148 PROCESSES CHAP. 2

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1 148 PROCESSES CHAP. 2 Interprocess communication primitives can be used to solve such problems as the producer-consumer, dining philosophers, reader-writer, and sleeping barber. Even with these primitives, care has to be taken to avoid errors and deadlocks. Many scheduling algorithms are known, including round-robin, priority scheduling, mu1 tilevel queues, and policy-driven sc hedulers. MINIX supports the process concept and provides messages for interprocess communication, Messages are not buffered, so a SEND succeeds only when the receiver is waiting for it. Similarly, a RKEIVE succeeds only when a message is already available. If either operation does not succeed, the caller is blocked. When an interrupt occurs, the lowest Level of the kernel creates and sends a message to the task associated with the interrupting device. For example, the disk task calls receive and is blocked after writing a command to the disk controller hardware requesting it to read a block of data. The controller hardware causes an interrupt to occur when the data are ready. The low-level software then builds a message for the disk task and marks it as runnable. When the scheduler chooses the disk task to run, it gets and processes the message. It is also possible for the interrupt handler to do some work directly, such as a clock interrupt updating the time. Task switching may follow an interrupt. When a process is interrupted, a stack is created within the process table entry of the process, and all the information needed to restart it is put on the new stack. Any process can be restarted by getting the stack pointer to point to its process table entry and initiating a sequence of instructions to restore the CPU registers, culminating with an iretd instruction. The scheduler decides which process table entry to put into the stack pointer. Interrupts also occur when the kernel itself is running. The CPU detects this, and the kernel stack, rather a stack within the process table, is used. Thus nested interrupts can occur, and when a later interrupt service routine terminates, the one below it can complete. When dl interrupts have been serviced, a process is restarted. The MINIX scheduling algorithm uses three priority queues, the highest one for tasks, the next one for the file system, memory manager, and other servers, if any, and the lowest one for user processes. User processes are run round robin for one quantum at a time. All the others are run until they block or are preempted. PROBLEMS 1. Suppose that you were to design an advanced computer architecture that did process switching in hardware, instead of having interrupts. What information would the CPU need? Describe how the hardware process switching might work. 2. On all current computers, at least part of the intermpt handlers are written in assembly language. Why?

2 CHAP. 2 PROBLEMS In the text it was stated that the model of Fig. 2-6(a) was not suited to a file server using a cache in memory. Why not? Could each process have its own cache? 4. In a system with threads, is there one stack per thread or one stack per process'? Explain. 5. What is a race condition? 6. Write a shell script that produces a file of sequential numbers by reading the last number in the file, adding 1 to it, and then appending to the file. Run one instance of the script in the background and one in the foreground, each accessing the same file. How long does it take before a race condition manifests itself? What is the critical section? Modify the script to prevent the race (hint: use In file file.lock to lock the data file). 7. Is a statement like In file file.lock an effective locking mechanism for a user program like the scripts used in the previous problem? Why (or why not)? 8. Does the busy waiting solution using the turn variable (Fig. 2-8) work when the two processes are running on a shared-memory multiprocessor, that is, two CPUs, sharing a common memory? 9. Consider a computer that does not have a TEST AND SET LOCK instruction but does have an instruction to swap the contents of a register and a memory word in a single indivisible action. Can that be used to write a routine enter-region such as the one found in Fig. 2-1 O? 10. Give a sketch of how an operating system that can disable interrupts could implement semaphores. 11. Show how counting semaphores (i-e., semaphores that can hold an arbitrarily large value) can be implemented using only binary semaphores and ordinary machine instructions. 12. In Sec , a situation with a high-priority process, H, and a low-priority process, L, was described, which led to H looping forever. Does the same problem occur if round-robin scheduling is used instead of priority scheduling? Discuss. 13. Synchronization within monitors uses condition variables and.two special operations, WAIT and SIGNAL. A more general form of synchronization would be to have a single primitive, WAITUNTIL that had an arbitrary Boolean predicate as parameter. Thus, one could say, for example, WAITUNTIL x < 0 or y + t < n The SIGNAL primitive would no longer be needed. This scheme is clearly more general than that of Hoare or Brinch Hansen, but it is not used. Why not? (Hint: think about the implementation.)

