The message passing model of communication is typically used in distributed memory systems, where each processor node owns private memory, and is linked by an interconnection network. In the case of the SP, its switch provides the interconnection network needed for high-speed exchange of messages. With message passing, each task operates exclusively in a private environment, but must cooperate with other tasks in order to interact. In this situation, tasks must exchange messages in order to interact with one another.
The challenge of the message passing model is in reducing message traffic over the interconnection network while ensuring that the correct and updated values of the passed data are promptly available to the tasks, when required. Optimizing message traffic is one way of boosting performance.
Synchronization is the act of forcing events to occur at the same time or in a certain order. Synchronization also requires taking into account the logical dependence and the order of precedence among the tasks. We can describe the message passing model as self-synchronizing because the mechanism of sending and receiving messages involves implicit synchronization points. To put it another way, a message cannot be received if it has not already been sent.