1. Deadlock Problem
Deadlock is a situation in which two or more processes are unable to proceed because each process is waiting for a resource held by another process.
Simple meaning: Deadlock means processes are stuck permanently.
2. Deadlock Characterization
Deadlock can occur only when all four conditions are true at the same time.
Mutual Exclusion
Hold and Wait
No Preemption
Circular Wait
- Mutual Exclusion: One resource can be used by only one process at a time.
- Hold and Wait: A process holds one resource and waits for another resource.
- No Preemption: Resource cannot be forcibly taken from a process.
- Circular Wait: Processes wait in a circular chain.
3. Deadlock Prevention
Deadlock prevention stops deadlock before it happens by breaking at least one necessary condition.
- Remove hold and wait condition.
- Allow resource preemption.
- Prevent circular wait using resource ordering.
- Reduce mutual exclusion where possible.
4. Deadlock Avoidance
Deadlock avoidance checks the system state before allocating resources. Resource is allocated only when the system remains safe.
Banker's Algorithm is the most important deadlock avoidance algorithm for RGPV exams.
5. Deadlock Detection
In deadlock detection, the operating system allows resource allocation and later checks whether deadlock has occurred.
- Resource Allocation Graph can be used.
- Detection algorithm checks waiting processes.
- After detection, recovery method is applied.
6. Recovery from Deadlock
Recovery from deadlock means bringing the system back to normal working condition after deadlock is detected.
- Terminate deadlocked processes.
- Preempt resources from selected processes.
- Rollback process to safe state.
7. Methods for Deadlock Handling
| Method | Meaning | Importance |
|---|---|---|
| Prevention | Stops deadlock before occurrence. | High |
| Avoidance | Allocates resource only in safe state. | Very High |
| Detection | Checks whether deadlock has occurred. | High |
| Recovery | Removes deadlock from system. | Medium |
8. Concepts of Memory Management
Memory management is an important function of the operating system. It manages main memory and decides how memory is allocated to processes.
- Tracks used and free memory.
- Allocates memory to processes.
- Deallocates memory after process completion.
- Protects one process memory from another.
9. Logical and Physical Address Space
| Basis | Logical Address | Physical Address |
|---|---|---|
| Generated By | CPU | Memory Unit |
| Also Called | Virtual Address | Real Address |
| Visible To User | Yes | No |
| Used For | Program execution view | Actual memory location |
10. Swapping
Swapping is a technique in which a process is temporarily moved from main memory to secondary memory and later brought back into main memory.
Swapping increases multiprogramming but too much swapping can reduce performance.
11. Fixed and Dynamic Partitions
Fixed Partitioning
Memory is divided into fixed-size partitions. It is simple but may cause internal fragmentation.
Dynamic Partitioning
Memory partitions are created according to process size. It reduces internal fragmentation but may cause external fragmentation.
12. First-Fit, Best-Fit and Worst-Fit Allocation
| Method | Working | Key Point |
|---|---|---|
| First-Fit | Allocates first free block that is large enough. | Fast |
| Best-Fit | Allocates smallest suitable free block. | Less wastage |
| Worst-Fit | Allocates largest free block. | Leaves large remaining space |
13. Paging
Paging divides logical memory into fixed-size pages and physical memory into fixed-size frames.
- Page size and frame size are equal.
- Page table maps pages to frames.
- Paging removes external fragmentation.
14. Segmentation
Segmentation divides a program into logical parts such as main program, functions, stack and data. Each part is called a segment.
- Segments are variable-sized.
- Segment table contains base and limit.
- It represents user's logical view of memory.
15. Paging Combined with Segmentation
Paging combined with segmentation uses both techniques. Each segment is divided into pages. This gives logical division of segmentation and efficient allocation of paging.
Important comparison: Paging vs Segmentation vs Paging with Segmentation.