An Operating Systems Vade Mecum

Exercises 25and a case study for the PDP-11 is described by Popek (1975).8 EXERCISES1. Compute the utilization for open shop. We define the utilization u as the fractionof time used for computation. Assume that a typical job requires r = 10 seconds toread in from cards, c = 3 seconds to compute and p = 30 seconds to print theresults on paper. Programmers sign up for 15-minute slots and run their programstwice per slot.2. Compute the utilization for the same situation, using operator-driven shop.Assume that an operator takes s = 30 seconds to remove the output from one joband set up the next job. There are enough jobs to ensure that the operator alwayshas another job to start as soon as one finishes.3. Compute the utilization for the same situation, using offline transput. Assume thatit takes only 1/100 as long to read information from tape as from cards and only1/100 as long to write information to tape as to paper. The resident monitor takess = 0.1 seconds to reset the machine between jobs. The operator spends 60seconds to unload and load tapes after every ten jobs. There are several offlinecomputers, so there is no bottleneck reading jobs and printing results.4. Compute the utilization for the same situation, using spooling. Assume that thecomputer has enough card readers and printers so that there are always jobs waitingto be run and printing is not a bottleneck. It takes only 1/1000 as long to reador write information from or to disk as from or to cards or paper. The computerspends 1 percent of its time servicing interrupts for transput; this time is notcounted as computation time. It takes s = 0.01 seconds to reset the machinebetween jobs.5. Construct formulas for exercises 1−4 that describe the utilization in terms of theparameters r , c , p , and s .6. Find out what computers are available at your institution and discover whether theyuse spooling, batch, interactive, or some other kind of computing.7. In an interactive multiprogramming situation, several people could be running thesame program at once. Would there be one process or many to support this situation?8. If two processes are using the very same main storage, what data do they notshare?9. Throughout the book, we will suggest various service calls that the kernel may provide.Suggest a few that you think every kernel should have.10. How can a user submit a service call?