We discussed broadly the concurrency primitives that the operating systems provide us. But a runtime system like a virtual machine directly manages user-level threads. There are various models that map user-level threads to kernel-level threads like one-to-one or many-to-one. On the other hand, we also have user-level threads that are supported in the user-space, part of the system memory allocated to the running applications: This makes kernel-level threads slow and inefficient, resulting in costly thread operations. However, the kernel has full control of these threads, including their scheduling. The kernel does provide system calls to create and manage these threads from outside. The operating system kernel supports and manages the kernel-level threads directly.

This makes inter-thread communication much easier. While threads share the same file handles and address spaces, they maintain their own programming stacks. A process can typically have multiple threads.

Threads are separate lines of execution within a process. This is where kernel-level threads bring relief for concurrent programming:

Kotlin with java