> What is a hardware thread? I thought threads were a software abstraction? This is a great question! Threads are a programming abstraction, but not just in s

> What is a hardware thread? I thought threads were a software abstraction? This is a great question! Threads are a programming abstraction, but not just in software. Processors present an "architected state" to the programmer. Internally, the machine itself has a much more complex set of logic managing that presented state. The feature we're talking about here is generally called Simultaneous Multi-threading or SMT.[1] POWER was the first architecture to support this in commercial devices back in 2004.[2] What's important to understand is that the processor core itself is made up of multiple units, each of which have their own internal state, task management, and methods of communication with each other. They don't just run a linear "fetch/decode/execute/store" cycle, even in a pipeline. If you were to do that, you'd end up with an extreme under-utilization of the silicon (or put a different way, someone else would figure out how to get better performance than you). With SMT enabled, multiple real programming threads can execute at the exact time, while the core "figures out" how to share resources between them, increasing the effective throughput of the device at the expense of silicon. With the newest POWER cores you can get up to SMT-8, and (if you want) even tune cores down to SMT-4, SMT-2, or just turn the feature off and dedicate all resources to a single thread while the operating system is running. All of that is presented to the programmer as by way of the architected registers and behaviors in the processor. In reality there's a huge number of other registers and machinery running behind the scenes that isn't visible to user-space, the operating system, or in some cases even the hypervisor (if present). Source: I worked in firmware for a couple of generations of these devices, was lead for a simulator team, and was functional owner for the launch vehicle for POWER7. [1] https://en.wikipedia.org/wiki/Simultaneous_multithreading https://en.wikipedia.org/wiki/Simultaneous_multithreading [2] https://en.wikipedia.org/wiki/POWER5 https://en.wikipedia.org/wiki/POWER5