Workstation networks and interfaces are approaching the performance characteristics of MP systems [93]. However, network protocols have not kept up with this trend [1]. Illinois Fast Messages [72] and U-net [99] are two examples of approaches to reduce the software overhead.
Fast messages (FM) recognize the need of overlapping communication with computation, precise control of the hardware involved (busses and network interface), and efficient buffer management. Using Myrinet interfaces, this approach goes as far as replacing the program in the network interface coprocessor (LANai) with one that is specific to the FM protocol. Data is moved directly into user space, avoiding a costly memory-to-memory copy. The protocol further assumes a reliable network and puts the burden of message content verification (checksums) onto higher level layers. (Measurements under Amoeba indicate that user level protocols incur only small additional costs, but provide increased flexibility [70].)
The U-net approach is to map the network interface into user space and avoid protection domain crossings for message transfers. To send or receive a message, the kernel does not have to be invoked.
Both FM and U-net lower message passing latencies, but do not directly address the problem of handler response time. In both cases, a polling user application can quickly respond to user messages. A general, interrupt driven solution still has a high cost associated with it.