shared memory

1. Memory in a parallel computer, usually RAM, which can

be accessed by more than one processor, usually via a shared

bus or network.

It usually takes longer for a processor to access shared

memory than to access its own private memory because of

contention for the processor-to-memory connections and because

of other overheads associated with ensuring synchronised

access. Computers using shared memory usually have some kind

of local cache on each processor to reduce the number of

accesses to shared memory. This requires a cacheconsistency protocol to ensure that one processor's cached

copy of a shared memory location is invalidated when another

processor writes to that location.

The alternative to shared memory is message passing where

all memory is private to some particular processor and

processors communicate by sending messages down special

links. This is usually slower than shared memory but it

avoids the problems of contention for memory and can be

implemented more cheaply.

2. Memory which can be access by more than one process in a

multitasking operating system with memory protection.

Some Unix variants, e.g. SunOS provide this kind of shared

memory.

Unix manual pages: shmop(2), shmctl(2), shmget(2).

(1994-10-20)