🧩 Topic: Process Concepts

🧠 What is a Process?

A process is a program in execution.
It is an active entity, unlike a program (which is just a passive code file).
A process includes:
Program code
Current activity (Program Counter)
Data, stack, heap, and registers

📦 Process Components

Component	Description
Text Section	The program code (instructions)
Data Section	Global/static variables
Heap	Dynamic memory allocation (e.g., malloc)
Stack	Function calls, return addresses, local variables
Program Counter	Shows the address of the next instruction
Registers	Hold intermediate data during execution

🔄 Process vs Program

Aspect	Program	Process
Nature	Passive (code on disk)	Active (code in execution)
Stored In	Secondary memory (disk)	Main memory (RAM)
Lifespan	Exists until deleted	Exists during execution

🔁 Process States

State	Description
New	Process is being created
Ready	Process is waiting to be assigned to CPU
Running	Process is currently being executed by CPU
Waiting/Blocked	Process is waiting for an event (e.g., I/O)
Terminated	Process has finished execution

👉 OS uses a state transition diagram to manage these changes.

🔁 Process Control Block (PCB)

A data structure used by the OS to store information about a process.

Field	Description
Process ID (PID)	Unique identifier for the process
Process State	Current state (Running, Waiting, etc.)
Program Counter	Next instruction to execute
CPU Registers	Contents of registers when switched out
Memory Info	Info about memory assigned
I/O Info	List of open files/devices

👨‍👩‍👧‍👦 Types of Processes

Type	Description
User Process	Initiated by a user (e.g., running a program)
System Process	Created and managed by OS (e.g., background services)

👯 Process Creation

A process can create child processes using system calls like fork() (UNIX/Linux).

Parent and child processes can:

Execute concurrently
Share resources or have separate memory

🧩 Process Termination

A process ends by calling exit()
Can be:
Normal termination (completed successfully)
Abnormal termination (error, killed by parent, failed I/O, etc.)

📌 Summary

A process is a program in execution with code, data, and system resources.
It moves through states (New, Ready, Running, Waiting, Terminated).
Managed by the OS through PCB.
Understanding processes is essential for multitasking, scheduling, and resource management.

🧩 Topic: Concurrent Processes

🧠 What are Concurrent Processes?

Concurrent processes are two or more processes that are executed during overlapping time periods.
They may run:
Truly simultaneously on multi-core systems
Sequentially interleaved on a single-core CPU, but appear to run in parallel

🔄 Why Concurrency is Important

Allows multiple tasks to be active at the same time
Increases CPU utilization and system efficiency
Enables responsive systems (e.g., web servers, OS multitasking)

🧩 Examples of Concurrent Processes

Example	Description
Running a music player while browsing	Both applications are active concurrently
Downloading a file while editing text	One process uses I/O, the other uses CPU
OS handling multiple users	User processes handled concurrently by the OS

🔁 Types of Concurrency

Type	Description
Process-based	Multiple independent programs running simultaneously
Thread-based	Multiple threads within the same process sharing memory

⚙️ Challenges in Concurrency

Problem	Description
Race Condition	Two or more processes access shared data simultaneously, leading to inconsistent results
Critical Section Problem	Section of code that accesses shared data must not be executed by more than one process at a time
Deadlock	Two or more processes wait for resources held by each other, blocking all
Starvation	A process waits indefinitely for a resource

🔐 OS Responsibilities in Concurrency

Process synchronization using:
Semaphores
Mutexes
Monitors
Scheduling to ensure fair CPU access
Deadlock detection and prevention

📌 Summary

Concurrent processes run during overlapping times to improve system performance.
May share resources and need synchronization to avoid errors.
The OS uses scheduling and synchronization mechanisms to manage concurrency safely and efficiently.

🧩 Topic: Scheduling Concepts

🧠 What is Scheduling in OS?

Scheduling is the process of deciding which process runs next on the CPU when there are multiple ready processes.
The component that makes this decision is called the CPU scheduler.

