Shared Memory Organization

• A number of basic issues in the design of shared memory systems have to be taken into consideration.
• These include access control, synchronization, protection, and security.
  • Access control determines which process accesses are possible to which resources.
  • Synchronization constraints limit the time of accesses from sharing processes to shared resources.
  • Protection is a system feature that prevents processes from making arbitrary access to resources belonging to other processes.
• The simplest shared memory system consists of one memory module that can be accessed from two processors.
• Requests arrive at the memory module through its two ports.

Depending on the interconnection network, a shared memory system leads to systems can be classified as:

• Uniform Memory Access (UMA). A shared memory is accessible by all processors through an interconnection network in the same way a single processor accesses its memory.
  • Therefore, all processors have equal access time to any memory location.
• Nonuniform Memory Access (NUMA). Each processor has part of the shared memory attached.
  • However, the access time to modules depends on the distance to the processor. This results in a nonuniform memory access time.
• Cache-Only Memory Architecture (COMA). Similar to the NUMA, each processor has part of the shared memory in the COMA.
  • However, in this case the shared memory consists of cache memory.
  • A COMA system requires that data be migrated to the processor requesting it.