What is shared memory?

In computer hardware, shared memory refers to a (typically) large block of random access memory that can be accessed by several different central processing units (CPUs) in a multiple-processor computer system. The issue with shared memory systems is that the many CPUs need fast access to memory and will likely cache memory. 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. The alternatives to shared memory are distributed memory and distributed shared memory, with another, similar set of issues. See also Non-Uniform Memory Access. In software the term shared memory refers to memory that is accessible by more than one process, where a process is a running instance of a program. In this context, shared memory is used to facilitate inter-process communication.