Hyperthreading

Motivating Hyperthreading

• Recall each thread's state is maintained by the following:

  • A single program counter
  • A set of registers
• Classically, each CPU core could only support a single thread of execution
• Meaning, each CPU core only had enough resources (i.e. registers) for a single thread

• Therefore, the CPU would be idle while fetching data from the main memory

  • These are examples of I/O bound threads
• This would be especially slow if the data wasn't cached
• Thus, someone had the idea to have two sets of thread states
• Meaning, there were two sets of a program counter and registers

Describing Hyperthreading

• By doing this, another thread can get work done while another thread is waiting on the main memory

  • This thread could be in the same process or a different process
• This idea of modifying a CPU core to include an extra set of registers and program counter for another thread is called hyperthreading
• Hyperthreaded CPU cores genuinely support 2 threads per core
• Non-hyperthreaded CPU cores can only support 1 thread per core

References

• Python Essential Reference
• Python in a Nuteshell
• History of Hyperthreading
• Hyperthreading and Hardware Threads
• Hyperthreading and the Operating System