What is meant by shared memory parallel computer? Explain its advantages and disadvantages.

General Characteristics:

> all processors can access all memory as global address space

> Multiple processors can operate independently

> If any processor modify any memory location, visible to other processors

> Historically, shared memory machines have been classified as UMA and NUMA, based upon memory access times.

Fig. UMA Fig. NUMA

Advantages:

> Global address space provides a user-friendly programming perspective to memory

> Data sharing between tasks is both fast and uniform due to the proximity of memory to CPUs

Disadvantages:

> Lack of scalability between memory and CPUs.

Adding more CPUs can geometrically increases traffic on the shared memory-CPU path, and for cache coherent systems, geometrically increase traffic associated with cache/memory management.

> Programmer responsibility for synchronization constructs that ensure "correct" access of global memory.