Thread

• The word "thread" has multiple meanings depending on the context

Hardware Threads

• Each CPU Core has 2 Threads thanks to Hyper-Threading (Intel) or SMT (AMD)
• Example: if a CPU has 4 Cores, the OS sees 8 logical CPUs

OS/Kernel Threads

• An OS thread is a unit of work created by the OS (via system calls like pthread_create on Linux).
• Each process can create many OS threads, far more than the number of hardware threads. The OS scheduler decides which OS Threads run on which Hardware Threads
• If there are more OS threads than hardware threads, the scheduler will time-slice (interleave)

Virtual Threads / Coroutine

• It's a lightweight thread managed by a runtime (e.g., JVM, PVM) instead of the OS. Millions os threads are possible.
• They're much lighter and don't map directly to OS Threads. The runtime can schedule many virtual threads onto a much smaller pool of OS threads
• This concept appear with different namings in different programming languages
• Java
  • new Thread(...): OS Thread
  • Thread.startVirtualThread(...): Virtual Thread (Java 19+)
• Python
  • Thread(...).start(): OS Thread
  • async def() + await <coroutine>: asyncio Coroutine
• Go
  • Goroutine