Threads
There are two types of threads:
- Physical threads: Threads that are executing on a hardware core
- Logical threads: Thread abstractions managed by the OS. Multiple logical threads can map to a single physical thread using time sharing
Hyper-threading
Hyper-threading is a form of SMT that allows one physical core to appear as multiple logical cores
To achieve this, each core has multiple copies of registers, PC, but a single ALU
Context Switch
A context switch involves saving and restoring the process control block (PCB), including:
- Registers
- Stack pointer
- Program counter
- Page table info
Imagine a single-threaded program being relocated from one core to another; from a programmer’s perspective, the program should behave as if nothing changed (program order). Because registers are saved in RAM, cache coherency ensures that another core will see all changes (even to registers), so the program behaves as expected