A method and processing apparatus are provided for executing a program. The processing apparatus comprises memory and a processor. The processor is configured to dispatch a parent work group of a program to be executed and execute a spawn work group instruction to enable a child work group of the parent work group to be executed. The processor is also configured to dispatch the child work group for execution when a sufficient amount of resources are determined to be available to execute the child work group and execute the child work group on one or more compute units. The spawn work group instruction comprises a pointer to a synchronization variable, and the processor is also configured to execute a join workgroup instruction which comprises the pointer to the synchronization variable in the spawn work group instruction.