44. Concurrent programming theory

1. Procedures and processes

 A program is a code file , Process refers to the running process of a program .
 The same program is executed many times , It's multiple processes .

 process ：CUP The process of executing a program .
 There are many tasks to be performed at the same time , however cpu There can only be one thing at a time .

2. The program runs in three states

 When the program is created and executed , The first thing to enter is the ready state , Wait for the operating system scheduling to start running .
 Encountered in execution I/O The event request of the operation will enter the blocking state , Wait for the result .
 After getting the return, instead of running immediately, it enters the ready state and waits for the operating system to call ready to run again .

3. Concurrency and parallelism

 Concurrent ： It's pseudo parallelism , It feels like it's running at the same time , Single cpu+ Implementation of multi-channel technology , Parallel also belongs to concurrent .

 Multi channel technology can realize cpu Division of time slice of each program process , Each program is executed for a period of time , Switch between different programs , Too fast , It makes people feel that they are running at the same time .

 parallel ： Really run at the same time , need cpu With multi-core .
 A kernel performs a task . When there are more tasks than the kernel , Only when the first few tasks are executed I/O Interrupt execution , Will be assigned to other tasks for temporary execution ,I/O The end operating system will call it again .

 Today's computers run multiple tasks at the same time , A task can be understood as a process .

 The concurrent implementation of a process is that the hardware interrupts a running process , Save all the states in the program at this time , The operating system will record in a process table （ Each process occupies a process representation , These entries are also called process control modules ） Ensure that the program starts again as if it had never been interrupted .

4. Synchronous asynchronous

 Synchronous and asynchronous ： Describes how the task is submitted .

 Sync ： After the task is submitted , Wait in place for the return result of the task , Don't do anything while waiting .

 asynchronous ： After the task is submitted , Don't wait for the returned result , To do something else , The return result of the task will be automatically processed by an asynchronous callback mechanism .

5. Blocking non blocking

 Blocking non blocking ： Describe the running state of the program .

 Blocking ： Before the call result returns , The current thread will be suspended , Only after the result is obtained will it return （ Blocking state ）.
 Non blocking ： Before you can't get the results immediately , Go straight back to .（ The ready state , Running state ）.

ps:
 Combination of appeal concepts ： The most efficient combination ： Asynchronous non-blocking .
 Ideal state ： The code written is always switched between ready state and running state , Difficult to achieve .

6. Process creation

 Hardware requires operating system management , As long as there is an operating system, there is the concept of process .
 New processes are created by an existing process .

 Subprocesses :  A process created by a process .
 The parent process ： The process that created the new process .

windows The lower parent process creates child processes with different address spaces ,( Multichannel technology requires the physical layer to realize the memory isolation between processes ), Any modification of a process in its address space will not affect another process .

7. Zombies and orphans

 Zombie process ： When you start a child process , The occupied process number will not be released immediately after the process ends .
 The parent process needs to be able to view the basic information of its child processes , The amount of pid No. running time, etc .

 All processes will enter the zombie process , Wait for the parent process to view .
 The parent process does not die and creates child processes infinitely , And the subprocess will not end .

 Orphan process : Child processes survive , The parent process died unexpectedly , The operating system will start a management program , Manage orphan processes and recycle related resources .

8. Termination of process

1. The normal exit （ Exit after the normal execution of the program , Or execute the end command of the program ).
2. Error exit （ The execution file does not exist ）
3. Serious mistakes （ Execution of illegal orders , Reference does not exist , Be able to catch exceptions ）.
4. Forced to end by other processes .

9. Process scheduling

 Process scheduling ： Long and short assignments are cpu The division of .
 First come first serve scheduling algorithm ： Good for long work , It's useless to work too often ,
 Short job first algorithm ： It's good for short work , Long homework is useless .

 Time slice rotation + Multistage feedback queue ：
 Time slice ： Divide the fixed time into N Multipoint , Each copy represents a time slice  .

 Set the multi-level ready queue , Queue priority at all levels from high to low , Time slice is divided from small to large .
 The new process first enters the first level queue , Waiting in line to be allocated time slice .
 If you run out of time , The process is not over , Then the process enters the end of the next level queue .