) • Response time – Each process gets a small unit of time on CPU (time – Time from submission until first output/input quantum or time slice) – May approximate by time from submission until first – For now, assume a FIFO queue of processes access to CPU FCFS: First-Come First-Served Solution: Gantt Chart Method P1 P2 P3 P4 P5 • Implement with a FIFO ready queue 20 32 40 56 60 • Major disadvantage can be long wait times • Example • Waiting times: • P1: 0 – Draw Gantt chart • P2: 20 – Compute the average wait time for processes • P3: 32 with the following burst times and queue order: • P4: 40 • P1: 20, P2: 12, P3: 8, P4: 16, P5: 4 • P5: 56 • Average wait time: 29.6 1
  2. 2. SJF: Shortest Job First SJF Solution P5 P3 P2 P4 P1 • The job with the shortest next CPU burst 4 12 24 40 60 time is selected • Example (from before): • Waiting times: – CPU job burst times: • P1: 40 • P1: 20, P2: 12, P3: 8, P4: 16, P5: 4 • P2: 12 – Draw Gantt chart and compute the average • P3: 4 waiting time given SJF CPU scheduling • P4: 24 • P5: 0 • Average wait time: 16 SJF Example Estimate • Provably shortest average wait time Say, α = 0.5 • However, requires future knowledge • τ0 = 10 • May have an estimate, to predict next CPU burst • – E.g., base on last CPU burst and a number summarizing • CPU burst, t = 6 history of CPU bursts τn+1 = α * t + (1 - α) * τn • What is estimate of next CPU burst? – Where t is the last CPU burst value, α is a constant τ1 = 0.5 * 6 + 0.5 * 10 = 8 indicating how much to base estimate on last CPU burst, and τn is the last estimate Which Scheduling Algorithms Priority Scheduling Can be Preemptive? • Have to decide on a numbering scheme – 0 can be highest or lowest • FCFS (First-come, First-Served) • FCFS as priority: all have equal priorities – Non-preemptive • SJF as priority: priority is reciprocal of predicted • SJF (Shortest Job First) CPU burst – Can be either • Priorities can be – Choice when a new job arrives – Internal – Can preempt or not • according to O/S factors (e.g., memory requirements) – External: e.g., User importance • Priority – Static: fixed for the duration of the process – Can be either – Dynamic – Choice when a processes priority changes or when a higher priority process arrives • Changing during processing • E.g., as a function of amount of CPU usage, or length of time waiting (a solution to indefinite blocking or starvation) 2
  3. 3. RR (Round Robin) Scheduling Solution completes completes completes completes completes • Give each process a unit of time (time slice, quantum) of execution on CPU P1 P2 P3 P4 P5 P1 P2 P3 P4 P1 P2 P4 P1 P4 P1 • Then move to next process 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 • Waiting times: • Continue until all processes completed • P1: 16 + 12 + 8 + 4 = 40 • Example • P2: 4 + 16 + 12 = 32 – CPU job burst times & order in queue • P3: 8 + 16 = 24 • P1: 20, P2: 12, P3: 8, P4: 16, P5: 4 • P4: 12 + 16 + 8 = 36 – Draw Gantt chart, and compute average wait time • P5: 16 • Average wait time: 29.6 Calculate Other Measurements Response Time Calculations • Response time – Estimate by time from job submission to time to first CPU dispatch Job FCFS SJF RR – Assume all jobs submitted at same time, in order given P1 0 40 0 • Turnaround time – Time interval from submission of a process until P2 20 12 4 completion of the process FCFS P3 32 4 8 P1 P2 P3 P4 P5 20 32 40 56 60 P4 40 24 12 SJF P5 P3 P2 P4 P1 P5 56 0 16 4 12 24 40 60 Average 29.6 16 8 RR P1 P2 P3 P4 P5 P1 P2 P3 P4 P1 P2 P4 P1 P4 P1 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 Performance Characterstics of Turnaround Time Calculations Scheduling Algorithms Job FCFS SJF RR P1 20 60 60 • Different algorithms will have different performance characteristics P2 32 24 44 • RR (Round Robin) P3 40 12 32 – Good average response time • Important for interactive or timesharing systems P4 56 40 48 • SJF P5 60 4 20 – Best average waiting time – Some overhead w.r.t. estimates of CPU burst length Average 41.6 28 40.8 Assume processes submitted at same time 3
  4. 4. Context Switching Issues Example • These calculations have not taken context switch • Calculate average wait time for RR (round duration into account robin) scheduling, for – In general, the context switch will take time – Processes: P1: 24, P2: 4, P3: 4 – Just like the CPU burst of a process takes time – Assume this arrival order – Response time, wait time etc. will be affected by context switch time – Quantum = 4; context switch time = 1 • RR (Round Robin) & quantum duration – The smaller the time quantum, the better the average response time, but the more system overhead – Want the quantum large compared to context switch time Solution: Average Wait Time Multi-level Ready Queues With Context Switch Time • Multiple ready queues P1 P2 P3 P1 P1 P1 P1 P1 – For different types of processes (e.g., system, vs. user processes) 45 9 10 14 15 19 20 24 25 29 30 34 35 39 – For different priority processes (e.g., Mach) • P1: 0 + 11 + 4 = 15 • Each queue can • P2: 5 – Have a different scheduling algorithm • P3:10 – Receive a different amount of CPU time • Average: 10 – Have movement of processes to another queue (feedback); • e.g., if a process uses too much CPU time, put in a lower (This is a case for dynamically varying the priority queue time quantum, as in Mach.) • If a process is getting too little CPU time, put it in a higher priority queue Synchronization Issues Multiprocessor Scheduling • Symmetric multiprocessing • When a computer has more than one processor, • Involves synchronization of access to global ready need a method of dispatching processes queue • Types of ready queues – E.g., only one processor must execute a job at one time – Local: dispatch to a specific processor • Processors: CPU1, CPU2, CPU3, … – Global: dispatch to any processor (“load sharing”) • When a processor (e.g., CPU1) accesses the ready • Processor/process relationship queue – Run on only a specific processor (e.g., if it must use a – All other processors (CPU2, CPU3, …) must wait, and device on that processor’s private bus) be denied access to the ready queue – Run on any processor – The accessing processor (e.g., CPU1) will remove a • Symmetric: Each processor does own scheduling process from the ready, and dispatch it on itself – Then that processor will make the ready queue • Master/slave: available for use by the other CPU’s (CPU2, CPU3, …) – Master processor dispatches processes to slaves 4
  5. 5. Pre-emptive Scheduling & Operating System Design • With pre-emptive CPU scheduling, a new process can run when interrupt occurs • What if thread A was in the middle of updating data structures, and was put back in ready queue – Either on disk or in shared memory • If thread B also accesses same data structures – May access data structures in an inconsistent state • Need mechanisms for cooperative data access – Both in Kernel • Kernel, in general, needs to handle interrupts • Don’t want to loose interrupts • Real-time & multi-processor issues • May need preemption in the kernel itself – And by multiple processes/threads 5
  • ogirish

    Aug. 3, 2009

Chapter 6: CPU Scheduling

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