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UTD CS 5348 - Section 10: Scheduling

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Section 10: Scheduling Question 1 1. Describe the difference between an IO-Bound and a Compute-Bound process. 2. Describe Processor Burst Time. 3. What differences between IO and Compute bound process’s effect on Processor Burst Time? Answer 1. IO-Bound: Processes that make frequent I/O requests or system calls that cause the process to block e.g. an application that is retrieving and persisting data to files such as a database. 2. Compute-Bound: Processes that spend most of their time processing data and will run for extended periods without making blocking system calls. 3. Processor burst time is the time a process spends executing before being blocked (I/O) or preempted (timer interrupt). 4. A process that is compute-bound will have longer processor burst times. Compute bound processes will not often block for I/O and so are more likely to run for their entire scheduling quantum. Question 2 Consider Throughput vs Response Time as scheduling criteria. 1. Provide a definition of System’s Response Time. 2. How can the OS minimize an interactive process’s response time? 3. Is minimizing response time critical to a batch processing system? If not, which scheduling criteria is important to batch processing systems? Answer 1. The system’s response time is perceived by the user as the time between making a request of the system (i.e. mouse click, text entry, etc.) and the system’s response to the request. 2. Response time can be minimized by keeping the pages for an interactive processes resident in memory (not swapping out those pages) even if the pages are not currently active (in the working set). 3. Minimizing response time is not critical to a batch processing system. Maximizing system throughput is important to a batch processing system. Question 3 1. Describe Tr and Ts , and the meaning of the ratio Tr /Ts . 2. Describe how Non-Preemptive FIFO dispatching can produce an unfair schedule when there is a mix of long and short-lived processes.Answer 1. Tr /Ts is ratio of the Turnaround Time / Service Time. This ratio was described in class as the ‘Fairness Ratio”. The fairness ratio describes the delay the process experienced before completing. The minimum ratio is 1 meaning that the process executed to completion as soon as it was submitted for processing. The ratio grows if the process’s execution is delayed. In our examples, the delay is because of unfair scheduling policies. 2. With N-P FIFO, if a short-lived process is submitted behind one or more long-lived compute-bound processes, the execution of the short-lived process is delayed until the processes ‘in front of it’ have completed, increasing its Tr. This might be considered an unfair schedule for the short lived process and can lead to user dissatisfaction when the expectation is that short running processes should finish quickly. Question 4 Describe the problem with fairness with Preemptive Round-Robin scheduling WRT I/O-bound processes vs. compute-bound processes. Answer RR scheduling allows a process to complete its quantum (time-slice) and then schedules the next process at the head of the queue. A compute-bound process will likely execute for its full quantum without blocking. However, an I/O-bound process is more likely not to use its entire quantum before being blocked (I/O) and rescheduled when the I/O operation completes. Thus overall, an I/O-bound process will receive less processor time than a compute-bound process. Question 5 How does Virtual Round-Robin scheduling (as described in text) solve the problem described in question 5? Answer Virtual RR scheduling maintains a second queue for “Returned From Blocked” processes that have completed their I/O operation and have not completed their current quantum. See Figure 9.7. When a process is placed in the RFB queue, the dispatcher will preempt the currently executing process in favor of the returned-from-blocked process which will be allowed to complete the remainder of its previously interrupted quantum before being placed into the ready queue. Question 6 1. Briefly describe how the dispatcher selects the next ready process to execute when scheduling processes of different priorities.2. How does the slide suggest implementing a priority-aware dispatcher? Hint: dispatch queues. Answer 1. The dispatcher will give preference to (selects) processes of higher priority over processes of lower priority. 2. Suppose each process is assigned a priority of 1-N, where N is higher priority than N-1. The dispatcher can be implemented with N FIFO queues where each queue holds ready processes of priority i. The dispatcher will select from the highest priority, non-empty queue. For example, the dispatcher will select a process from Queue Lvl N-1 only if Lvl N is empty. Question 7 1. How does Feedback scheduling give preference to short lived processes over long lived processes? 2. How does Feedback scheduling keep long-lived processes from starving? Answer 1. Feedback scheduling assigns priorities to processes according to the number of times they have timed out (completes its quantum). The more times a processes times-out, the lower its assigned priority becomes. This will cause new processes to be scheduled ahead of long-lived processes. 2. The scheduling quantum assigned to a process is increased as its priority is decreased. This means that although a long-lived process is not scheduled as often, when it is scheduled it is allowed to execute for a longer duration before timing out. Question 8 What is the tradeoff between long and short scheduling quantum durations? Answer Long quantum durations will unfairly favor the execution of compute-bound processes as they will receive a larger percentage of the processor before being timed out. Recall that IO-bound processes run for short times and block. Short quantum durations more fairly schedule all type of processes (including short and I/O bound), but decreases processor utilization with an increase in the OS overhead caused by increased context switching. Question 9 1. In the context of Multiprocessor Scheduling, describe the difference between Static Assignment vs. Dynamic Assignment of threads to processors.2. As described in the slides, how are both strategies implemented? Hint: ready queues. Answer 1. With static assignment, newly created threads are attached to, and execute on, the same processor throughout is lifetime (unless moved by


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