Loosely Coupled vs Tightly Coupled Multiprocessor System

A multiprocessor is a computer system with two or more processors that work together to execute tasks within the same system.

• It includes multiple CPUs within a single computer and supports parallel execution.
• It improves overall processing capability and increases system efficiency and performance.

Tightly Coupled Multiprocessor System

It is a shared-memory architecture in which multiple processors are connected to a common main memory and operate under a single operating system.

• Communication is done through shared memory.
• Cache coherence mechanisms are required to maintain data consistency.
• All processors access the same global memory and are managed by a single operating system.

Advantages

• Due to shared memory communication between processes is very fast.
• As all processes have the same memory space, data sharing is easy.
• Suitable for parallel applications that require frequent interaction.
• Programming is simplified as no message passing is required.

Disadvantages

• Limited scalability as adding more processors increases memory traffic.
• Multiple processors access shared memory simultaneously, and memory contention occurs.
• Maintaining data consistency is difficult and requires complex cache coherence.
• Implementation of hardware is more complex and expensive.

Working Mechanism

Practically, this works like a modern multi-core computer.

• When a program runs, the operating system divides it into threads.
• These threads are assigned to different processors (or cores).
• All processors access the same RAM to read and write data.
• If Processor 1 updates a variable in memory, Processor 2 can directly access the updated value.
• To avoid conflicts, synchronization techniques (like mutex, semaphore, locks) are used.
• Cache coherence protocols ensure that all processor caches have consistent data.

Example: In a multi-core desktop CPU, when you run a large software like a video editor, different cores process different parts of the task (rendering, effects, background tasks) while using the same system memory.

Loosely Coupled Multiprocessor System

It is a distributed-memory architecture in which each processor has its own local memory and communicates with other processors through message passing.

• Each processor has its own private memory.
• Processors communicate through a network connection.
• Separate operating systems can run on different processors.
• The system can be expanded by adding more processors.
• The failure of one processor does not affect the entire system.

Advantages

• As new needs are added easily, it is highly scalable.
• The failure of one node doesn't affect the system, providing fault tolerance.
• Better for distributed and large-scale computing environments.
• Cost-effective because it can be built using independent systems connected through a network.

Disadvantages

• Communication between processors is slower due to network-based message passing.
• Programming is more complex because explicit message passing is required.
• Synchronization and coordination between processors is difficult.
• Sharing of data is not as efficient as a shared memory system.

Working Mechanism

These systems are connected through a communication network.

• Each processor executes its own task using its private memory.
• If one processor needs data from another, it sends a message request over the network.
• The receiving processor processes the request and sends back a response.
• Data sharing does not happen automatically — it must be explicitly communicated.
• Since there is no shared memory, synchronization is handled through communication protocols.

Example: In a computer cluster (like servers in a data center), each server processes part of a large job (e.g., searching billions of web pages). If needed, servers exchange results over the network instead of accessing the same memory.

Tightly Coupled vs Loosely Coupled Multiprocessor Systems