Synchronization with a host processor

A method of operating a processing system comprising a subsystem for acting as a work accelerator to a host processor, the subsystem comprising an arrangement of tiles each comprising a respective execution unit for executing machine code, and the processing system further comprising an interconnect for communicating between the tiles and for connecting the subsystem to an external host processor via an external interconnect;

• the method comprising:
• using synchronization logic in the form of hardware logic in the interconnect to coordinate barrier synchronizations between a group of some or all of said tiles, each barrier synchronization comprising:
• a) at a synchronization point in the code executed by each respective tile in the group, the respective tile sending a synchronization request to the synchronization logic and suspending instruction issue on the respective tile pending a synchronization acknowledgement being received back from the synchronization logic, and b) in response to a condition comprising at least receiving the synchronization request from all of the tiles of said group, the synchronization logic returning the synchronization acknowledgment back to each of the tiles in the group and thereby allow the instruction issue to resume;
• wherein at least some of the barrier synchronizations require host involvement, and wherein the synchronization logic comprises at least one host sync proxy module, the host sync proxy module comprising a counter
• wherein the method comprises operating the counter to receive a number of credits written by the host processor via said external interface, and to automatically decrement the number of credits each time one of said barrier synchronizations requiring host involvement is performed;
• and
• wherein the method comprises operating the synchronization logic, for the barrier synchronizations that require host involvement, so as when the number of credits in the counter is exhausted, said condition comprises an additional criterion that the synchronization acknowledgment is not returned until released by a further write from the host to the host sync proxy module via said external interface, but when the number is credits in the counter is not exhausted the synchronization acknowledgement is allowed to be returned without said additional criterion.