Shared Memory - in Hardware

In Hardware

In computer hardware, shared memory refers to a (typically large) block of random access memory (RAM) that can be accessed by several different central processing units (CPUs) in a multiple-processor computer system.

A shared memory system is relatively easy to program since all processors share a single view of data and the communication between processors can be as fast as memory accesses to a same location. The issue with shared memory systems is that many CPUs need fast access to memory and will likely cache memory, which has two complications:

• CPU-to-memory connection becomes a bottleneck. Shared memory computers cannot scale very well. Most of them have ten or fewer processors.
• Cache coherence: Whenever one cache is updated with information that may be used by other processors, the change needs to be reflected to the other processors, otherwise the different processors will be working with incoherent data (see cache coherence and memory coherence). Such coherence protocols can, when they work well, provide extremely high-performance access to shared information between multiple processors. On the other hand they can sometimes become overloaded and become a bottleneck to performance.

Technologies like crossbar switches, Omega networks, HyperTransport or Front-side bus can be used to dampen the bottleneck-effects.

The alternatives to shared memory are distributed memory and distributed shared memory, each having a similar set of issues. See also Non-Uniform Memory Access.