Nexus

According to the Nexus [24] developers,

Nexus is a portable library providing the multithreading, communication, and resource management facilities required to implement advanced languages, libraries, and applications in heterogeneous parallel and distributed computing environments. Its interface provides multiple threads of control, dynamic processor acquisition, dynamic address space creation, a global memory model via interprocessor references, and asynchronous events. Its implementation supports multiple communication protocols and resource characterization mechanisms that allow automatic selection of optimal protocols.

Nexus is a cooperative effort between Argonne National Laboratory, the USC/Information Sciences Institute, the Aerospace Corporation, and the High Performance Computing Laboratory at Northern Illinois University. Nexus is probably the one system most closely related to Converse. Both Nexus and Converse provide the same basic facilities, active messages and threads. However, there are some significant differences in design philosophy between the two. The following list summarizes the differences between the two.

• In Nexus, if two processors share memory, they share all variables. There is no mechanism to allow an individual processor to have private data. This makes it difficult to implement functions like malloc efficiently. Converse makes it possible to choose whether a resource (like the malloc heap) is to be shared between processors or private. This choice can be made on a resource-by-resource basis.

• Converse threads are nonpreemptive, whereas Nexus threads are usually preemptive. Nonpreemptive threading, in combination with the processor-private data described earlier, almost completely eliminates locking and the need for thread-safe libraries. On the other hand, the preemptive threads in Nexus will be more useful in real-time or interactive systems.

• Nexus thread-blocking primitives (mutexes and condition variables) are at a higher level of abstraction than Converse thread-blocking primitives (suspend, awaken, thread identifiers). The ones in Converse are less expensive, which may be relevant to language implementers.

• Nexus has two features not present in Converse. First, Nexus can convert the data format of one machine to the data format of another, enabling the use of heterogeneous networks. Second, Nexus can dynamically add workstations to the set of processors it is using. These two features are clearly advantageous in certain cases. On the other hand, both have a cost, in that they complicate the Nexus API significantly.

• Converse includes convenience modules for dynamic load-balancing, futures, automatic parameter marshalling, tag-based message lookup, and many of the other functions a language implementer may want. Nexus is more minimalist, providing only what is strictly needed.