Deadlock, thread and process explanation

One 、 Under what circumstances will the operating system deadlock ？

1.1 The necessary conditions for deadlock

• mutual exclusion ： That is, a resource can only be occupied by one process for a period of time , Cannot be occupied by two or more processes at the same time , Such exclusive resources as CD-ROM Driver , Printers, etc , It must be released after the process occupying the resource takes the initiative , Other processes can occupy this resource , This is determined by the attributes of the resource itself .
• Conditions that cannot be preempted ： The resources obtained by the process are not used up , Resource applicants cannot forcibly seize resources from occupiers , It can only be released by the owner process of the resource .
• Possession and application conditions ： The process already occupies at least one resource , But he applied for new resources . Because the resource is already occupied by another process , The process is blocked at this time ; But it is waiting for new resources , Continue to occupy existing resources .
• Loop wait condition ： There is a process waiting sequence {P1,P2,……,Pn}, among P1 wait for P2 Part of the resources occupied ,P2 wait for P3 The possession of a resource ,……, and Pn wait for P1 The possession of a resource , Form a process loop waiting ring .

1.2 Deadlock prevention

• Break the mutually exclusive condition ： That is, allow the process to access some resources at the same time . however , Some resources are not allowed to be accessed at the same time , Such as a printer .（ This is determined by the attributes of the resource itself , So this method has no practical value .）
• Break the inviolable conditions ： That is, allow the process to forcibly obtain some resources from the owner . That is to say , When a process already has some resources , It applies for new resources , But not immediately satisfied , It has to release all the resources it has , Apply again later . The resources it releases can be allocated to other processes . This is equivalent to the hidden possession of the resources occupied by the process . This method of preventing deadlock is difficult to realize , Will degrade system performance .
• Break possession and apply for conditions ： Can achieve Resource pre allocation strategy . That is, the process applies to the system for all the resources it needs at one time before running . If all the resources needed by a process cannot be met , No resources will be allocated , This process is not running , Only when the system can meet all the resource requirements of the current process , To allocate all the applied resources to the process at one time , Because the running process has occupied all the resources it needs , So there will be no phenomenon of occupying resources and applying for resources , So there is no deadlock . however , This strategy also has the following disadvantages ：

• unpredictable ： in many instances , A process cannot know all the resources it needs before execution , This is because the execution of the process is dynamic , unpredictable .
• Low resource utilization ： No matter when the allocated resources are used , A process can only be executed after it occupies all the resources it needs . Even if some resources are only used once by the process , But the process has occupied them throughout its lifetime , Cause the situation of long-term occupation . This is obviously a great waste of resources .
• Process concurrency decreases ： Because of limited resources , Plus there is waste , The number of processes that can allocate all the required resources is bound to be less .

• Break the cycle of waiting : implement Orderly resource allocation strategy . Use this strategy , That is, the resources are classified and numbered in advance , Assign by number , So that the process will not form a loop when applying for resources . All processes' requests for resources must be made strictly in the order of increasing resource serial numbers . Process occupation （ It should be used up ） Small resources , To apply for large resources , There will be no loops , So as to prevent deadlock . This strategy is compared with the previous , Resource utilization and system throughput have been greatly improved . But there are also some disadvantages ：

• Limits the process's requests for resources , At the same time, it is also difficult to number all resources in the system reasonably , And increases the overhead of the system .
• In order to follow the order of application by number , Resources not used for the time being also need to be applied in advance , This increases the resource consumption time of the process .

1.3 Deadlock avoidance

• Avoid one thread acquiring multiple locks at the same time ;
• Avoid a thread taking up multiple resources in the lock at the same time , Try to make sure that each lock occupies only one resource ;
• Try using a time lock , Use lock.tryLock(timeout) Instead of using the internal locking mechanism ;
• For database locks , Locking and unlocking must be in a database connection , Otherwise, the unlocking will fail ;

1.4 Practical application methods

• Security sequence
• Banker Algorithm

Two 、 How to understand distributed locks ？

Distributed lock is a way to control the synchronous access of shared resources between distributed systems . In distributed systems , It's often necessary to coordinate their movements . If different systems or different hosts of the same system share one or a group of resources , So when you visit these resources , Mutual exclusion is often needed to prevent mutual interference to ensure consistency , under these circumstances , You need to use distributed locks .

3、 ... and 、 There are several ways of interprocess communication ？

Be careful ： Interprocess communication is through the operating system , The communication between threads is based on processes , No need to go through the operating system .

• The Conduit ： Pipes can be used for communication between processes with kinship , Allow a process to communicate with another process that has a common ancestor .
• Command pipeline ： Named pipes overcome the limitation that pipes have no names , therefore , In addition to the functions of pipeline , It also allows unrelated processes to communicate , Named pipes have corresponding file names in the file system . Named pipe through command mkfifo Or system call mkfifo To create .
• The signal ： Signal is a more complex way of communication , Used to notify the receiving process of an event , Except for interprocess communication , The process can also signal the process itself .
• Message queue ： Message queue is the connection table of messages , Include Posix Message queue 、systern V Message queue . Processes with sufficient privileges can add messages to the queue , The process that is granted read permission can read the messages in the queue 、 Message queuing overcomes The signal carries less information , The pipeline can only carry unformatted byte streams and buffer size limitations .
• Shared memory ： Enable multiple processes to access the same block of memory space , Is the fastest available IPC form . It is designed for the low efficiency of other communication mechanisms . Often with other communication mechanisms , If used in combination with semaphores , To achieve synchronization and mutual exclusion between processes .
• Memory mapping ; Memory mapping allows multiple processes to communicate , Each process using this mechanism implements it by mapping a shared file to its own process address space .
• Semaphore ： Mainly as a means of synchronization between processes and between different threads of the same process .
• Socket （socket） More general interprocess communication mechanism , It can be used for process communication between different machines .

Four 、 The relationship between thread synchronization and blocking ？ Must synchronization be blocked ？ Does blocking have to be synchronized ？

• Thread synchronization has nothing to do with blocking .
• Synchronous and asynchronous focus on Message communication mechanism . So called synchronization , When you make a call , Before we get results , The call does not return . But once the call returns , You get the return value . Asynchrony is the opposite , After the call is made , This call directly returns , So no results returned . let me put it another way , When an asynchronous procedure call is issued , Callers don't get immediate results , But after the call is issued , The callee passes through the State , Notify the caller , Or the callback function handles the call .
• Blocking and non blocking focus on Program waiting for call result （ news , Return value ） The state of . Blocking call refers to the time between the return of call results , The current thread will be Hang up , The calling thread can only return after getting the result . A non blocking call is one that occurs before an immediate result cannot be obtained , The call will not block the current thread .

5、 ... and 、 The difference between a process and a thread

A process is an executing application , A thread is an execution sequence within a process .

• Address space and other resources ： Process independence , Resources are shared among threads of the same process , Threads within a process are not visible to other processes .
• signal communication ： Interprocess communication IPC（ A standard unix Communication mechanism , It can be seen that the communication between processes above ）, Threads can read and write directly Process data segment （ Such as global variables ） To communicate – Process synchronization and mutual exclusion are needed , To ensure data consistency .
• Switch ： Threads are much faster than process switching ;
• Dispatch ： A process is a running program （ For example, an application ：qq、 WeChat 、 Alipay ）, A thread is an execution sequence within a process （ such as ：qq One of the operations in , Chat write data , Check chat history ……）