For instance, a send that blocks till the receiver machine has received the message is blocking but not synchronous since the receiver process may not have received it. Similarly, we will see later a Xinu receive that is non-blocking but is not asynchronous. Asynchronous message passing allows more parallelism. In a synchronous system, such parallelism can be achieved by forking a separate process for each concurrent send, but this approach incurs the cost of extra process management. This cost is typically bearable with lwps but not hwps.
Asynchronous message passing introduces several problems.
The process never waits for delivery of the message, and thus never hears about the error. Software interrupts could be used to report such errors. Another problem related to asynchronous communication is to do with buffering. If messages sent asynchronously are buffered in a space managed by the OS, then a process may fill this space by flooding the system with a large number of messages.