🎯 Goals of CPU Scheduling

Maximize CPU utilization
Maximize throughput (number of processes completed in a time unit)
Minimize waiting time, turnaround time, and response time
Ensure fairness among processes

🧩 Types of Schedulers

Scheduler Type	Description
Long-Term Scheduler	Selects which processes are admitted into the ready queue (job scheduler)
Short-Term Scheduler	Selects which ready process gets the CPU next (CPU scheduler)
Medium-Term Scheduler	Temporarily removes and swaps processes to control memory load (swapper)

🔁 Process Scheduling Queues

Queue	Description
Job Queue	Contains all processes in the system
Ready Queue	Processes waiting to be assigned to CPU
Waiting (I/O) Queue	Processes waiting for I/O devices

🔄 Dispatcher

The dispatcher is a module that gives CPU control to the selected process.
Responsibilities:
Switching context
Switching to user mode
Jumping to the process's program counter

🕐 Scheduling Criteria

Criterion	Meaning
CPU Utilization	Keep CPU busy as much as possible
Throughput	Number of processes completed per unit time
Turnaround Time	Time from submission to completion of a process
Waiting Time	Total time a process spends in the ready queue
Response Time	Time from submission to the first response (especially in interactive systems)
Fairness	All processes get equal share of CPU

🧠 Preemptive vs Non-Preemptive Scheduling

Type	Description
Preemptive	CPU can be taken away from a running process (e.g., time slicing)
Non-Preemptive	Once a process gets the CPU, it runs until it finishes or blocks

📌 Summary

CPU Scheduling determines the order in which processes are executed.
It aims to improve performance by reducing waiting time, increasing throughput, and ensuring fairness.
The OS uses schedulers and queues to manage the execution of multiple processes.

🧩 Topic: CPU Scheduling

🧠 What is CPU Scheduling?

CPU Scheduling is the process of selecting a process from the ready queue and assigning it to the CPU for execution.
It's a core function of the short-term scheduler (also called CPU scheduler).

🎯 Objectives of CPU Scheduling

Maximize CPU utilization
Minimize waiting time and turnaround time
Increase system throughput
Ensure fairness and responsiveness (especially in interactive systems)

🧩 Key CPU Scheduling Algorithms

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

Non-preemptive
Processes are executed in the order they arrive

✅ Simple to implement ❌ May cause long waiting time (convoy effect)

2. Shortest Job First (SJF)

Non-preemptive or Preemptive (also called SRTF)
CPU is given to the process with the shortest burst time

✅ Minimum average waiting time ❌ Requires knowledge of burst time in advance ❌ May cause starvation

3. Priority Scheduling

Each process is assigned a priority number
CPU is allocated to the highest priority process

✅ Suitable for critical tasks ❌ Low-priority processes may starve

👉 Aging can be used to prevent starvation (gradually increase priority over time)

4. Round Robin (RR)

Preemptive
Each process gets a fixed time quantum
After the time expires, process is moved to the end of the ready queue

✅ Good for time-sharing systems ❌ Performance depends on time quantum size

5. Multilevel Queue Scheduling

Ready queue is divided into multiple queues, each for different types of processes
Each queue may have its own scheduling algorithm

✅ Separates processes based on priority/type ❌ Rigid – No movement between queues unless combined with aging

6. Multilevel Feedback Queue

Processes can move between queues based on behavior
Interactive processes get higher priority

✅ Flexible and adaptive ✅ Good for general-purpose systems

🧪 Common Scheduling Criteria

Metric	Description
CPU Utilization	% of time CPU is busy
Throughput	# of processes completed per unit time
Waiting Time	Time spent in ready queue
Turnaround Time	Time from submission to completion
Response Time	Time from submission to first CPU response
Fairness	Equal chance for all processes

📊 Comparison Table of Scheduling Algorithms

Algorithm	Preemptive	Starvation	Good For
FCFS	✘	✘	Simple batch systems
SJF	Optional	✔	Short tasks, minimum wait
Priority	Optional	✔	Time-critical applications
RR	✔	✘	Interactive users
MLQ	Optional	✔	System type separation
MLFQ	✔	✘ (uses aging)	General OS with mixed load

📌 Summary

CPU Scheduling improves performance by efficiently allocating CPU to processes.
Several algorithms exist, each suited to different system goals.
Important to balance between efficiency, fairness, and responsiveness.

🧩 Topic: Scheduling Algorithms

🧠 What are Scheduling Algorithms?

Scheduling algorithms determine how the CPU selects processes from the ready queue for execution. Each algorithm has its own way of improving CPU utilization, response time, turnaround time, and fairness.

