CS604 Short Notes Subjective by Anie Satti

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LEC WATCHING · Cs604 OPERATING SYSTEM IMP TOPICS · MOST IMP AND MOST REPEATED subjective

CS604 - Operating Systems (Lectures 19 to 45) - Full Concepts with Diagrams

1. Critical Section Problem

A critical section is part of a program where shared resources are accessed. If multiple processes enter their critical sections simultaneously, it may result in data inconsistency.

Requirements for Correct Synchronization:

1. Mutual Exclusion: No two processes are in the critical section at the same time.
2. Progress: If no process is in the critical section, the decision of which process enters next must not be delayed.
3. Bounded Waiting: A limit must exist on the number of times a process is bypassed before entering.

Software Solutions:

Peterson's Algorithm (uses turn and flag variables)

Hardware Solutions:

Test-and-Set Compare-and-Swap

Diagram: Critical Section Cycle

| Entry Section |

v

| Critical Section |

v

| Exit Section |

v

| Remainder Section|

2. Deadlocks

A deadlock is a situation where a group of processes are blocked, each waiting for resources held by others.

4 Necessary Conditions for Deadlock:

1. Mutual Exclusion – At least one resource is held in a non-shareable mode.
2. Hold and Wait – Processes hold some resources and wait for others.
3. No Preemption – Resources can't be forcibly taken.
4. Circular Wait – A closed chain of processes exists.

Handling Methods:

Prevention: Eliminate one of the conditions. Avoidance: Use Banker's Algorithm. Detection & Recovery: Check resource allocation graph for cycles.

Diagram: Circular Wait

P1 ---> R1 | v R2 <--- P2

3. Semaphores & Classical Problems

A Semaphore is a synchronization tool to control access to critical sections.

Types:

Binary Semaphore (mutex): values 0 or 1 Counting Semaphore: range over an unrestricted domain

Operations:

wait(S): Decrements S. If S < 0, process waits. signal(S): Increments S. If S <= 0, a waiting process is resumed.

Classical Problems Solved with Semaphores:

1. Producer-Consumer: Synchronize buffer usage
2. Readers-Writers: Multiple readers or one writer
3. Dining Philosophers: Avoid deadlock while eating

4. Memory Management

Responsible for keeping track of memory usage and managing allocation/deallocation.

Contiguous Memory Allocation:

Single Partition: One process in memory Multiple Fixed Partition: Fixed size blocks Multiple Variable Partition: Allocate based on process size

Paging:

Divide memory into fixed-size blocks Logical address = page number + offset Uses page table to map pages to frames

Diagram: Paging

Logical Address: Page # + Offset Page Table -> Frame #