Hardware Reference
In-Depth Information
As a consequence of these and other factors, there is a great deal of interest in
building and using parallel computers in which each CPU has its own private mem-
ory, not directly accessible to any other CPU. These are the multicomputers. Pro-
grams on multicomputer CPUs interact using primitives like
send
and
receive
to
explicitly pass messages because they cannot get at each other's memory with
LOAD
and
STORE
instructions. This difference completely changes the pro-
gramming model.
Each node in a multicomputer consists of one or a few CPUs, some RAM
(conceivably shared among the CPUs at that node only), a disk and/or other I/O de-
vices, and a communication processor. The communication processors are con-
nected by a high-speed interconnection network of the types we discussed in Sec.
8.3.3. Many different topologies, switching schemes, and routing algorithms are
used. What all multicomputers have in common is that when an application pro-
gram executes the
send
primitive, the communication processor is notified and
transmits a block of user data to the destination machine (possibly after first asking
for and getting permission). A generic multicomputer is shown in Fig. 8-36.
Node
CPU
Memory
…
…
Disk
and
I/O
Disk
and
I/O
…
Local interconnect
Local interconnect
Communication
processor
High-performance interconnection network
Figure 8-36.
A generic multicomputer.
In Fig. 8-36 we see that multicomputers are held together by interconnection
networks. Now it is time to look more closely at these interconnection networks.
Interestingly enough, multiprocessors and multicomputers are surprisingly similar
in this respect because multiprocessors often have multiple memory modules that
must also be interconnected with one another and with the CPUs. Thus the mater-
ial in this section frequently applies to both kinds of systems.
The fundamental reason why multiprocessor and multicomputer intercon-
nection networks are similar is that at the very bottom both of them use message
