KEY POINTS
◆ A traditional way to increase system performance is to use multiple processors
that can execute in parallel to support a given workload.The two most
common multiple-processor organizations are symmetric multiprocessors
(SMPs) and clusters. More recently, nonuniform memory access (NUMA)
systems have been introduced commercially.
◆ An SMP consists of multiple similar processors within the same computer,
interconnected by a bus or some sort of switching arrangement. The most
critical problem to address in an SMP is that of cache coherence. Each
processor has its own cache and so it is possible for a given line of data to be
present in more than one cache. If such a line is altered in one cache, then
both main memory and the other cache have an invalid version of that line.
Cache coherence protocols are designed to cope with this problem.
◆ When more than one processor are implemented on a single chip, the configuration
is referred to as chip multiprocessing. A related design scheme is
to replicate some of the components of a single processor so that the
processor can execute multiple threads concurrently; this is known as a
multithreaded processor.
◆ A cluster is a group of interconnected, whole computers working together
as a unified computing resource that can create the illusion of being one
machine.The term whole computer means a system that can run on its own,
apart from the cluster.
◆ A NUMA system is a shared-memory multiprocessor in which the access
time from a given processor to a word in memory varies with the location of
the memory word.
◆ A special-purpose type of parallel organization is the vector facility, which
is tailored to the processing of vectors or arrays of data.