Otherwise, we must request all the resources at the beginning of both protocols. In the above-given example, for instance, we can release the DVD drive and disk file and again request the disk file and printer only if we can be sure that our data will remain on the disk file. Utilization of resources may be low, since resources may be allocated but unused for a long period. After copying the disk file to the printer, the process releases these two resources as well and then terminates. The process must then again request the disk file and printer. It copies the data from the DVD drive to the disk and then releases both the DVD drive and the disk file. While the second method allows the process to request initially only the DVD drive and disk file. It will hold the printer during its entire execution, even though the printer is needed only at the end. If all the resources must be requested at the beginning of the process according to the first protocol, then the process requests the DVD drive, disk file, and printer initially. We will consider a process that mainly copies data from a DVD drive to a file on disk, sorts the file, and then prints the results to a printer. Let us illustrate the difference between these two protocols: The second protocol allows a process to request resources only when it does not occupy any resource. There are some protocols that can be used in order to ensure that the Hold and Wait condition never occurs:Īccording to the first protocol Each process must request and gets all its resources before the begining of its execution.
Thus if we did not want the occurrence of this condition then we must guarantee that when a process requests a resource, it does not hold any other resource. Hold and wait condition occurs when a process holds a resource and is also waiting for some other resource in order to complete its execution. Generally, deadlocks cannot be prevented by denying the mutual exclusion condition because there are some resources that are intrinsically non-sharable. A good example of a sharable resource is Read-only files because if several processes attempt to open a read-only file at the same time, then they can be granted simultaneous access to the file.Ī process need not to wait for the sharable resource. In contrast, Sharable resources do not require mutually exclusive access and thus cannot be involved in a deadlock. For example, a printer cannot be simultaneously shared by several processes. This condition must hold for non-sharable resources. We will elaborate deadlock prevention approach by examining each of the four necessary conditions separately. The same is the situation with the deadlock if we become able to violate any condition among the four and do not let them occur together then there can be prevented from the deadlock problem. If anyone leg of the chair gets broken, then definitely it will fall. Likewise, for the deadlock problem, all the above given four conditions are needed.
Let us take an example of a chair, as we know that chair always stands on its four legs. So, in that situation, we use the Banker’s algorithm to determine the deadlock.In this tutorial, we will elaborate on the deadlock prevention approach.Īs we are already familiar with all the necessary conditions for a deadlock. There will be the possibility of forming the circle, but it may lead to a deadlock.
If there is more than one instance of a resource, it’ll not be certain in determining the deadlock. There is a limitation with the RAG algorithm that it’ll only work if all the resources have a single instance. Mostly, we represent vertices with a rectangular shape and edges with a circular shape: RAG has two vertices: process vertex and resource vertex, and two edges: assignment edge and request edge. Thus, we can easily avoid the deadlock.Īs every graph has vertices and edges, in the same way, RAG also has vertices and edges. We can figure out how many resources are allocated to each process and how many resources will be needed in the future. The resource allocation graph is the pictorial view of all allocated resources, available resources, and OS’s current state. Using RAG, it’s possible to predict the occurrence of deadlock in an OS.
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