现代操作系统作业答案课件.ppt

Modern Operating Systems Solutions;Chapter 2;8. A worker thread will block when it has to read a Web page from the disk. If user-level threads are being used, this action will block the entire process, destroying the value of multithreading. Thus it is essential that kernel threads are used to permit some threads to block without affecting the others. 11. Threads in a process cooperate. They are not hostile to one another. If yielding is needed for the good of the application, then a thread will yield. After all, it is usually the same programmer who writes the code for all of them. ;Figure 2-8 A multithreaded Web server;14. The biggest advantage is efficiency. No traps to the kernel are needed to switch threads. The biggest disadvantage is that if one thread blocks, the entire process blocks. 28. With kernel threads, a thread can block on a semaphore and the kernel can run some other thread in the same process. Consequently, there is no problem using semaphores. With user-level threads, when one thread blocks on a semaphore, the kernel thinks the entire process is blocked and does not run it ever again. Consequently, the process fails. 31. It does not lead to race conditions (nothing is ever lost), but it is effectively busy waiting. ;37. (a) For round robin, during the first 10 minutes each job gets 1/5 of the CPU. At the end of 10 minutes, C finishes. During the next 8 minutes, each job gets ? of the CPU, after which time D finishes. Then each of the three remaining jobs gets 1/3 of the CPU for 6 minutes, until B finishes, and so on. The finishing times for the five jobs are 10, 18, 24, 28, and 30, for an average of 22 minutes. Average = (10 + 18 + 24 + 28 + 30) / 5 = 22 minutes ;37. (b) priority Turn around time of B = 6 minutes Turn around time of E = 14 minutes Turn around time of A = 24 minutes Turn around time of C = 26 minutes Turn around time of D = 30 minutes Average = (6 + 14 + 24 + 26 + 30) / 5 = 20 minutes ;37. (c) FCFS Turn around t