3 150 PROCESSES CHAP A fast food restaurant has four kinds of employees: (I) order takers, who take customer's orders; (2) cooks, who prepare the food; (3) packaging specialists, who stuff th2 food into bags; and (4) cashiers, who give the bags to customers and take their money. Each employee can be regarded as a communicating sequential process. What form of interprocess communication do they use? Relate this model to processes in MINIX. 15. Suppose that we have a message-passing system using mailboxes. When sending to a full mailbox or trying to receive from an empty one, a process does not block. Instead, it gets an error code back. The process responds to the error code by just trying again, over and over, until it succeeds. Does this scheme lead to race conditions? 16. In the solution to the dining philosophers problem (Fig. 2-20), why is the state variable set to HUNGRY in the procedure take-forks? 17. Consider the procedure put-forks in Fig Suppose that the variable stateli] was set to THINKING ajler the two calls to test, rather than before. How would this change affect the solution for the case of 3 philosophers? For 100 philosophers? 18. The readers and writers problem can be formulated in several ways with regard to which category of processes can be started when. Carefully describe three different variations of the problem, each one favoring (or not favoring) some category of processes. For each variation, specify what happens when a reader or a writer becomes ready to access the data base, and what happens when a process is finished using the data base. 19. The CDC 6600 computers could handle up to 10 VO processes simultaneously using an interesting forrn of round-robin scheduling called processor sharing. A process switch occurred after each instruction, so instruction 1 came from process 1, instruction 2 came from process 2, etc. The process switching was done by special hardware, and the overhead was zero. If a process needed T sec to complete in the absence of competition, how much time would it need if processor sharing was used with n processes? 20. Round robin schedulers normally maintain a list of all runnable processes, with each process occurring exactly once in the list. What would happen if a process occurred twice in the list? Can you think of any reason for allowing this? 21. Measurements of a certain system have shown that the average process runs for a time T before blocking on VO. A process switch requires a time S, which is effectively wasted (overhead). For round-robin scheduling with quantum Q, give a formula for the CPU efficiency for each of the following. 22. Five jobs are waiting to be run. Their expected run times are 9,6, 3, 5, and X. In what order should they be run to minimize average response time? (Your answer will depend on X.)

4 CHAP. 2 PROBLEMS Five batch jobs A through E, arrive at a computer center at almost the same time. They have estimated running times of 10, 6. 2, 4. and 8 minutes. Their (externally determined) priorities are 3, 5. 2, 1. and 4, respectively, with 5 being the highest priority. For each of the following scheduling algorithms, determine the mean process turnaround time. Ignore process switching overhead. (a) Round robin. (b) Priority scheduling. (c) First-come, first-served (run in order 10, 6. 2, 4,8). (d) Shortest job first. For (a), assume that the system is multiprogramrned, and that each job gets its fair share of the CPU.. For (b) through (d) assume that only one job at a time runs, until it finishes. All jobs are completely CPU bound. 24. A process running on CTSS needs 30 quanta to complete. How many times must it be swapped in, including the very first time (before it has run at all)? 25. The aging algorithm with a = 1/2 is being used to predict run times. The previous four runs, from oldest to most recent, are 40, 20, 40, and 15 msec. What is the prediction of the next time? 26. A soft real-time system has four periodic events with periods of 50, 100, 200, and 250 rnsec each. Suppose that the four events require 35, 20, 10, and x msec of CPU time, respectively. What is the largest value of x for which the system is schedulable? 27. Explain why two-level scheduling is commonly used. 28. During execution, MlNIX maintains a variable procptr that points to the process table entry for the current process. Why? 29. MINIX does not buffer messages. Explain how this design decision causes problems with clock and keyboard interrupts. 30. When a message is sent to a sleeping process in MIYIX, the procedure reudjl is called to put that process on the proper scheduling queue. This procedure starts out by disabling interrupts. Explain. 31. The MINIX procedure mini-rec contains a loop. Explain what it is for. 32. MINIX essentially uses the scheduling method in Fig. 2-23, with different priorities for classes. The lowest class (user processes) has round-robin scheduling, but the tasks and servers always are allowed to run until they block. Is it possible for processes in the lowest class to starve? Why (or why not)? 33. kmlnlx suitable for real-time applications. such as data logging? If not, what could be done to make it so? 34. Assume that you have an operating system that provides semaphores. Implen~ent a message system. Write the procedures for sending and receiving messages. 35. A student majoring in anthropology and minoring in computer science has embarked on a research project to see if African baboons can be taught ahout deadlocks. He locates a deep canyon and fastens a ropc across it. so the baboons can cross handover-hand. Several baboon\.:,iil. I!,,,.! t;~:.*!me time. provided that they dre all

5 PROCESSES CHAP. 2 going in the same direction. If eastward moving and westward moving baboons ever get onto the rope at the same time, a deadlock will result (the baboons will get stuck in the middle) because it is impossible for one baboon to climb over another one while suspended over the canyon. If a baboon wants to cross the canyon, he must check to see that no other baboon is currently crossing in the opposite direction. Write a program using semaphores that avoids deadlock. Do not worry about a series of eastward moving baboons holding up the westward moving baboons indefinitely. 36. Repeat the previous problem, but now avoid starvation. When a baboon that wants to cross to the east amves at the rope and finds baboons crossing to the west, he waits until the rope is empty, but no more westward moving baboons are allowed to start until at least one baboon has crossed the other way. 37, Solve the dining philosophers problem using monitors instead of semaphores. 38. Add code to the MINIX kernel to keep track of the number of messages sent from process (or task) i to process (or task) j. Print this matrix when thi: F4 key is hit. 39. Modify the MINIX scheduler to keep track of how much CPU time each user procegs has had recently. When no task or server wants to run, pick the user process that has had the srnallest share of the CPU. 40. Redesign MINIX so each process has a priority level field in its process table that can be used to give higher or lower priorities to individual processes. 41. Modify the hwint-master and hwint-slave macros in mpx386.s so the operations now performed by the save function are performed inline. What is the cost in code size? Can you measure an increase in performance?