The send, receive, and reply operations may be synchronous or asynchronous. A synchronous operation blocks a process till the operation completes. An asynchronous operation is non-blocking and only initiates the operation. The caller could discover completion by polling, by software interrupt, or by waiting explicitly for completion later. (Does it make sense to have an RPC send not block?) An asynchronous operation needs to return a call/transaction id if the application needs to be later notified about the operation. At notification time, this id would be placed in some global location or passed as an argument.
The notion of synchronous operations requires an understanding of what it means for an operation to complete. In the case of remote assignment, both the send and receive complete when the message has been delivered to the receiver. In the case of remote procedure call, the send, receive, and reply complete when the result has been delivered to the sender, assuming there is a return value. Otherwise, the send and receive complete when the procedure finishes execution. During the time the procedure is executing, the sender and receiver are in a rendezvous, as mentioned before.
Note that synchronous/asynchronous implies blocking/not blocking but not vice versa, that is, not every blocking operation is synchronous and not every non blocking operation is asynchronous. 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. These definitions of synchronous/asynchronous operations are similar but not identical to the ones given in your text books, which tend to equate synchronous with blocking.
Asynchronous message passing allows more parallelism.
it can do some computation while the message is in transit. 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.
What happens if the message cannot be delivered?
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.