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Which of the following conditions is broken when an OS requires processes to acquire resources in a strictly ascending numerical order (e.g. Resource 1 before Resource 2)?

Circular Wait. Enforcing a global total ordering on resource allocation mathematically eliminates the possibility of a circular dependency graph (Circular Wait).

In Round Robin CPU scheduling, what happens if the time quantum (slice) is set to an extraordinarily large value?

The scheduling algorithm degenerates into First-Come, First-Served (FCFS). When the time quantum is larger than the burst time of every ready process, each process finishes completely without preemption, behaving exactly like FCFS.

Computer Science · College · BCA, BTech & MCA

Operating Systems: Process Scheduling, Context Switching & Deadlocks

Master OS core internals: Process Life Cycle, Process Control Blocks (PCB), CPU Scheduling Algorithms (FCFS, SJF, Round Robin), and the 4 Coffman Deadlock Conditions.

1Process States & The Process Control Block (PCB)

Operating Systems: Process Scheduling, Context Switching & Deadlocks: Subject, Target Audience, Topic Core — Operating Systems: Process Scheduling, Context Switching & Deadlocks
Master OS core internals: Process Life Cycle, Process Control Blocks (PCB), CPU Scheduling Algorithms (FCFS, SJF, Round Robin), and the 4 Coffman Deadlock Conditions.
1Subject — Computer Science
2Target Audience — BCA, BTech & MCA Students
3Topic Core — Process Lifecycle, CPU Scheduling & Deadlocks

AProcessis a program in active execution. Unlike passive code stored on secondary storage, a process possesses an allocated address space (Text, Data, Heap, Stack) and runtime resources managed by the kernel via a data structure called theProcess Control Block (PCB).

Process Address Space Memory Segments

Segment	Role in Memory	Growth Direction	Lifecycle / Allocation
Text Segment	Compiled executable binary machine instructions	Fixed size	Read-only, loaded at program start
Data Segment	Initialized and uninitialized (BSS) global and static variables	Fixed size	Persists for entire process lifetime
Heap Segment	Dynamically allocated memory via $malloc()$ , $\text{new}\$ , etc.	Grows upward toward higher memory addresses	Managed explicitly by programmer or runtime GC
Stack Segment	Local variables, function arguments, and stack activation frames	Grows downward toward lower memory addresses	Automatically allocated and freed on function entry/exit

Text Segment
Role in Memory
Compiled executable binary machine instructions
Growth Direction
Fixed size
Lifecycle / Allocation
Read-only, loaded at program start
Data Segment
Role in Memory
Initialized and uninitialized (BSS) global and static variables
Growth Direction
Fixed size
Lifecycle / Allocation
Persists for entire process lifetime
Heap Segment
Role in Memory
Dynamically allocated memory via $malloc()$ , $\text{new}\$ , etc.
Growth Direction
Grows upward toward higher memory addresses
Lifecycle / Allocation
Managed explicitly by programmer or runtime GC
Stack Segment
Role in Memory
Local variables, function arguments, and stack activation frames
Growth Direction
Grows downward toward lower memory addresses
Lifecycle / Allocation
Automatically allocated and freed on function entry/exit

5-State Process Lifecycle Transitions

1
New
The process is being created and its PCB initialized in memory
2
Ready
Process resides in RAM in the Ready Queue, waiting to be assigned a CPU core by the scheduler
3
Running
CPU instructions are currently executing on a physical processor core
4
Waiting / Blocked
Process yielded CPU waiting for an I/O event, disk read, or network response
5
Terminated
Execution finished; kernel reclaims memory and closes open file descriptors

2CPU Scheduling Algorithms: FCFS, SJF, Priority & Round Robin

The CPU Scheduler selects a process from the ready queue and allocates the CPU. Schedulers are classified asNon-Preemptive(process keeps the CPU until it yields or terminates) orPreemptive(OS timer interrupt forces context switch back to ready queue).

CPU Scheduling Algorithms Comparison Matrix

Algorithm	Type	Decision Metric	Pros	Cons / Pitfalls
First-Come, First-Served (FCFS)	Non-Preemptive	Arrival time	Simple to implement with FIFO queue	Convoy Effect: short processes get blocked behind long CPU-bound process
Shortest Job First (SJF)	Non-Preemptive	Smallest CPU burst time	Provably optimal minimum average waiting time	Impossible to know upcoming burst time in advance; starves long jobs
Shortest Remaining Time First (SRTF)	Preemptive	Smallest remaining burst time	Extremely responsive for short tasks	High context switching overhead; starvation for CPU-intensive processes
Round Robin (RR)	Preemptive	Time Quantum (q) slice (e.g. 10–50ms)	Fair, starvation-free; optimal for interactive time-sharing systems	Performance heavily dependent on quantum size: too small = excessive context switch overhead; too large = degenerates into FCFS
Priority Scheduling	Both	Numerical priority integer	Accommodates critical OS kernel tasks	Indefinite blocking / starvation of low-priority jobs (mitigated by aging)

