Scalability

The primary goal in the design of Portals is scalability. Portals are designed specifically for an implementation capable of supporting a parallel job running on tens of thousands of nodes. Performance is critical only in terms of scalability. That is, the level of message passing performance is characterized by how far it allows an application to scale and not by how it performs in micro-benchmarks (e.g., a two node bandwidth or latency test).

The Portals API is designed to allow for scalability, not to guarantee it. Portals cannot overcome the shortcomings of a poorly designed application program. Applications that have inherent scalability limitations, either through design or implementation, will not be transformed by Portals into scalable applications. Scalability must be addressed at all levels. Portals do not inhibit scalability, but do not guarantee it either.

To support scalability, the Portals interface maintains a minimal amount of state. Portals provide reliable, ordered delivery of messages between pairs of processes. They are connectionless: a process is not required to explicitly establish a point-to-point connection with another process in order to communicate. Moreover, all buffers used in the transmission of messages are maintained in user space. The target process determines how to respond to incoming messages, and messages for which there are no buffers are discarded.