Silicon Graphics 2400, Owner'S Manual

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About the SGI 2400 and 2800
9
About the SGI 2400 and 2800
As illustrated in Figure 1-7, the server is a number of processing modules linked together
by the CrayLink Interconnect. Each processing module contains either one or two
processors, a portion of main memory, a directory to maintain cache coherence, and two
interfaces: one that connects to I/O devices and another that links system nodes through
the CrayLink Interconnect.
Cache coherence is the ability to keep data consistent throughout a system. In the
symmetrical multiprocessor (SMP) server system, data can be copied and shared among
all the processors and their caches. Moving data into a cache may cause the cached copy
to become inconsistent with the same data stored elsewhere. The cache coherence
protocol is designed to keep data consistent and to propagate the most recent version of
the data to wherever it is being used.