Precedence Graph in Operating System

Operating systems utilize a data structure called a precedence graph to show the interdependencies between various tasks or processes. Another name for it is a Task Dependency Graph. Several processes may be running at once in a multi-tasking operating system, and some of these processes may wait for others to finish before they can start executing. These dependencies are represented by a Precedence graph, which is a directed graph with each node being a process or task and edges denoting dependencies between tasks. In the precedence graph, each node's label indicates which process or task it corresponds to, and each edge's label indicates the kind of dependence that exists between tasks.

Take into account the following list of project-related tasks −

• Create the interface.

• Create the database's code.

• Create the front-end code.

• Create the back-end code.

• Check the system as a whole.

We may create a Precedence Graph to show how these jobs are interdependent. Each job will be represented as a node in this graph, and dependencies will be shown as edges connecting the nodes.

Precedence Graph

Since the user interface design must be finished before implementing the database code, job A in the graph is a requirement for task B. Similar to job B, tasks C and D are dependent on it since before either the front-end or back-end code can be written, the database needs to be set up. Last but not least, task E depends on every other task since it tests a fully working system.

The graph's edges include labels to show what type of dependency there is between the jobs. For instance, the edge from A to B may be labelled "Design UI," which denotes that job A is the creation of the user interface and that task B, which is the writing of code, is dependent upon task A.

There are two typical forms of dependencies −

When one process must complete before another can start, this is referred to as control dependence. When one task needs the results of another task in order to begin, data dependence occurs. Scheduling algorithms employ precedence graphs to decide the sequence in which activities should be finished, assuring effectiveness and prompt completion.

Functionalities of a Precedence Graph

An operating system's precedence graph can perform a number of functions, some of which are given below −

Representation of Task Dependency

Precedence graphs are frequently used in multi-tasking operating systems to show the relationships between tasks or processes. The graph gives a visual depiction of the relationships between tasks as well as the sequence in which they must be completed.

Task scheduling

To decide the order in which tasks should be completed, task scheduling algorithms also employ precedence graphs. The graph enables the scheduler to optimize task execution and boost system performance by revealing which jobs may be completed concurrently and which ones must wait for others to finish.

The code underneath illustrates how to create an elementary Precedence graph and use it to prioritize tasks within the queue −

from queue import PriorityQueue
graph = {
   'A': set([]),
   'B': set(['A']),
   'C': set(['A']),
   'D': set(['B', 'C']),
   'E': set(['D'])
}
queue = PriorityQueue()
queue.put(('A', 0))
while not queue.empty():
   task, priority = queue.get()
   print(“ Undergoing Task", task)
   for dependent_task in graph[task]:
      queue.put((dependent_task, priority+1))

Output

Undergoing Task A
Undergoing Task B
Undergoing Task C
Undergoing Task D
Undergoing Task E

The programme creates a graph containing nodes and edges and then does a breadth-first search of the graph beginning at node 'A' using a priority queue. Every node that is visited, in the order that they are visited, is printed out. The nodes are visited in this scenario in the following order: A, B, C, D, and E.

Detection of Deadlocks

Precedence graphs can be employed to identify deadlocks in a system. When two or more tasks become dependent on one another in a cyclic manner, a stalemate results. A stalemate can be found and broken by removing one or more circular dependencies by examining the Precedence graph.

def detect_deadlock(graph):
   for task in graph.keys():
      visited = set([])
      if dfs(graph, task, visited):
         return True
   return False
def dfs(graph, task, visited):
   visited.add(task)
   for dependent_task in graph[task]:
      if dependent_task in visited or dfs(graph, dependent_task, visited):
         return True
   visited.remove(task)
   return False
if detect_deadlock(graph):
   print("Deadlock has been detected!")
else:
   print("No deadlock cannot be detected.")

No deadlock cannot be detected.

Allocating resources

Priority graphs can be employed to distribute system resources including memory, CPU time, and I/O devices. The system may analyze the graph to identify which resources are needed by each job and then distribute them accordingly, ensuring that each task has the resources it needs to finish.

Analysis of Performance

Precedence graphs may be used for performance analysis by spotting bottlenecks and inefficient regions of the system. The system can make improvements to its performance by analyzing the graph to discover tasks that are taking longer than planned or using more resources than necessary.

Future Prospects of Precedence Graphs

Operating systems have long utilized precedence graphs to depict task interdependence and enhance resource allocation and scheduling. However, as technology continues to advance, it is anticipated that the future application of Precedence graphs in operating systems will increase. Precedence graphs in operating systems may be used in the following scenarios in the future −

Big Data Processing

Precedence graphs are anticipated to be used increasingly frequently in big data processing systems as the volume of data collected keeps increasing exponentially. The relationships between jobs in a data processing pipeline may be represented using precedence graphs, enabling effective scheduling and resource allocation.

Real-time Systems

Real-time systems, such as those used in robotics and autonomous cars, call for exact resource allocation and scheduling. A real-time system's mutual dependence between operations may be represented using precedence graphs, enabling scheduling and resource allocation more effective.

Machine learning

Algorithms that use machine learning frequently assess huge datasets and demand a lot of computer power. The relationships between jobs in a machine learning pipeline may be represented using precedence graphs, enabling effective scheduling and resource allocation.

Distributed systems

Appropriate resource distribution and job scheduling across several nodes is essential for distributed systems, such as those utilized in cloud computing. The interdependence between jobs in a distributed system may be depicted using precedence graphs, enabling effective scheduling and resource distribution across several nodes.

Fault Tolerance

By identifying key activities and ensuring that those foresaid activities are planned and equipped with a proper fund appropriately, precedence graphs may be used to raise the fault tolerance of OS. In the case of a failure, the Precedence graph may be utilized to take the vital decision of which tasks need to be rescheduled or resumed.

Precedence graphs are hence, an effective tool for displaying the task dependencies in operating systems, and as these systems continue on improving and new applications keep on getting created, their potential will undoubtedly grow.

Summary

In a nutshell, Operating systems utilize precedence graphs to show how tasks in a system are interdependent. These networks, which depict the interactions between tasks as directed acyclic graphs, include nodes that each represent a task and edges between nodes that reflect dependencies between tasks. Due to their ability to let the operating system decide which actions should be executed in what order based on their dependencies, precedence graphs are helpful for task scheduling and resource allocation. By identifying key activities and ensuring that they are planned and resourced appropriately, they are also helpful for enhancing the fault tolerance of operating systems. A number of applications, including huge data processing, real-time systems, etc., can employ precedence graphs since they are simple to see and comprehend.