A data processing apparatus has processing circuitry for processing threads each having thread state data. The threads may be processed in thread groups, with each thread group comprising a number of threads processed in parallel with a common program executed for each thread. Several thread state storage regions are provided with fixed number of thread state entries for storing thread state data for a corresponding thread. At least two of the storage regions have different fixed numbers of entries. The processing circuitry processes as the same thread group threads having thread state data stored in the same storage region and processes threads having thread state data stored in different storage regions as different thread groups.