CPU has threads OS has threads Programming languages has their own threads
From what I understand from Reddit, Say a Quad Core processor with 4 core and 4 thread, can only run 4 threads at the same time. The Programming lanaguges/OS thread can spin up tens, and thousands of threads. Only 4 can be run at the same time. The OS juggles between them in sub milliseconds so, it feels like they all happen at the same time. concurrency-parallelism, concurrency enabled parallelism
- CPU threads = hardware execution contexts (like 4 cores → 4 threads, or 8 with hyper-threading). That’s the max number of instructions executed at the same instant.
- OS threads = abstractions managed by the kernel. You can create thousands, but only as many as hardware threads can run in parallel. The rest get time-sliced.
- Language threads (green threads, goroutines, fibers, etc.) = scheduled by runtime, often multiplexed onto fewer OS threads.