Difference between CPU threads, Kernel threads, OS threads, user threads

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I've been reading a lot about how parallelism is different than concurrency, but I'm having a lot of questions like how can I achieve a parallelism with a programming language like Java?

What I understood so far is with a VM like Java we can only achieve concurrency and parallelism is not in scope of JVM, it's threads mapped to ( I'm confused here ) OS threads or Kernel threads?

To understand the multi-threading one needs to understand how the threads of a VM ( JVM ) or a process are actually mapped down to the CPU hardware. Please help here.

Correct me if I'm wrong, here are few things I think I understand right:

1. CPU threads ( Intel hyper-threading ): Hardware threads provided by adding more registers in CPU having same ALU, so do not actually behave as a dual-CPU ( if 2 threads are there in CPU ) and a "just"-concurrent use of ALU can just mask ( hide by an interleaved MUXing ) a principal inability to deliver indeed a true parallel code-execution.

2. Kernel threads: Started by Kernel and these can be more in number than CPU threads but are a bit expensive to start.

3. OS threads: I don't understand it. I'm guessing this is what you mean when you call native threads and probably same as previous #2 ie Kernel threads.

4. User threads: Just like new Thread() in JVM or .NET. I think they are not recognized as threads by OS so nothing much can be done here from the parallelism perspective.

I need to understand how threads created in #4 are mapped to #1, may be with examples to Windows/Linux/MacOS.

Guys, I understand how wide this question can be but I can live with brief info and link to any detailed documentation. Thanks in advance.

multitasking parallel-processing concurrency multithreading java
1 Answers

1. Intel Hyperthreading - It is a technology developed by Intel to improve the efficiency of one core, because in a lot of its execution time, the CPU waits for retrieving data & code from RAM. The operating system views hyperthreading-cores as two separate logical processors, and each contains separate registers.

2. Kernel Threads - This are managed by the OS-kernel and are scheduled in kernel-mode only. They are attached to registers & EIP along with some attributes. These are created and managed by the process by using some "defined" interfaces by the kernel.

3. User Threads - These are created by the userspace process are executed on a kernel thread (because only a kernel one can be scheduled). But multiple user-threads can take turns for executing on a kernel thread, and giving higher performance for 'temporary' threads that go & come, due to time saved by not doing a system call.

4. OS Threads - OS threads don't have a proper definition in my knowledge. They can be threads used by the OS, or just kernel threads.

There are multiple models by which user-threads execute on kernel-threads - one-to-one, m-to-n & all-to-one, you can google these.

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