Communication between host and accelerator over network

A host system compiles a set of local programs which are provided over a network to a plurality of subsystems. By defining the synchronisation activity on the host, and then providing that information to the subsystems, the host can service a large number of subsystems. The defined synchronisation activity includes defining the synchronisation groups between which synchronisation barriers occur and the points during program execution at which data exchange with the host occurs. Defining synchronisation activity between the subsystems allows a large number of subsystems to be connecting whilst minimising the required exchanges with the host.