Administration Guide

Introducing the LAPI

The LAPI is a non-standard application programming interface designed to provide optimal communication performance on an SP switch. It is based on an active message programming mechanism that provides a one-sided communications model (that is, one process initiates an operation and the completion of that operation does not require any other process to take a complementary action). The LAPI library provides the functions PUT, GET, and a general active message function that allows programmers to supply extensions by means of additions to the notification handlers. The LAPI is designed for use by libraries and power programmers for whom performance is more important than code portability.

Why use the LAPI?

The LAPI provides the following advantages:

• Performance - The LAPI provides basic function for optimal performance. It is designed to especially provide low latency on short messages.
• Flexibility - The LAPI's one-sided communications model provides flexibility because the completion of an operation by one process does not require any other process to take a complementary action. Also, the LAPI provides a more primitive interface to an SP switch than either MPI or IP, giving the programmer the choice of how much additional communications protocol needs to be added.
• Extendibility - The LAPI supports programmer-defined handlers that are invoked when a message arrives. Programmers can customize the LAPI to their specific environments.

Some general characteristics of the LAPI software are the following:

• Reliability - using the LAPI guarantees delivery of messages. Errors not directly related to the application are not propagated back to the application.
• Flow control.
• Support for large messages.
• Non-blocking calls.
• Interrupt and polling modes.
• Efficient exploitation of switch function.
• Ordering is not guaranteed.

General LAPI functions

LAPI functions are divided into three parts:

1. A basic active message infrastructure that allows programmers to install a set of handlers that are invoked and executed in the address space of a target process on behalf of the process originating the active message. This generic interface allows programmers to customize the LAPI function to their unique environment.
2. A set of defined functions that is complete enough to satisfy the requirements of most programmers. These defined functions make the LAPI more usable and at the same time lend themselves to efficient implementation because their syntax and semantics are known.
3. A set of control functions for the initialization and eventual orderly shutdown of the LAPI layer.