Concepts such as kernel space, processes, and threads

User space and kernel space

Now that the operating system is using virtual memory, the 32-bit operating system, its addressing space (virtual storage space) is 4G (2 of 32). The core of the operating system is the kernel, which is independent of the normal application, has access to protected memory space, and has all the permissions to access the underlying hardware device. In order to ensure that the user process can not directly manipulate the kernel (kernel), to ensure the security of the kernel, the operating system divides the virtual space into two parts, part of the kernel space, part of the user space. For the Linux operating system, the highest 1G bytes (from the virtual address 0xc0000000 to 0xFFFFFFFF) for the kernel to use, called the kernel space, and the lower 3G bytes (from the virtual address 0x00000000 to 0xBFFFFFFF) for each process to use, Called User space.

Process switching

To control the execution of a process, the kernel must have the ability to suspend a process that is running on the CPU and resume execution of a previously suspended process. This behavior is called process switching. So it can be said that any process that runs under the support of the operating system kernel is closely related to the kernel.
The process of moving from one process to another runs through the following changes:
Save the processor context, including program counters and other registers.
Update PCB information.
The PCB of the process is moved into the appropriate queue, such as ready, in an event blocking queue. Select another process to execute and update its PCB.
Update the data structure of memory management.
Restore the processing machine context.

Blocking of processes

The executing process, because some expected events did not occur, such as requesting system resources failed, waiting for the completion of an operation, new data has not arrived or no new work to do, etc., the system automatically executes the blocking primitive (block), making itself from the running state into a blocking state. It can be seen that the blocking of a process is an active behavior of the process itself, and therefore it is possible to turn it into a blocking state only if the process is in a running state (acquiring the CPU). When a process goes into a blocking state, it does not consume CPU resources.

File descriptor

File descriptor, a term in computer science, is an abstraction that describes a reference to a file.
The file descriptor is formally a non-negative integer. In fact, it is an index value that points to the record table in which the kernel opens a file for each process maintained by the process. When a program opens an existing file or creates a new file, the kernel returns a file descriptor to the process. In programming, some of the underlying programming often revolves around file descriptors. However, the concept of file descriptors is often applied only to operating systems such as UNIX and Linux.

Cache io

Cache Io is also known as standard IO, and most file system default IO operations are cache io. In the Linux cache IO mechanism, the operating system caches the IO data in the file system's page cache, which means that the data is copied into the buffer of the operating system kernel before it is copied from the operating system kernel buffer to the application's address space.
Disadvantages of Cache IO:
Data is required to perform multiple copies of data in the application address space and the kernel during transmission, and the CPU and memory overhead of these data copy operations is very large.

Processes and Threads

For the operating system, a task is a process, such as open a browser is to start a browser process, open a notepad started a Notepad process, open two Notepad started the two Notepad process, open a word started a word process
Some processes do more than one thing at the same time, such as word, which can be typed, spell-checked, and printed at the same time. Within a process, to do multiple tasks at the same time, you need to run multiple "subtasks" at the same time, and we refer to these "subtasks" in the process as threads (thread).
So an application can have one or more processes, a process can have one or more threads, one of which is the main thread (the thread is the entity in the process, and a thread must have a parent process)

Multi-process

Unix/linux Multi-process
The Unix/linux operating system provides a fork () system call, which is very special. A normal function call, called once, is returned once, but the fork () is called once and returned two times because the operating system automatically copies the current process (called the parent process), which is then returned within the parent and child processes, respectively.
The child process always returns 0, and the parent process returns the ID of the child process. The reason for this is that a parent process can fork out many child processes, so the parent process has to note the ID of each child process, and the child process only needs to call Getppid () to get the ID of the parent process.

threads, processes, CPUs, memory, hard disk relationships

The CPU is the control release instruction and the operation processing data
The hard disk stores the data and mainly saves the data. Common IO operations refer to the operation of the hard disk
Memory temporarily stores data, mainly connecting the hard disk and CPU, is the performer, according to the instructions to work
Threads are part of a process
The CPU is scheduling the thread
The system allocates resources (memory) for the process and does not assign resources to threads

Concepts such as kernel space, processes, and threads