1. A method of controlling constraint sharing in a problem solver executed on a processing element, comprising: receiving a set of shared constraints from a further problem solver executed on a further processing element during a time interval, wherein the set of shared constraints comprises one or more constraints on a solution to a common problem being solved by the problem solver and the further problem solver in parallel; determining a total number of shared constraints received at the problem solver during the time interval; comparing the total number of shared constraints received during the time interval to a target number of shared constraints and determining a feedback parameter for the further problem solver in dependence thereon, wherein the feedback parameter regulates a total number of constraints in a subsequent set of constraints received from the further problem solver in a subsequent time interval; and providing the feedback parameter to the further problem solver.