Managing global and local execution phases

A computer system comprising one or more processors an memory comprising one or more memory units arranged to store code arranged to run on the one or more processors, the code being configured so as when run to perform the operations of running a program comprising a plurality of concurrent threads on a computer, wherein:

• at any given time the program is in a current one of a sequence of global execution phases;
• each of the threads is divided into a respective sequence of local execution phases each corresponding to a different corresponding one in the sequence of global execution phases, wherein at any given time the thread is in a current one of the respective sequence of local execution phases, and the current local execution phase is not allowed to progress beyond the local execution phase in the respective sequence that corresponds to the current global execution phase;
• any of the threads is able to advance the global execution phase to the next in the sequence of global execution phases on condition that the current local execution phase of all of the threads has reached the local execution phase in the respective sequence that corresponds to the current global execution phase;
• one of the threads comprises code to perform an internal acquire to acquire a lock on its respective local execution phase;
• and
• at least one other of the threads comprises code to perform an external release to force advancement of the current local execution phase of said one of the threads, but wherein the external release will be blocked if said one of the threads has performed the internal acquire.