2.2 Topologies

How can we connect the processors of a parallel computer ? Especially, we are interested in machines with distributed memory. A detailed discussion of most of the topics presented in this section can be found in [KGGK94].

Link :
A connection between 2 processes.

• unidirectional link: can be used only in one direction at the same time.
• bidirectional link: can be used in both directions at any time.

Topology :
Generally, an interconnection network of the processes.

Physical topology :
Interconnection network of the processors (nodes of the graph) given in hardware by the manufacturer.

• Configurable by hardware (Xplorer, MultiCluster-I).
• Configurable by software (crossbar switches in MultiCluster-II)
  

Ex.: Transputer T805 had 4 hardware links.

Logical topology [virtual topology] :
Connection of the processes (nodes) given by the user or the operating system. It will be usually derived from the data relations or communication structure of the program.

The mapping from logical to physical topology will be done via parallel operating systems or parallel extensions of OS, see Sec. 2.3.

Remark : The identity of logical and physical topology is the most efficient
$\Longrightarrow$ kontrary to data relations in applications.


Diameter :
Max. number of links which have to be used by a message sent from an arbitrary node $p$ to another node $q$.