Sync group selection

A gateway implementing multiple independent sync networks. The independent sync networks can be used to allow for synchronisation between different synchronisation groups of accelerators. The independent sync networks allow synchronisations to be carried out asynchronously and simultaneously. The gateway has sync propagation circuitry that receives a first synchronisation request for an upcoming exchange phase and propagates this sync request through a first sync network. The first synchronisation request is a request for synchronisation between subsystems of a first synchronisation group. The sync propagation circuitry of the gateway also receives a second synchronisation request for a different exchange phase and propagates this sync request through the second sync network. The second synchronisation request is a request for synchronisation between subsystems of a second synchronisation group. The two exchange phases overlap in time. Therefore, the syncs are simultaneous and asynchronous.