The CM-5 is a distributed memory machine based on a fat tree network. Each processor is a Sparc CPU together with four vector units. (In the experiments described here the vector units are not used.) The basic communication library is CMMD 3.0 Beta which is based on a re-implementation of the Berkeley CMAM Active Message layer . The active message layer provides very good support for short messages that consist of a pointer to a local function to execute and three words of argument. In addition, the system can be made to interrupt a processor upon the receipt of a message, or it can be done by polling. One can switch between these two modes at run time. Our experience indicates that using a combination of polling and interrupts works the best. During barrier synchronization, the interrupt mechanism is used. The CMMD barrier operation is very fast (4 microseconds).