Processor Management

1. Process Overview

A process is a program in execution. It goes through various states during its lifecycle in an operating system. Processes require resources like CPU time, memory, files, and I/O devices to execute.

2. Process States

A process moves through several states during its lifetime:

1. New: The process is being created.
2. Ready: The process is waiting to be assigned to a CPU.
3. Running: Instructions are being executed by the CPU.
4. Waiting (Blocked): The process is waiting for an event (like I/O) to occur.
5. Terminated: The process has finished execution.

3. Process State Transitions

Processes change states depending on various events like the completion of an I/O request or the allocation of a CPU.

4. Process Control Block (PCB)

The Process Control Block (PCB) is a data structure maintained by the operating system to keep track of process information. It includes:

• Process ID: Unique identifier for the process.
• Process State: Current state of the process.
• Program Counter: Address of the next instruction to execute.
• CPU Registers: Holds the current working registers.
• Memory Limits: Information about the allocated memory.
• List of Open Files: Keeps track of files in use.

5. Operations on Processes

1. Process Creation: A new process is created using system calls like fork() or exec().
2. Process Execution: The operating system schedules the process to run on the CPU.
3. Process Termination: A process completes and is removed from the system.
4. Suspend and Resume: The operating system can suspend (temporarily halt) or resume a process.

6. Interrupt Processing

An interrupt is a signal that temporarily halts the CPU's current activities to execute a high-priority task (like I/O completion).

1. Hardware Interrupt: Generated by hardware devices (e.g., keyboard).
2. Software Interrupt: Generated by executing a system call or an error (e.g., divide by zero).

7. Scheduling Algorithms

Scheduling algorithms decide the order in which processes access the CPU. Common algorithms include:

1. First-Come, First-Served (FCFS)

Processes are executed in the order they arrive.

Example:

2. Shortest Job First (SJF)

The process with the shortest burst time is executed first.

Example:

3. Round Robin (RR)

Processes are assigned a fixed time slice or quantum and rotate in a cyclic manner.

Example: For time quantum = 2s

4. Priority Scheduling

Processes are executed based on their priority. The process with the highest priority is scheduled first.

Example:

8. Multiple Processor Scheduling

In systems with more than one processor, scheduling algorithms need to ensure optimal CPU utilization. Some strategies include load balancing, where processes are evenly distributed across all processors to prevent any single processor from being overwhelmed.

Deadlock

1. Deadlock Problem

A deadlock occurs when two or more processes are unable to proceed because each is waiting for the other to release resources.

2. Deadlock Characterization

Four necessary conditions must be met for a deadlock to occur:

1. Mutual Exclusion: At least one resource must be held in a non-shareable mode.
2. Hold and Wait: A process holding at least one resource is waiting to acquire additional resources held by other processes.
3. No Preemption: Resources cannot be forcibly taken from a process.
4. Circular Wait: A circular chain of processes exists where each process is waiting for a resource held by the next process.

3. Deadlock Prevention

The system can prevent deadlocks by ensuring that at least one of the four necessary conditions does not hold.

Example: Eliminate the Hold and Wait condition by requiring processes to request all resources at once before execution.

4. Deadlock Avoidance

Banker’s Algorithm

This algorithm ensures that the system never enters an unsafe state by granting a resource only if it leaves the system in a safe state.

Example:

• If a process requests resources that exceed the system's capacity to fulfill the request without going into deadlock, the request is denied.

5. Deadlock Detection

Deadlock detection algorithms run periodically to check if a deadlock has occurred by examining resource allocation and the wait-for graphs.

6. Recovery From Deadlock

Once deadlock is detected, the system can recover by:

1. Preempting Resources: Taking resources away from some processes.
2. Killing Processes: Terminating one or more processes to break the deadlock.

Deadlock Avoidance Diagram

This diagram represents a circular wait condition in a deadlock scenario.