to somewhere between 16 and 64 processors. For example, Silicon Graphics’ Power
Challenge SMP is limited to 64 R10000 processors in a single system; beyond this
number performance degrades substantially.
The processor limit in an SMP is one of the driving motivations behind the development of cluster systems. However, with a cluster, each node has its own private
main memory; applications do not see a large global memory. In effect, coherency is
maintained in software rather than hardware.This memory granularity affects performance and, to achieve maximum performance, software must be tailored to this environment. One approach to achieving large-scale multiprocessing while retaining the
flavor of SMP is NUMA. For example, the Silicon Graphics Origin NUMA system is
designed to support up to 1024 MIPS R10000 processors [WHIT97] and the Sequent
NUMA-Q system is designed to support up to 252 Pentium II processors [LOVE96].
The objective with NUMA is to maintain a transparent system wide memory
while permitting multiple multiprocessor nodes, each with its own bus or other
internal interconnect system.
Organization