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Deadlocks
In a multiprogramming environment, several processes may compete for a finite number of resources. A process requests resources; if the resources are not available at that time, the process enters a waiting state. Sometimes, a waiting process is never again able to change state, because the resources it has requested are held by other waiting processes. This situation is called a deadlock.
System Model
? A system consists of a finite number of resources to be distributed among a number of competing processes. The resources are partitioned into several types, each consisting of some number of identical instances. Memory space, CPU cycles, files, and I/O devices (such as printers and DVD drives) are examples of resource types.
? If a system has two CPUs, then the resource type CPU has two instances.
? A process must request a resource before using it and must release the resource after using it.
? A process may request as many resources as it requires carrying out its designated task.
? Obviously, the number of resources requested may not exceed the total number of resources available in the system. In other words, a process cannot request three printers if the system has only two.
A process may utilize a resource in only the following sequence:
1. Request. The process requests the resource. If the request cannot be granted immediately (for example, if the resource is being used by another process), then the requesting process must wait until it can acquire the resource.
2. Use. The process can operate on the resource (for example, if the resource is a printer, the process can print on the printer).
Deadlocks
In a multiprogramming environment, several processes may compete for a finite number of resources. A process requests resources; if the resources are not available at that time, the process enters a waiting state. Sometimes, a waiting process is never again able to change state, because the resources it has requested are held by other waiting processes. This situation is called a deadlock.
System Model
? A system consists of a finite number of resources to be distributed among a number of competing processes. The resources are partitioned into several types, each consisting of some number of identical instances. Memory space, CPU cycles, files, and I/O devices (such as printers and DVD drives) are examples of resource types.
? If a system has two CPUs, then the resource type CPU has two instances.
? A process must request a resource before using it and must release the resource after using it.
? A process may request as many resources as it requires carrying out its designated task.
? Obviously, the number of resources requested may not exceed the total number of resources available in the system. In other words, a process cannot request three printers if the system has only two.
A process may utilize a resource in only the following sequence:
1. Request. The process requests the resource. If the request cannot be granted immediately (for example, if the resource is being used by another process), then the requesting process must wait until it can acquire the resource.
2. Use. The process can operate on the resource (for example, if the resource is a printer, the process can print on the printer).
3. Release. The process releases the resource. The request and release of resources are system calls
? A system table records whether each resource is free or allocated; for each resource that is allocated, the table also records the process to which it is allocated. If a process requests a resource that is currently allocated to another process, it can be added to a queue of processes waiting for this resource.
? A set of processes is in a deadlocked state when every process in the set is waiting for an event that can be caused only by another process in the set.
? The resources may be either physical resources (for example, printers, tape drives, memory space, and CPU cycles) or logical resources (for example, files, semaphores, and monitors).
? Deadlocks may also involve different resource types. For example, consider a system with one printer and one DVD drive. Suppose that process Pi is holding the DVD and process Pj is holding the printer. If Pi requests the printer and Pj requests the DVD drive, a deadlock occurs.
Necessary Conditions A deadlock situation can arise if the following four conditions hold simultaneously in a system:
1. Mutual exclusion. At least one resource must be held in a nonsharable mode; that is, Only one process at a time can use the resource. If another process requests that resource, the requesting process must be delayed until the resource has been released.
2. Hold and wait. A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes.
3. No preemption. Resources cannot be preempted; that is, a resource can be released only voluntarily by the process holding it, after that process has completed its task.
3. Release. The process releases the resource. The request and release of resources are system calls
? A system table records whether each resource is free or allocated; for each resource that is allocated, the table also records the process to which it is allocated. If a process requests a resource that is currently allocated to another process, it can be added to a queue of processes waiting for this resource.
? A set of processes is in a deadlocked state when every process in the set is waiting for an event that can be caused only by another process in the set.
? The resources may be either physical resources (for example, printers, tape drives, memory space, and CPU cycles) or logical resources (for example, files, semaphores, and monitors).
? Deadlocks may also involve different resource types. For example, consider a system with one printer and one DVD drive. Suppose that process Pi is holding the DVD and process Pj is holding the printer. If Pi requests the printer and Pj requests the DVD drive, a deadlock occurs.
Necessary Conditions A deadlock situation can arise if the following four conditions hold simultaneously in a system:
1. Mutual exclusion. At least one resource must be held in a nonsharable mode; that is, Only one process at a time can use the resource. If another process requests that resource, the requesting process must be delayed until the resource has been released.
2. Hold and wait. A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes.
3. No preemption. Resources cannot be preempted; that is, a resource can be released only voluntarily by the process holding it, after that process has completed its task.