Shared Memory Architecture

Figure 4.1: Shared memory systems.

• Shared memory systems form a major category of multiprocessors. In this category, all processors share a global memory (See Fig. 4.1).
• Communication between tasks running on different processors is performed through writing to and reading from the global memory.
• All interprocessor coordination and synchronization is also accomplished via the global memory.
• Two main problems need to be addressed when designing a shared memory system:
  1. performance degradation due to contention. Performance degradation might happen when multiple processors are trying to access the shared memory simultaneously. A typical design might use caches to solve the contention problem.
  2. coherence problems. Having multiple copies of data, spread throughout the caches, might lead to a coherence problem. The copies in the caches are coherent if they are all equal to the same value. However, if one of the processors writes over the value of one of the copies, then the copy becomes inconsistent because it no longer equals the value of the other copies.
• Scalability remains the main drawback of a shared memory system.