PROCESS SCHEDULING AND COORDINATION

CPU Scheduling

is the sequence in which processes are executed as they are allotted the processor.

short-term scheduler

The scheduler responsible for this task is called.

CPU Utilization

is the ratio of the total CPU burst time to the total time spent processing all jobs in the ready queue. It is the percentage of time the processor is busy.

Throughput

is the number of processes that have completed their execution per time unit. It measures how much work is being performed in a given time.

Turn-around Time

- is the amount of time to execute particular processes. It is the time from when the process was submitted to its completion.

Waiting time

is the amount of time a process has been waiting in the ready queue before it is allotted to the processor.

Response time

is the amount of time it takes from when a request was submitted until the first response is produced.

Balanced Utilization

evaluates the percentage utilization of not only the CPU but also the memory, I/O devices, and other system resources.

Predictability

refers to consistency in response time.

Fairness

in scheduling refers to the equal opportunity given to all processes to execute such that no one is allowed to suffer from starvation.

Preferential Treatment

to processes with higher priorities may also be considered as a criterion.

Non-preemptive

- means that once a process is running, it continues to execute until it terminates.

Preemptive

means that currently running processes may be interrupted and moved to the ready state by the operating system, even if the total burst time (the allotted time to execute) has not been consumed.

First Come First Serve (FCFS)

Also known as First In, First Out (FIFO), this allocation method assigns the CPU based on time of arrival. It is one of the simplest and most commonly used CPU scheduling algorithms.

Convoy Effect

in which a long process at the front of the queue forces all shorter processes behind it to wait, reducing overall system performance.

Shortest Job First (SJF)/Shortest Job Next (SJN)

allocates the processor based on the shortest CPU burst. The ready queue is treated as a priority queue based on the smallest time requirement.

Non-Preemptive Priority (NPP) -

Its processing order is based on assigned priority: the highest-priority job is processed first, and incoming jobs in the ready queue are considered candidates.

Starvation

in which lower-priority processes may wait a long time if higher-priority processes continue to arrive.

Shortest Remaining Time First (SRTF)/ Shortest Time to Completion (STFC)

allocates the processor based on the shortest remaining burst and preempts if a shorter burst is found in the ready queue.

Round Robin (RR)

Allocates the processor based on a given time slice (quantum). This also uses a circular queue for the ready queue, wherein arriving jobs or preempted jobs are placed at the tail of the queue.

Time Slice

or quantum, defines the amount of time that a job is allocated to the processor.

Process Coordination

The operating system's method of organizing and managing concurrent processes so they can execute efficiently, communicate effectively, and share resources safely.

Process Synchronization

the mechanism used by an operating system to coordinate the execution of concurrent processes so that they access shared resources in an orderly and controlled manner.

Semaphores

A signaling mechanism that controls access to shared resources using an integer counter.

Mutex Locks

A synchronization mechanism that allows only one process or thread to access a critical section or shared resource at any given time.

Mutual Exclusion

A synchronization principle that allows only one (1) process at a time to access a shared resource or execute a critical section

Critical section

is the portion of a program where shared variables, files, or hardware devices are accessed or modified.

Interprocess Communication (IPC)

The methods that allow two or more processes to exchange information and coordinate their activities.

Shared Memory

Processes communicate by accessing the same memory region.

Pipes

Provide one-way communication between related processes.

Named Pipes

Unlike ordinary pipes, ___ allow communication between unrelated processes.

Message Queues

Processes exchange messages through queues managed by the operating system.

Signals

Notify a process that a particular event has occurred.

Sockets

Enable communication between processes running on different computers through a network.

Resource Sharing

The controlled allocation and utilization of system resources among multiple processes.

Deadlock

occurs when two or more processes wait indefinitely for resources held by one another, preventing any process from continuing.

Deadlock Prevention: The operating system prevents one of the necessary conditions for deadlock from occurring.

Deadlock Avoidance

The operating system examines each resource request and grants it only if doing so keeps the system in a safe state

Deadlock Detection

Instead of preventing deadlocks, the operating system periodically checks whether one has occurred. If detected, corrective action is taken.

Deadlock Recovery

Once a deadlock is detected, the operating system attempts to eliminate it

Reclaiming allocated resources

Process A is waiting for a printer that Process B holds. The operating system forcibly reclaims the printer from Process B and assigns it to Process A, breaking the deadlock.

Rolling back affected processes

The operating system restores Process B to a previously saved checkpoint before it acquired the scanner. The scanner is released, allowing Process A to complete. Once the deadlock is resolved, Process B restarts from the saved checkpoint