The present disclosure relates to a method of scheduling messages to be exchanged between tiles in a computer where there is a fixed transmission time between sending and receiving tiles. According to the method a total size of message data to be sent or received by each tile is determined. One of the tiles is selected based at least on the size of the message data to schedule a first message. The first message to be scheduled is selected from the set of messages on that tile. In order to schedule the message the other end points of this selected message are determined, and then respective time slots are allocated at the sending and receiving tiles for that message. The size of the selected message is then deducted from each of the tiles acting as end points for the message, and then the sequence is carried out again until all messages have been scheduled. This technique optimises message exchange in an exchange phase of a BSP system.