MC78 Homework 1 (Fall 1998)

Due: September 21, 1998

1. Do exercise 5.9 from page 152.
2. Suppose we have a system that does preemptive priority-based scheduling using externally defined priorities. Consider the following scenario. A low-priority process is using some resource that only one process can use at a time. The low-priority process is preempted by an newly arriving medium-priority process, which just runs and runs forever (totally CPU-bound). Now the medium-priority process is in turn preempted by an arriving high-priority process. However, the high-priority process needs the same resource that the low-priority process was in the middle of using, so the high-priority process goes into a waiting state, waiting for the resource. Which process will run now? Why is this a bad thing? Give at least two completely different modifications that could be made to the priority-based scheduler to solve this problem. (Do not simply scrap the priority scheduling entirely in favor of round-robin, for example. The goal is to still generally give the high-priority process precedence.)
3. Windows NT Workstation versions 3.x and 4.0 both provide some scheduling preference for the user's "foreground" application - the application with the window that is currently "in focus" (i.e., the window that would receive any keyboard input from the user). However, the nature of this scheduling preference was changed in NT 4.0 versus 3.x. As background information, I'll summarize the scheduling algorithm used in both versions of NT. (This description is simplified, but gives the essential features.) The basic scheduling algorithm is a preemptive multilevel queue, with one queue for each priority level and round-robining among the processes at any one priority level. Under normal circumstances, all a user's processes will be at the same priority level (8, as it happens) and will use the same time quantum for round-robining (normally 30 milliseconds). Of course, a process may not run for its full time quantum - it can be preempted by an arriving higher-priority process, in which case it will get the remainder of its quantum when it resumes, or it can go into a waiting state before the quantum is up, in which case it gets a fresh quantum when it again becomes ready to run. With this information as background, here is the difference between NT 3.x and 4.0: in 3.x, the foreground application gets boosted to a higher priority level, whereas in 4.0 it instead gets double the normal time quantum. (In 3.x it keeps the normal quantum and in 4.0 it keeps the normal priority.) Explain what you would expect the user-visible results of this difference to be, and why. How would the foreground application behave differently? How would other (background) applications behave differently?

Instructor: Max Hailperin