A computer program is a collection of instructions used to perform a certain task.
A process is an abstraction used to represent a program while it is in execution.
A process typically exists in five states: New, Ready, Running, Blocked, or Finished
Context switching allows CPU cores to alternate between ready and blocked processes to best take advantage of limited computing resources.
Preemption occurs when a process is temporarily interrupted by an external scheduler to prioritize a more important task.
A process is blocked when it has to wait for a contested, limited, or slow resource, such as accessing a specific file or waiting for a network request.
The layout of a process in memory has four distinct sections:
- A text section for the compiled code
- A data section for initialized variables
- A stack for local variables
- A heap for dynamic memory allocation
Process Control Block
Every process is initialized with a process control block that is required by the operating system to be able to identify and control the process.
Process Parent-Child Relationship
When a process launches another process, the original process enters a parent-child relationship with the new process. This relationship facilitates the sharing of common data and signals along the hierarchy as well as the arrangement of which process may terminate first.
A thread represents the sequence of programmed instructions that are actively being executed. They share resources which allows for faster communication and context switching as well as requiring fewer system resources when compared to processes.
Multithreading is the capability for a single CPU core to execute multiple threads at once. This improves system utilization and responsiveness by more efficiently splitting up tasks
Kernel threads are threads created in kernel space using kernel code and libraries through a system call. The kernel is fully aware of these threads and can properly manage them.
User threads are threads created in user space using local code and function calls. The kernel is not aware of these threads and cannot directly control them. User threads allow for more fine-grained control by developers and are more efficient than kernel threads as they do not need to make system calls.
User vs Kernel Threads
User threads can be mapped to kernel threads in a variety of ways: 1:1 Kernel-Level threading, N:1 User-Level threading, or M:N Hybrid threading.