Thread selection for multithreaded processing

A processor 4 interleaves program instructions from different threads to perform fine grained multithreading. Thread monitoring circuitry 30 monitors the performance of the threads. Issue control circuitry 28 selects a thread based on the performance of the threads and issues instructions from the selected thread. The monitoring circuitry may measure the proportion of clock cycles in which the thread has an instruction available. The instructions may be issued from a combination of threads to ensure that there is always an instruction available from one of the threads. An instruction may be considered unavailable if it has not been fetched or it has a dependency on a result, which is not yet available. For each thread, the monitoring circuitry may have a counter 36 for the number of clock cycles for which there is an instruction available in a predetermined period. The period may be set in a register 38.