⭐ Symmetric Multiprocessing (SMP)

1. Definition

Symmetric Multiprocessing (SMP) is a computer architecture in which two or more identical processors are connected to a single shared main memory and operate under a single operating system, sharing I/O and memory resources equally.

In SMP, all processors are peers—any processor can execute any task, and the OS can schedule tasks on any processor.

2. Key Features of SMP

1. Multiple processors: Two or more identical CPUs.
2. Shared memory: All processors have equal access to the main memory.
3. Single OS instance: One operating system controls all processors.
4. Uniform access time: Memory access time is the same for all processors (unlike NUMA).
5. Common I/O subsystem: All processors share the same I/O devices.

3. Benefits of SMP

1. Improved performance and throughput: Multiple processors can execute tasks concurrently.
2. Scalability: Adding more processors increases system performance (up to limits).
3. Reliability and fault tolerance: If one processor fails, others can continue processing (though not fully fault-tolerant).
4. Simpler programming model: Single memory space makes it easier to write concurrent programs.

4. Challenges of SMP

1. Cache coherency: Each processor typically has its own cache, so mechanisms (like MESI protocol) are needed to maintain consistent data.
2. Contention for memory and I/O: Multiple processors accessing the same memory or I/O can cause bottlenecks.
3. Synchronization overhead: Threads/processes must be synchronized to avoid race conditions.
4. Scalability limits: Performance gain diminishes as the number of processors increases due to contention.

5. Typical SMP Architecture

• Processors (P1, P2, …): Identical CPUs, each with private registers and possibly private cache.
• Shared Main Memory: Single memory accessible by all CPUs.
• System Bus / Interconnect: Connects processors to memory and I/O.
• Operating System: Single OS schedules tasks across all processors.

6. Comparison with Other Architectures

7. Example Scenario

• Suppose an SMP system has 4 CPUs and shared memory.
• OS can schedule Thread 1 on CPU1, Thread 2 on CPU2, and so on.
• All threads access shared variables in memory.
• Cache coherency protocols ensure all CPUs see the most recent value of shared variables.

8. Exam-Friendly Summary

• SMP: Multiple identical processors sharing memory and I/O under a single OS.
• Goal: Increase throughput and parallelism for multiple tasks/threads.
• Key challenges: Cache coherency, memory contention, synchronization.
• Advantages: Simplicity, performance, scalability (moderate), reliability.