Memory Reader-Writer Problem

Memory Reader-Writer Problem Visually

The Reader-Writer Problem is a classic synchronization problem in operating systems that demonstrates the need for controlled access to shared resources. Multiple readers can access the resource simultaneously, but writers require exclusive access.

Shared Reads Exclusive Writes Priority Solutions Concurrency Control
Reader-Writer Synchronization Control
Readers: 0
Writers: 0
Step: 0
Shared Resource (Database/File)
Available
Data Entry 1
Data Entry 2
Data Entry 3
Data Entry 4
Data Entry 5

Readers

R1
R2
R3
R4
R5

Writers

W1
W2
W3
Mutex (Binary Semaphore)
1
Write Mutex (Binary Semaphore)
1
Read Count (Integer)
0
Waiting Queue
No processes waiting
Select priority and click Start to begin simulation

Reader-Writer Problem

The Reader-Writer Problem is a classic synchronization problem that occurs when multiple processes need to access a shared resource:

  • Readers: Only read the shared resource (no modifications)
  • Writers: Read and modify the shared resource

The challenge is to ensure data consistency while maximizing concurrency.

Problem Constraints

  • Multiple readers can access simultaneously (shared reads)
  • Only one writer can access at a time (exclusive writes)
  • Writers must have exclusive access (no readers or other writers)
  • No process should wait indefinitely (no starvation)

Solution Approaches

Reader Priority: Readers are given priority. Once a reader starts, other readers can join.

wait(mutex); readCount++; if (readCount == 1) wait(writeMutex); signal(mutex); wait(mutex); readCount--; if (readCount == 0) signal(writeMutex); signal(mutex);

Writer Priority: Writers are given priority. Once a writer arrives, no new readers are allowed.

wait(readTry); wait(writeMutex); signal(writeMutex); signal(readTry);

Fair Solution: Uses a service queue to ensure fairness between readers and writers.

do { wait(serviceQueue); wait(resourceCount); if (resourceCount == 0) wait(writeMutex); resourceCount++; signal(resourceCount); signal(serviceQueue); wait(resourceCount); resourceCount--; if (resourceCount == 0) signal(writeMutex); signal(resourceCount); } while (TRUE);

State Diagram

1

Reader Request

Reader requests access to shared resource

2

Check Writers

Check if any writers are active or waiting

3

Grant Access

If no writers, grant read access; otherwise, wait

4

Release Resource

Reader releases resource when finished

Key Concepts

  • Mutual Exclusion: Writers must have exclusive access
  • Shared Reads: Multiple readers can access simultaneously
  • No Starvation: Neither readers nor writers should wait indefinitely
  • Priority Solutions: Different approaches to handle reader/writer priority