How does more than one thread execute on a processor core

Question

I wanted to know how does a multi-threaded program with more number of threads executes on a processor core. For example, my program has 12 threads and I am running it on a intel core-i5 machine. It has four CPUs. Will each core run 3 threads? I am confused because I have seen programs with 30 threads running on a 4 core machine.

Thanks

Answer 1

A core is responsible for executing thread cycles. The more cores you have, the more threads you may run simultaneously. Each core can execute only one instruction at a time, however it’s so fast it seems as if you are running multiple threads simultaneously. Intel Processors supports Hyper Threading enabling a single core to support multiple Threads as operating system sees twice the amount of logical cores per physical core. For instance, a Core i3, which is only a dual core, can actually serve two threads per core i.e. total of four threads can run simultaneously. However even if Core i5 processors are quad cores, since they don’t support Hyper-Threading (except the i5-661) the number of threads they can serve at the same time is just about equal to those of their Core i3 counterparts.

Answer 2

Each core would be able to execute one thread simultaneously. So if there are 30 threads and 4 cores, 26 threads will be waiting to get context switched to get executed. Something like, thread 1-4 runs for 200ms and then 5-8 runs for 200 ms and so on