For large database, deadlock prevention method is suitable. A deadlock can be prevented if the resources are allocated in such a way that deadlock never occur. The DBMS analyzes the operations whether they can create deadlock situation or not, If they do, that transaction is never allowed to be executed.

2 Hold and Wait. To prevent this condition processes must be prevented from holding one or more resources while simultaneously waiting for one or more others. There are several possibilities for this: Require that all processes request all resources at one time.

How can we avoid a deadlock in Java?

1. Avoid Nested Locks: A deadlock mainly happens when we give locks to multiple threads. Avoid giving a lock to multiple threads if we already have given to one.
2. Avoid Unnecessary Locks: We can have a lock only those members which are required. Having a lock unnecessarily can lead to a deadlock.
3. Using Thread.

The three basic techniques to control deadlocks are:

• Deadlock preventation . A transaction requesting a new lock is aborted when there is the possibility that a deadlock can occur.
• Deadlock detection. The DBMS periodically tests the database for deadlocks.
• Deadlock avoidance.

For this, we use two methods:

1. (a). Abort all the Deadlocked Processes: Aborting all the processes will certainly break the deadlock, but with a great expenses.
2. (b). Abort one process at a time untill deadlock is eliminated: Abort one deadlocked process at a time, untill deadlock cycle is eliminated from the system.

Resource Allocation Graphs

Four Necessary and Sufficient Conditions for Deadlock

• mutual exclusion. The resources involved must be unshareable; otherwise, the processes would not be prevented from using the resource when necessary.
• hold and wait or partial allocation.
• no pre-emption.
• resource waiting or circular wait.

Once a deadlock is detected, there needs to be a way to recover several alternatives exists: Temporarily prevent resources from deadlocked processes. Back off a process to some check point allowing preemption of a needed resource and restarting the process at the checkpoint later.

Deadlock is a situation where two or more processes are waiting for each other. This is called Deadlock. If a process is in the waiting state and is unable to change its state because the resources required by the process is held by some other waiting process, then the system is said to be in Deadlock.

Distributed deadlock detection Processes request resources from the current holder of that resource. Some processes may wait for resources, which may be held either locally or remotely. Cross-machine arcs make looking for cycles, and hence detecting deadlock, difficult.

Distributed approach – In the distributed approach different nodes work together to detect deadlocks. No single point failure ( that is the whole system is dependent on one node if that node fails the whole system crashes) as the workload is equally divided among all nodes.

When responses are received from all processes, then process can enter its Critical Section. When process exits its critical section, the process sends reply messages to all its deferred requests.

Abstract: Detecting a nonexistent deadlock in distributed systems has been referred to as false deadlock detection. This correspondence shows that false deadlock wi1l never occur in a system of two-phase locking transactions.

Phantom Deadlock is a deadlock that occurs in a Distributed DBMS due to communication delays between different processes and leads to unnecessary process abortions. Such a deadlock is called Phantom Deadlock . …

Discussion Forum

A directory of Objective Type Questions covering all the Computer Science subjects….Discussion Forum.

Definition. Tanenbaum and Woodhull define deadlock as: A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause. If the other process is itself waiting for a resource held by the first then deadlock occurs.

Explanation : If deadlocks occur frequently, then the detection algorithm should be invoked frequently. Resources allocated to deadlocked processes will be idle until the deadlock can be broken. In addition, the number of processes involved in the deadlock cycle may grow.

Deadlock Detection Algorithm helps decide if in scenario of multi instance resources for various processes are in deadlock or not. In cases of single resource instance we can create wait-for graph to check deadlock state.

When processes enter the system, they are put in the job queue. Long-term scheduler selects processes from the job queue and puts them in the ready queue. It is also known as Job Scheduler. 5.

Process Scheduling Queues When the state of a process is changed, its PCB is unlinked from its current queue and moved to its new state queue. Job queue − This queue keeps all the processes in the system. Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute.

Three types of the scheduler are 1) Long term 2) Short term 3) Medium-term.