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If there is some way for all processes to have their pending requests satisfied and run to completion, then deadlock does not exist. Otherwise, deadlock does exist, and some action must be taken to break the deadlock. The deadlock detection algorithm works by maintaining several data structures. First, the algorithm maintains a vector with the count of available resources of each type. This vector is called Available. If a system has seven printers, two CD-ROM drives, and six plotters, and all devices are allocated to one or another process (none is available), then the vector Available = [0, 0, 0] A matrix called Allocated records the counts of resources of each type already allocated to each active process. In a particular case of five active processes, where all of the resources listed in the previous paragraph have been allocated, the matrix Allocated might look like this: Allocated CD 1 0 0 1 0
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A matrix called Requests maintains the counts of all pending resource requests, at this particular moment, by each process. Suppose this is the matrix Requests for the same set of five processes at a particular instant: Requests CD 0 0 0 0 0
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With this information, the system can determine whether deadlock has occurred. All it needs to do is determine whether there is some way all processes can have their requests met, and then continue to execute. If there is some way all processes can complete, then there is no deadlock. In this case, no resources are available; all resources are already allocated. However, P0 and P2 are not requesting any more resources. Suppose P0 and P2 run to completion; P0 will release one CD-ROM drive, and P2 will release three printers and three plotters. At that point Available = [3, 1, 3]. Now the requests of P1 for two more printers and two more plotters can be satisfied; P1 can complete; and P1 can return the two printers it already has, plus the two more it requested. Now Available = [5, 1, 3]. It s obvious that similar reasoning shows that the requests of the remaining processes can be satisfied, and therefore there is no deadlock at this time. Suppose, however, that one small change is made to the matrix Requests. Suppose that P2 has a pending request for one plotter. Now the matrix Requests looks like this: Requests CD 0 0 0 0 0
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P0 P1 P2 P3 P4
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OPERATING SYSTEMS
[CHAP. 6
P0 can still proceed as in our earlier analysis, and when it completes it will return one CD ROM drive. Then Available = [0, 1, 0]. Unfortunately, there is no way to satisfy the requests of any other process, and P1, P2, P3, and P4 are deadlocked. Deadlock recovery Once the system detects a deadlock, the next question is what to do about it. Some resource could be preempted from one of the processes, but the operating system probably will not know the consequences of such an action. One of the processes could be terminated, but again the consequences will probably be unknown. What if the deadlocked process had just withdrawn $1000 from a savings account, and was now blocked as it attempted to deposit that money in a checking account One wouldn t want to terminate the process; nor would one want to restart it. An exception might be if one of the processes were a compiler or some other process whose computation could be restarted from the beginning. Since such a process could be safely restarted, it could be terminated in order to free resources. Such action might or might not, depending on the circumstances, remove the deadlock. Most data-base systems have the ability to roll back transactions that fail, and a data-base management system may provide for rollback in the face of deadlocked access to data-base tables and rows. In fact, modern data-base management systems are the most likely application area to find implementations of deadlock detection and recovery. In the end, it may be best for the system to simply announce that a deadlock has occurred and let the operator take some action the operator thinks appropriate. Deadlock detection is potentially expensive of computing time, especially if run frequently, and the question of what action to take when a deadlock occurs is usually too difficult to automate. As a result, general-purpose operating systems almost never try to manage deadlock. On the other hand, data-base management systems often do try to detect deadlocks and recover from them within the narrower purview of data-base management. SCHEDULING One of the very most important tasks of an operating system is to schedule processes and threads for execution. In the early days of computing, the programmer would load the program into memory somehow (front panel switches, paper tape, etc.), and then press the start button. Very soon the task of starting, interrupting, and restarting processes became the bailiwick of the OS. The question of which task to schedule for execution at any moment has been the subject of much research and considerable design effort. The answer depends in part on the type of operating system. If the system is a batch system, the scheduling plan should optimize throughput (the number of jobs completed per hour), CPU utilization, and/or turnaround time (the average elapsed time between the submission of a job and its completion). If the system is an interactive system, the scheduler should provide responsiveness and the impression of fairness to all users. Interactive systems often have less well-defined boundaries of jobs, so the criteria mentioned for batch systems are not as relevant. A more important criterion is response time to the user, the time delay between input from the user and some response back from the process. If the system is a real-time system, with important constraints on execution time, the scheduler will be judged by the consistency with which applications meet their deadlines, and the predictability of performance. Some real-time systems are hard real-time systems, meaning that important absolute limits bound performance requirements (imagine a system that coordinates process equipment in a chemical plant). Other real-time systems are soft real-time systems, meaning the highest priority processes should execute as fast as possible, but absolute limits on processing time do not exist. An important way in which schedulers differ is in whether or not the scheduler is preemptive. A preemptive scheduler can interrupt an executing process and switch to a different one. Such a change is called a context switch, and entails a substantial amount of system overhead in changing the contents of registers, memory maps, and cache memory. First come first served (FCFS) Early batch systems used a simple FCFS (FCFS also called FIFO, for first in first out) scheduling system. With FCFS, the first process to the system is scheduled, and it runs to completion or until it is blocked
CHAP. 6]
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