Symmetric Multithreading (SMT) or Hyperthreading

A processor with hyperthreading looks like multiple CPU to the OS. They are not true multicore machine as SMT CPU shares most of the on-chip resources and contends for each other. Each logical processor has its own register set and instruction pipeline only. It shares the on-chip cache, MMU, TLB and all other execution units. Therefore, the SMT CPU cannot process instruction twice as fast as a multicore machine. SMT is an opportunity for parallelism unique to CISC processor. Because the pipeline is deeper in CISC, the effect of stall is more significant. Providing multiple pipelines help to reduce stalling. Hyperthreading gets its name because it is more effective at accelerating multithread software. Thread, unlike process, shares memory and page table entries which make them optimal to distribute across logical CPUs. Because there is only one MMU on a SMT CPU, threads experience more of a performance boost than processes do.