Processes, threads, semaphores, and mutexes

This article is from the blog 《 A simple explanation of process and thread 》

• The core of a computer is CPU, It does all the computing . It's like a factory , Running at all times .
  

• Suppose the power of the factory is limited , It can only be used in one workshop at a time . in other words , When a workshop starts , All other workshops have to be shut down . The meaning behind it is , Single CPU You can only run one task at a time .
  

• The process is like the workshop of a factory , It represents CPU The single task that can be handled . Any moment ,CPU Always run a process , Other processes are not running .
  

• In a workshop , There can be a lot of workers . They worked together on a task .
  

• Threads are like workers in a workshop . A process can include multiple threads .
  

• The workshop space is shared by the workers , For example, many rooms are accessible to every worker . This symbolizes that the memory space of a process is shared , Each thread can use these shared memory
  

• But , The size of each room is different , Some rooms can only hold one person at most , Such as toilet . When there are people inside , No one else can go in . This means that when a thread uses some shared memory , Other threads have to wait for it to finish , To use this memory .
  

• A simple way to prevent others from entering , Just add a lock to the door . First come lock the door , The next person sees the lock , Just line up at the door , Don't go in until the lock is open . This is called " The mutex "（Mutual exclusion, abbreviation Mutex）, Prevent multiple threads from reading and writing a block of memory area at the same time .
  

• And some rooms , Can accommodate at the same time n personal , For example, kitchen. . in other words , If the number of people is greater than n, The extra people can only wait outside . It's like some memory area , Only a fixed number of threads can be used .
  

• Solution at this time , Just hang it at the door n Key to . Those who go in take a key , Hang the key back when you get out . Those who arrived later found that the key was on the air , I knew I had to wait in line at the door . This is called " Letter Number quantity "（Semaphore）, To ensure that multiple threads do not conflict with each other .
  It's not hard to see. ,mutex yes semaphore A special case of （n=1 when ）. in other words , We can replace the former with the latter . however , because mutex It's simpler , And high efficiency. , So when we have to guarantee the exclusive use of resources , Or this design .
  
  The design of the operating system , So it comes down to three points ：

（1） In the form of multi process , Allow multiple tasks to run at the same time ;

（2） In the form of multithreading , Allow a single task to run in different parts ;

（3） Provide coordination mechanisms , On the one hand, it prevents conflicts between processes and threads , On the other hand, it allows resources to be shared between processes and threads .