Python Full stack Development Foundation "19th" process

I. What is a process

Process: a process in progress or a task. The CPU is responsible for performing the task.

Example: (Single core + multi-channel, to achieve concurrency of multiple processes):

For example, you are a CPU, you have some work to do in the afternoon, eat, wash clothes, toilet and so on. But just to get everything done in the afternoon (and the CPU can only do one thing at a time), how can multiple tasks achieve concurrent execution? Well, you should do this, you can cook first, in the process of waiting for the cooked meal you can go to wash clothes, wash the almost rice is ripe, then you can go to the toilet.

Ii. the difference between process and procedure

The program is just a bunch of code, and process refers to the process of running the program

Need to emphasize is: the same program executes two times, that is also the process, such as login QQ, although all are the same software, but one can video chat, one can stroll space.

Third, concurrency and parallelism

Concurrency: Single CPU, multi-process concurrency

Whether it is parallel or concurrent, the user appears to be ' simultaneously ' running, whether it is a process or a thread, is only a task, the real work is cpu,cpu to do these tasks, and a CPU can only perform one task at a time

Concurrency: Pseudo-parallel, which appears to be running at the same time. Concurrency can be achieved with a single cpu+ multi-channel technology (concurrent concurrency)

You're a CPU and you talk about three girlfriends at the same time, each can be a love task, you are three tasks to share 2 to play the effect of concurrent love, 3 should be you first with your girlfriend 1 to see a movie, see A will say: not good, I want to pull belly, and then run to a second girlfriend to eat, ate a will say: that what, I 4 Go to the bathroom and run to a room with girlfriend 3.

Parallel: Multi-CPU (running concurrently, only with multiple CPUs to achieve parallelism)

Single-core, multi-channel technology can be used, multiple cores, each core can also use multi-channel technology ( Multi-channel technology is for single-core )

There are four cores, six missions, so that four tasks are executed at the same time, assuming they were assigned to CPU1,CPU2,CPU3,CPU4,

Once task 1 encounters I/O and is forced to interrupt execution, Task 5 takes the cpu1 time slice to execute, which is the multi-channel technology under single core.

Once the I/O for Task 1 is over, the operating system calls it back ( knowing the schedule of the process, which CPU to run, the operating system ), and may be assigned to any of the four CPUs to execute

All modern computers often do many things at the same time, and a user's PC (either single or multi-CPU) can run multiple tasks simultaneously (a task can be understood as a process).

Multi-Channel technology: In-memory multi-channel (multiple) programs, the CPU from one process quickly switch to another, so that each process each run dozens of or hundreds of milliseconds, so that, although in a moment, a CPU can only perform a task, but in 1 seconds, the CPU could run multiple processes, This gives rise to a parallel illusion, pseudo concurrency, to differentiate the true hardware parallelism of a multiprocessor operating system (multiple CPUs share the same physical memory)

Iv. Synchronous and asynchronous

Synchronous execution: When a process executes a task, another process must wait for it to finish before it can continue execution
Asynchronous execution: When a process executes a task, another process does not have to wait for it to finish, it can continue execution, and when a message returns, the system notifies the latter for processing, which can improve execution efficiency

For example, the telephone is synchronous communication, the short interest is asynchronous communication.

V. Creation of processes

But all hardware, need to have the operating system to manage, as long as there is an operating system, there is the concept of process, you need to have a way to create processes, some operating systems for a single application program design, such as the microwave oven controller, once the microwave oven, all processes are already there.

For general-purpose systems (running many applications), there is a need to have the ability to create or revoke processes in the process of running the system, mainly divided into 4 forms to create new processes

1. System initialization (view process Linux in the PS command, Windows with Task Manager, the foreground process is responsible for interacting with the user, the background running process is not user-independent, running in the background and only when needed to wake up the process, called daemons, such as e-mail, Web pages, news, printing)

2. A process in the process of running a child process (such as Nginx open multi-process, os.fork,subprocess. Popen, etc.)

3. User's interactive request, and create a new process (such as user double-click Storm Video)

4. Initialization of a batch job (applied only in a mainframe batch system)

Either way, the creation of a new process is created by an already existing process that executes a system call to create the process:

1. In Unix the system call is: Fork,fork creates a copy that is identical to the parent process, with the same storage image, the same environment string, and the same open file (in the shell interpreter process, executing a command creates a child process)

2. In Windows, the system call is: Createprocess,createprocess processes both the creation of the process and the process of loading the correct program into a new one.

About creating child processes, UNIX and Windows

1. The same is true: After a process is created, the parent and child processes have their own different address spaces (the multi-channel technology requires the physical plane to implement the isolation of the memory between processes ), and the modification of any one process in its address space does not affect another process.

2. The difference is that, in Unix, the initial address space of a child process is a copy of the parent process, which indicates that the child process and the parent process can have read-only shared memory areas. However, for Windows systems, the address space of the parent process and the child process is different from the beginning.

VI. termination of the process

1. Normal exit (voluntary, such as the user clicks on the interactive page of the cross, or the completion of the program call to initiate system calls to exit normally, in Linux with exit, in Windows with ExitProcess)

2. Error exiting (voluntary, a.py does not exist in Python a.py)

3. Serious error (involuntary, execution of illegal instructions, such as referencing non-existent memory, 1/0, etc., can catch the exception, try...except ... ）

4. Killed by other processes (involuntary, e.g. kill-9)

VII. Hierarchical structure of processes

Regardless of Unix or Windows, the process has only one parent process, and the difference is:

1. All processes in Unix are rooted in the init process and are formed into a tree structure. Parent-child processes collectively form a process group so that when a signal is emitted from the keyboard, the signal is sent to all members of the current keyboard-related process group.

2. In Windows, there is no process hierarchy concept, all processes are of the same status, and the only hint similar to the process level is that when the process is created, the parent process gets a special token ( called a handle) that can be used to control the child process, But the parent process has the right to pass the handle to the other child processes, so there is no hierarchy.

Viii. Status of the process

Tail-f access.log |grep ' 404 '

Execute the program tail, open a subprocess, execute the program grep, open another subprocess, two processes based on pipe ' | ' Communication, tail the result as input to grep.

The state of process grep while waiting for input (i.e. I/O) is called blocking, when the grep command cannot be run

In fact, in both cases, a process cannot be logically run,

1. Process hangs is its own cause, encountering I/O blocking, it will let the CPU to allow other processes to execute, so that the CPU has been working

2. Regardless of the process, it is the operating system level that may invoke other processes to use the CPU because one process takes up too much time, or a priority, and so on.

Thus a process consists of three states

Nine, the phenomenon of process concurrency

The implementation of the concurrency of the process is that the hardware interrupts a running process, saving all the state at which the process is running, and for this reason, the operating system maintains a table, the process table, which consumes a process table entry (these are also referred to as Process Control blocks)

Python Full stack Development Foundation "19th" process