NONUNIFORM MEMORY ACCESS

COMPUTER ORGANIZATION AND ARCHITECTURE DESIGNING FOR PERFORMANCE NINTH EDITION

In terms of commercial products, the two common approaches to providing a
multiple-processor system to support applications are SMPs and clusters. For some
years, another approach, known as nonuniform memory access (NUMA), has been
the subject of research and commercial NUMA products are now available.
Before proceeding, we should define some terms often found in the NUMA
literature.
• Uniform memory access (UMA): All processors have access to all parts of
main memory using loads and stores. The memory access time of a processor
to all regions of memory is the same. The access times experienced by differ-
ent processors are the same. The SMP organization discussed in Sections 17.2
and 17.3 is UMA.
• Nonuniform memory access (NUMA): All processors have access to all parts
of main memory using loads and stores. The memory access time of a proces-
sor differs depending on which region of main memory is accessed. The last
statement is true for all processors; however, for different processors, which
memory regions are slower and which are faster differ.
• Cache-coherent NUMA (CC-NUMA): A NUMA system in which cache
coherence is maintained among the caches of the various processors.
A NUMA system without cache coherence is more or less equivalent to a cluster.
The commercial products that have received much attention recently are CC-NUMA
systems, which are quite distinct from both SMPs and clusters. Usually, but unfortu-
nately not always, such systems are in fact referred to in the commercial literature as
CC-NUMA systems. This section is concerned only with CC-NUMA systems.