User level communication software

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Processes on different CPUs on a multicomputer communicate by sending messages to one another. In the simplest form, this message passing is exposed to the user processes. In other words, the operating system provides a way to send and receive messages, and library procedures make these underlying calls available to user processes.

Send and Receive

For Send:

Blocking send (CPU idle during message transmission)

Nonblocking send with copy. "have the kernel copy the message to an internal kernel buffer and then allow the the process to continue" (CPU time wasted for the extra copy)

Nonblocking send with interrupt. "interrupt the sender when the message has been fully sent to inform that the buffer is once again available" (makes programming difficult)

Copy on Write "mark the buffer as read only until the message has been sent, if it is reused before that a copy is made" (extra copy probably needed eventually)

For receive:

Interrupt can be used to signal the receiver that the message has arrived.

Poll procedure

Pop-up thread, it runs a procedure specified in advance and whose parameter is a pointer to the incoming message. After processing the message, it simply exits and is automatically destroyed.

Active message. Run the receiver code directly in the interrupt handler. To make this scheme faster, the message itself contains the address of the handler, so when a message arrives, the handler can be called in a few instructions.