operating system
- 2. DEADLOCK The deadlocks problem System model Deadlock characterization Methods for handling deadlocks Deadlock prevention Deadlock avoidance Deadlock detection Recovery from deadlock
- 3. INTRODUCTION TO DEADLOCK Every process needs some resources to complete its execution. However, the resource is granted in a sequential order. The process requests for some resource. OS grant the resource if it is available otherwise let the process waits. The process uses it and release on the completion.
- 4. A Deadlock is a situation where each of the computer process waits for a resource which is being assigned to some another process. In this situation, none of the process gets executed since the resource it needs, is held by some other process which is also waiting for some other resource to be released.
- 6. METHODS FOR HANDLING DEADLOCKS Generally speaking, we can deal with the deadlock problem in one of three ways: • We can use a protocol to prevent or avoid deadlocks, ensuring that the system will never enter a deadlocked state. • We can allow the system to enter a deadlocked state, detect it, and recover. • We can ignore the problem altogether and pretend that deadlocks never occur in the system.
- 7. The third solution is the one used by most operating systems, including Linux and Windows. It is then up to the application developer to write programs that handle deadlocks. Next, we elaborate briefly on each of the three methods for handling deadlocks. Then, in Sections .we present detailed algorithms.
- 8. Before proceeding, we should mention that some researchers have argued that none of the basic approaches alone is appropriate for the entire spectrum of resource-allocation problems in operating systems. The basic approaches can be combined, however, allowing us to select an optimal approach for each class of resources in a system.
- 9. To ensure that deadlocks never occur, the system can use either a deadlockprevention or a deadlock-avoidance scheme. Deadlock prevention provides a set of methods to ensure that at least one of the necessary conditions (Section 7.2.1) cannot hold. These methods prevent deadlocks by constraining how requests for resources can be made. We discuss these methods in Section.
- 10. DEADLOCK AVOIDANCE Deadlock avoidance requires that the operating system be given additional information in advance concerning which resources a process will request and use during its lifetime. With this additional knowledge, the operating system can decide for each request whether or not the process should wait. To decide whether the current request can be satisfied or must be delayed, the system must consider the resources currently available, the resources currently allocated to each process, and the future requests and releases of each process. We discuss these schemes in Section.
- 11. If a system does not employ either a deadlock-prevention or a deadlockavoidance algorithm, then a deadlock situation may arise. In this environment, the system can provide an algorithm that examines the state of the system to determine whether a deadlock has occurred and an algorithm to recover from the deadlock (if a deadlock has indeed occurred). We discuss these issues in Section
- 12. In the absence of algorithms to detect and recover from deadlocks, we may arrive at a situation in which the system is in a deadlocked state yet has no way of recognizing what has happened. In this case, the undetected deadlock will cause the system’s performance to deteriorate, because resources are being held by processes that cannot run and because more and more processes, as they make requests for resources, will enter a deadlocked state. Eventually, the system will stop functioning and will need to be restarted manually.
- 13. Although this method may not seem to be a viable approach to the deadlock problem, it is nevertheless used in most operating systems, as mentioned earlier. Expense is one important consideration. Ignoring the possibility of deadlocks is cheaper than the other approaches. Since in many systems, deadlocks occur infrequently (say, once per year), the extra expense of the be put to use to recover from deadlock. In some circumstances, a system is in a frozen state but not in a deadlocked state. other methods may not seem worthwhile. In addition, methods used to recover from other conditions may
- 14. We see this situation, for example, with a real-time process running at the highest priority (or any process running on a nonpreemptive scheduler) and never returning control to the operating system. The system must have manual recovery methods for such conditions and may simply use those techniques for deadlock recovery.