Active messages are a type of inter-process communication used in computing that allow a subroutine or function to be executed by a different computer. They are asynchronous and can override existing IP network communication protocols, allowing for continuous overlapping of sent messages with computations on remote machines. Active messages are commonly used in parallel processing environments to efficiently process large amounts of data. However, custom-designed interface drivers may be needed for implementation.
An active message (AM) is a type of inter-process communication used in computing, whereby the subroutine or function of a computer program can be executed by a completely different computer than the one in which it resides. This type of message is of such a basic format that it allows for greater utilization of network bandwidth over standard Internet protocols (IP). It is also considered an asynchronous message passing system, as it runs immediately instead of waiting for a particular moment to synchronize with the program.
With the active messaging communication model, much of the work takes place at the hardware level, where the hardware interacts with the operating system drivers. Since this lower level is normally reserved for operating system activity, messages are constructed in such a way that they can carry instructions for accessing a sequence of higher level user functions. Inside the header of an AM is the address of what is called the manager. The handler is a special user-level instruction that allows the message body to be processed by a computation performed by the processor. The body of an active message, therefore, is the argument of the function or the data on which the calculation must act.
Given the essential nature of active message construction, it is capable of overriding existing IP network communication protocols. The main difference, however, is that it is done immediately upon receipt, rather than requiring a multi-step delivery, dispatch, acknowledgment of receipt method. In this way, an active message treats the network as simply an open pipe for traversal. Its only limitations are those of the physical network, where the distance between nodes can take into account message latency.
The way an active messaging system works is through a sender filling the network with messages. There is no buffering on the receiving end, whose only response to an AM is to briefly interrupt its computation for the message handler, get the message off the network, and continue on with its business. The only time message buffering occurs is from the sender’s end, where messages are held back depending on whether the network is reaching its limits. A polling method monitors the network so the system knows when to post more messages. Active messages therefore allows for the continuous overlapping of sent messages with computations occurring on remote machines, as well as keeping them all coordinated.
While the active message method may be simple in nature, it faces some implementation issues. Even though they are of such a primitive design, custom-designed interface drivers are often needed to handle messages for a computer system. Active messages are also designed to operate between computer nodes all running the same program. This way, the instructions sent and received are specific to the program already running on the target computer.
Due to their nature, active messages have found widespread use in parallel processing environments, such as single process, multiple data (SPMD) applications. These programs run on large networks of computer systems where active messages are used to pass instructions and data between machines. Such distributed systems use the method to efficiently process huge amounts of data that would otherwise take too much time on a single machine.
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