In one embodiment a set of threads are assigned in a particular order to an order group. The first assigned thread is treated as being, at least initially, at a head-of-line (HOL) for the order group. Each thread of the set is assigned a separate sequence number, each sequence number indicating the order in which the respective thread was assigned to the order group. A given thread is prevented from performing at least some of the given thread's instruction sequence until the given thread reaches the HOL of the order group as indicated by a modifiable HOL sequence value.