Simultaneous Multithreading

Simultaneous multithreading, often abbreviated as SMT, is a technique for improving the overall efficiency of superscalar CPUs. SMT permits multiple independent threads of execution to better utilize the resources provided by modern processor architectures. Simultaneous multithreading allows multiple threads to execute different instructions in the same clock cycle, using the execution units that the first thread left spare. This is done without great changes to the basic processor architecture: the main additions needed are the ability to fetch instructions from multiple threads in a cycle, and a larger register file to hold data from multiple threads. The number of concurrent threads can be decided by the chip designers, but practical restrictions on chip complexity usually limit the number to 2, 4 or sometimes 8 concurrent threads.