Shared Memory Organization

• Each processor may have registers, buffers, caches, and local memory banks as additional memory resources.
• 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.
  • Sharing and protection are incompatible; sharing allows access, whereas protection restricts it.
• 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), nonuniform memory access (NUMA), and cache-only memory architecture (COMA).
  • In the UMA system, 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.
    • The interconnection network used in the UMA can be a single bus, multiple buses, a crossbar, or a multi-port memory.
  • In the NUMA system, each processor has part of the shared memory attached.
    • The memory has a single address space. Therefore, any processor could access any memory location directly using its real address.
    • However, the access time to modules depends on the distance to the processor. This results in a nonuniform memory access time.
  • 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.