Introduction to Multiprocessors - Fundamentals of Computer Organization and Architecture - page 247

11.4.1. Shared Memory Organization

There has been recent growing interest in distributed shared memory systems. This is
because shared memory provides an attractive conceptual model for interprocess inter-
action even when the underlying hardware provides no direct support. A sharedmemory
model is one in which processors communicate by reading and writing locations in a
shared memory that is equally accessible by all processors. 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. Access control models make the required check for every access
request issued by the processors to the shared memory, against the contents of
the access control table. The latter contains flags that determine the legality of
each access attempt. If there are access attempts to resources, then until the desired
access is completed, all disallowed access attempts and illegal processes are
blocked. Requests from sharing processes may change the contents of the access
control table during execution. The flags of the access control with the synchroniza-
tion rules determine the system's functionality. Synchronization constraints limit the
time of accesses from sharing processes to shared resources. Appropriate synchro-
nization ensures that the information flows properly and ensures system functiona-
lity. Protection is a system feature that prevents processes from making arbitrary
access to resources belonging to other processes. Sharing and protection are incom-
patible; sharing allows access, whereas protection restricts it.
Running two copies of the same program on two processors will decrease the per-
formance relative to that of a single processor, due to contention for shared memory.
The performance degrades further as three, four, or more copies of the program
execute at the same time.
A shared memory computer system consists of (1) a set of independent processors,
(2) a set of memory modules, and (3) an interconnection network. The simplest shared
memory system consists of one memory module (M) that can be accessed from two
processors P a and P b (Fig. 11.8). Requests arrive at the memory module through its
two ports. An arbitration unit within the memory module passes requests through to
a memory controller. If the memory module is not busy and a single request arrives,
then the arbitration unit passes that request to the memory controller and the request
is satisfied. The module is placed in the busy state while a request is being serviced.

P a P b

M

Figure 11.8 A simple shared memory scheme