📊 Major CPU Scheduling Algorithms

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

Non-preemptive
Executes processes in the order they arrive

📌 Characteristics:

Simple and easy to implement
Can cause convoy effect (slow process blocks faster ones)

🧮 Example:

Processes:  P1  P2  P3  
Arrival:    0   1   2  
Burst:      5   3   1  

Gantt Chart: | P1 | P2 | P3 |

2. Shortest Job First (SJF)

Selects the process with the shortest CPU burst
Can be:
Non-preemptive (waits till current job finishes)
Preemptive (Shortest Remaining Time First – SRTF)

📌 Pros:

Best average waiting time ❌ Needs exact burst time in advance ❌ Starvation for long jobs

3. Round Robin (RR)

Preemptive
Each process gets a fixed time quantum (e.g., 2ms)
After quantum, process goes to back of ready queue

📌 Pros:

Fair for all processes
Good for time-sharing systems ❌ Too small quantum → high context switching ❌ Too large quantum → like FCFS

🧮 Example:

Processes: P1(4), P2(3), P3(5)  
Quantum: 2  
Gantt: | P1 | P2 | P3 | P1 | P2 | P3 | P3 |

4. Priority Scheduling

Each process is assigned a priority number
CPU is given to the highest-priority process

🟡 Can be:

Preemptive (interrupts running lower-priority process)
Non-preemptive

📌 Pros:

Useful for time-critical tasks ❌ Starvation possible for low-priority processes ✔️ Aging can be used to avoid starvation

5. Multilevel Queue (MLQ)

Processes are divided into multiple queues (e.g., system, interactive, batch)
Each queue has its own scheduling algorithm

📌 Rigid – no movement between queues

Example: OS uses FCFS for one queue, RR for another

6. Multilevel Feedback Queue (MLFQ)

Like MLQ, but processes can move between queues
Prioritizes interactive tasks with short bursts

📌 Pros:

Adaptive, prevents starvation
Complex but effective for mixed systems (real-world OS)

📈 Performance Metrics (Used to Compare Algorithms)

Metric	Description
CPU Utilization	% of time CPU is active
Throughput	# of processes completed per unit time
Turnaround Time	Total time from submission to completion
Waiting Time	Time spent in ready queue
Response Time	Time from submission to first response

📌 Summary Table of Scheduling Algorithms

Algorithm	Preemptive	Starvation	Ideal For
FCFS	✘	✘	Simple, batch jobs
SJF	✘/✔	✔	Short tasks
RR	✔	✘	Time-sharing
Priority	✘/✔	✔	Critical processes
MLQ	✘/✔	✔	Grouped scheduling
MLFQ	✔	✘	General OS (adaptive)

🧮 Bonus: Formulas to Remember

Turnaround Time = Completion Time – Arrival Time
Waiting Time = Turnaround Time – Burst Time
Response Time = First Execution Time – Arrival Time

🧩 Topic: Multiple Processor Scheduling

🧠 What is Multiple Processor Scheduling?

It refers to CPU scheduling in systems with more than one processor (also called multiprocessor systems).
The goal is to efficiently use all processors while ensuring fairness, responsiveness, and load balancing.

⚙️ Types of Multiprocessor Systems

Symmetric Multiprocessing (SMP)
All processors are identical and run the same OS.
Each processor may schedule its own tasks.
Most modern OS (like Linux, Windows) use SMP.
Asymmetric Multiprocessing (AMP)
One processor is master; others are slaves.
Master handles all scheduling and task assignment.
Simpler, but less efficient than SMP.

📊 Key Concepts in Multiple Processor Scheduling

Concept	Description
Processor Affinity	A process prefers to run on the same CPU it used before (caches stay warm)
Load Balancing	Distribute processes evenly across all CPUs
Scheduling Algorithms	Same algorithms (like RR, Priority) can be applied on multiprocessor setup

🔁 Types of Scheduling Approaches

Approach	Description
Common Ready Queue	One global ready queue for all processors – any CPU can pick a process
Separate Ready Queues	Each processor has its own queue – faster context switching
Dynamic Scheduling	Tasks are reassigned during runtime to balance load

🎯 Goals of Multiprocessor Scheduling

Maximize CPU utilization
Balance workload across processors
Minimize process migration (unless needed)
Improve scalability and responsiveness

🧩 Processor Affinity Types

Type	Meaning
Soft Affinity	OS tries to keep a process on the same CPU but may move if needed
Hard Affinity	Process is strictly bound to a specific CPU

📌 Summary

Multiple processor scheduling manages how processes are assigned to multiple CPUs.
Uses concepts like SMP/AMP, affinity, and load balancing.
Aims for better performance, efficiency, and parallelism in modern multicore systems.