4.4.5 Multicasting

Although a single <send_stmt> (see Sending) can specify multiple messages at once, these messages are sent one at a time. Multicasting is a form of collective communication in which a set of tasks collaborates to deliver a message from a source to multiple targets. With many messaging layers, multicasting a message to N tasks is more efficient than sending a sequence of N individual messages. coNCePTuaL supports multicasting as follows:

Unlike <send_stmt>s, <mcast_stmt>s do not support the UNSUSPECTING keyword. This is because MULTICASTS is a collective operation: all parties are active participants in delivering messages to the <target_tasks>.

<source_task> (see Source tasks) and <target_tasks> (see Target tasks) can be either disjoint or overlapping sets. That is, either of the following is legal:

TASK 0 MULTICASTS A 16 BYTE MESSAGE TO TASKS recip SUCH THAT recip<4
TASK 0 MULTICASTS A 16 BYTE MESSAGE TO TASKS recip SUCH THAT recip>=4

Note that in the first <mcast_stmt>, task 0 both sends and receives a message, while in the second <mcast_stmt>, task 0 sends but does not receive.

The MULTICASTS statement supports not only traditional one-to-many multicasting as shown in the preceding example but also many-to-many multicasting—and the less interesting but still valid many-to-one and one-to-one multicasting. The constituent pairs of communicating tasks are identical to those induced by the SENDS statement (see Sending). For example, the statement

TASKS s SUCH THAT s<3 MULTICAST A 1 KILOBYTE MESSAGE TO TASKS s SUCH
THAT s>6

performs the collective analogue of

TASKS s SUCH THAT s<3 SEND A 1 KILOBYTE MESSAGE TO TASKS s SUCH
THAT s>6

Specifically, task 0 will send a message to each of tasks 7, 8, 9, …; task 1 will send a message to each of tasks 7, 8, 9, … ; and task 2 will send a message to each of tasks 7, 8, 9, ...