First-Come, First-Served (FCFS)
Type
Non-Preemptive
Decision Metric
Arrival time
Pros
Simple to implement with FIFO queue
Cons / Pitfalls
Convoy Effect: short processes get blocked behind long CPU-bound process
Shortest Job First (SJF)
Type
Non-Preemptive
Decision Metric
Smallest CPU burst time
Pros
Provably optimal minimum average waiting time
Cons / Pitfalls
Impossible to know upcoming burst time in advance; starves long jobs
Shortest Remaining Time First (SRTF)
Type
Preemptive
Decision Metric
Smallest remaining burst time
Pros
Extremely responsive for short tasks
Cons / Pitfalls
High context switching overhead; starvation for CPU-intensive processes
Round Robin (RR)
Type
Preemptive
Decision Metric
Time Quantum (q) slice (e.g. 10–50ms)
Pros
Fair, starvation-free; optimal for interactive time-sharing systems
Cons / Pitfalls
Performance heavily dependent on quantum size: too small = excessive context switch overhead; too large = degenerates into FCFS
Priority Scheduling
Type
Both
Decision Metric
Numerical priority integer
Pros
Accommodates critical OS kernel tasks
Cons / Pitfalls
Indefinite blocking / starvation of low-priority jobs (mitigated by aging)

3Deadlocks: The 4 Coffman Conditions & Prevention

Definition

Deadlock — A situation in a multi-processing system where a set of processes are permanently blocked because each process holds a resource and waits for another resource held by another process in the set.

The Four Coffman Conditions for Deadlock (All 4 Must Hold Simultaneously)

Condition	Definition	Prevention Strategy
1. Mutual Exclusion	At least one resource must be held in a non-shareable mode (only one process uses it at a time)	Make resources shareable where possible (e.g. read-only files)
2. Hold and Wait	A process holding at least one resource is actively waiting to acquire additional resources held by other processes	Require processes to request all resources simultaneously before starting, or release current holds before requesting new ones
3. No Preemption	Resources cannot be forcibly seized; a resource can be released only voluntarily by the holding process	If a process holding resources requests an unavailable resource, preempt and release all of its currently held resources
4. Circular Wait	A closed cycle exists where P₀ waits for P₁, P₁ waits for P₂, and Pₙ waits for P₀	Impose a strict global numerical ordering on all resources; processes may request resources only in strictly increasing order

1. Mutual Exclusion
Definition
At least one resource must be held in a non-shareable mode (only one process uses it at a time)
Prevention Strategy
Make resources shareable where possible (e.g. read-only files)
2. Hold and Wait
Definition
A process holding at least one resource is actively waiting to acquire additional resources held by other processes
Prevention Strategy
Require processes to request all resources simultaneously before starting, or release current holds before requesting new ones
3. No Preemption
Definition
Resources cannot be forcibly seized; a resource can be released only voluntarily by the holding process
Prevention Strategy
If a process holding resources requests an unavailable resource, preempt and release all of its currently held resources
4. Circular Wait
Definition
A closed cycle exists where P₀ waits for P₁, P₁ waits for P₂, and Pₙ waits for P₀
Prevention Strategy
Impose a strict global numerical ordering on all resources; processes may request resources only in strictly increasing order

Exam tip: Banker’s Algorithm(developed by Edsger Dijkstra) is a classic deadlockavoidancealgorithm that tests for safety by simulating the allocation of predetermined maximum resources before actually committing the allocation.

Remember

A Process address space comprises Text (code), Data (globals), Heap (dynamic allocations), and Stack (activation records).
Process states transition through New → Ready → Running → Waiting → Terminated via context switches.
SJF achieves provably optimal average waiting time; Round Robin guarantees responsiveness via time quanta slices.
Deadlock occurs if and only if all 4 Coffman conditions hold: Mutual Exclusion, Hold & Wait, No Preemption, and Circular Wait.

Check yourself

Question 1
Which of the following conditions is broken when an OS requires processes to acquire resources in a strictly ascending numerical order (e.g. Resource 1 before Resource 2)?
Question 2
In Round Robin CPU scheduling, what happens if the time quantum (slice) is set to an extraordinarily large value?

1Process States & The Process Control Block (PCB)

2CPU Scheduling Algorithms: FCFS, SJF, Priority & Round Robin

3Deadlocks: The 4 Coffman Conditions & Prevention

Remember

Check yourself

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