Java threads and number of cores

Question

I just had a quick question on how processors and threads work. According to my current understanding, a core can only perform 1 process at a time. But we are able to prod

Answer 1

Java do not perform Thread scheduling, it leaves this on Operating System to perform Thread scheduling.

For computationally intensive tasks, It is recommended to have thread pool size equal to number of cores available. But for I/O bound tasks we should have larger number of threads. There are many other variations, if both type of tasks are available and needs CPU time slice.

a core can only perform 1 process at a time

Yes, but they can multitask and create an illusion that they are processing more than one process at a time

How is this possible if we are only have 4 cores? I am also able to run my 30 thread program on my local computer and also continue to perform other activities on my computer

This is possible due to multitasking (which is concurrency). Lets say you started 30 threads and OS is also running 50 threads, all 80 threads will share 4 CPU cores by getting CPU time slice one by one (one thread per core at a time). Which means on average each core will run 80/4=20 threads concurrently. And you will feel all threads/processes are running at the same time.

can someone explain how this works

All of this happens at OS level. If you are a programmer then you should not worry about this. But if you are a student of OS then pick any OS book & learn more about Multi-threading at OS level in detail or find some good research paper for depth. One thing you should know that each OS handle these things in different way (but generally concepts are same)

There are some languages like Erlang, which use green threads (or processes), due to which they get the ability to map and schedule threads on their own eliminating OS. So, do some research on green threads as well if you are interested.

Note: You can also research on actors which is another abstraction over threads. Languages like Erlang, Scala etc use actors to accomplish tasks. One thread can have hundred of actors; each actor can perform different task (similar to threads in java).

This is a very vast and active research topic and there are many things to learn.