操作系统概念 第5章答案.pdf

操作系统作业 5 2 Discuss how the following pairs of scheduling criteria confl ict in certain settings a CPU utilisation and response time b Average turnaround time and maximum waiting time c I O device utilisation and CPU utilisation a CPU utilisation and response time CPU utilisation is increased if the overheads associated with context switching is minimised The context switching overheads could be lowered by performing context switches infrequently This could however result in increasing the response time for processes b Average turnaround time and maximum waiting time Average turnaround time is minimised by executing the shortest tasks fi rst Such a scheduling policy could however starve long running tasks and thereby increase their waiting time c I O device utilisation and CPU utilisation CPU utilisation is maximised by running long running CPU bound tasks without performing context switches I O device utilisation is maximised by scheduling I O bound jobs as soon as they become ready to run thereby incurring the overheads of context switches 5 5 Which of the following scheduling algorithms could result in starvation a First come fi rst served b Shortest job fi rst c Round robin d Priority Answer Short time fi rst and Priority scheduling algorithms may contribute to starvation Starvation is situation that thread has arrived but have to wait for the CPU to be executed because of CPU occupied by other threads an the same time 5 7问 long running tasks 有什么区别 Consider a system running ten I O bound tasks and one CPU bound task Assume that the I O bound tasks issue an I O operation once for every millisecond of CPU computing and that each I O operation takes 10 milliseconds to complete Also assume that the context switching overhead is 0 1 millisecond and that all processes are long running tasks What is the CPU utilisation for a round robin scheduler when a The time quantum is 1 millisecond The time quantum is 1 millisecond Irrespective of which process is scheduled the scheduler incurs a 0 1 millisecond context switching cost for every context switch This results in a CPU utilisation of 1 1 1 100 91 b The time quantum is 10 milliseconds The time quantum is 10 milliseconds The I O bound tasks incur a context switch after using up only 1 millisecond of the time quantum The time required to cycle through all the processes is therefore 10 1 1 10 1 as each I O bound task executes for 1 millisecond and then incur the context switch task whereas the CPU bound task executes for 10 milliseconds before incurring a context switch The CPU utilisation is therefore 20 21 1 100 94 CPU利 用率计算 方法 假设 一段时间total内 该任务的运 行时间为idle cpu利 用 率的计算公式为 total idle total 这 一种 方法实现起来很简单 但是它只 能算得整个系统的cpu使 用情况 不能得到单个任务 的cpu使 用情况 同时它 会使cpu满负荷 工作 题 目中 提到的 that the I O bound tasks issue an I O operation once for every millisecond of CPU computing 意思是I O bound tasks 每个耗时是1毫秒 有 十个这样的任务 而 each I O operation takes 10 milliseconds to complete 意思是每个 CPU bound task每个耗时10毫秒 但是 只有 一个 优先级转换速率是什么 5 9 Consider a preemptive priority scheduling algorithm based on dynamically changing priorities Larger priority numbers imply higher priority When a process is waiting for the CPU in the ready queue but not running its priority changes at a rate when it is running its priority changes at a rate All processes are given a priority of 0 when they enter the ready queue The parameters and can be set to give many different scheduling algorithms a What is the algorithm that results from 0 b What is the algorithm that results from 0 Answer a FCFS 先来先服务 从就绪队列中选择 一个或若 干个最先进 入该队列的进程 将 他们调 入内存运 行 b LIFO 后进先出 从就绪队列中选择 一个或若 干个最后进 入该队列的进程 将它们调 入内存运 行 5 10 Explain the differences in the degree to which the following scheduling algorithms discriminate in favour of short processes a FCFS b RR c Multilevel feedback queues Answer a FCFS discriminates against short jobs since any short jobs arriving after long jobs will have a longer waiting time 秉承先来先服务原则 所以 短程任务如果在长程任务之后到来 就 必须等待 这样 子就等待蛮长时间的 所以对短程任务不利 b RR treats all jobs equally giving them equal