Hyperthreading SMT
Intel's name for Simultaneous Multithreading (SMT). One physical CPU core exposes two logical cores by sharing execution units between two threads. Launched in the Pentium 4 (2002). AMD has the same tech as SMT in Ryzen.
Idea: when one thread stalls on a cache miss, another thread can use the execution pipeline. ~30% throughput gain on mixed workloads, 0 for CPU-bound single-thread. Security issue: SMT shares the L1 cache → side-channel attacks (Foreshadow 2018, Zombieload, RIDL, the MDS family). OpenBSD disables by default. Intel disabled HT in 12th gen+ consumer CPUs (E-cores replace the need). Server CPUs still have it.