1. Operating system foundation

1.1 What is an operating system ：

Definition ： A program that manages computer hardware and software resources , It's the cornerstone of computers
Hardware resources ： Hardware equipment management
Software resources ： memory management , File system management and application management .

1.2 What is system call ：

According to Program access resources Characteristics , The operation of the process in the system is divided into two states ：
1) User mode ： The running process can access the data of the user program
2) The system state ： The process has access to all resources
3) system call ： In user mode , How to access system state data ？ Through system calls , Accessed by the operating system .
4） interrelation ： User mode <---- Process call ----> The system state

2. Processes and threads

2.1 Difference between process and thread ？

1. Process definition ： Programs executed concurrently during execution , The basic unit of resource allocation . Have system resources .
2. Thread definition ： Is an execution unit of a process , The basic unit of processor independent scheduling . No system resources .
3. Difference between process and thread ：
   1） address space ： The process has one ; Share this process address space
   2） resources ： Process resources are independent （ Easy to manage and protect }; Threads share the resources of this , Not independent .（ Not conducive to protection ）
   3） Robustness, ： There is no interaction between processes ; Interaction and coupling between threads
   4） Execution process ： Process execution cost is high , Low thread overhead
   5） Process is the basic unit of resource allocation , Threads are the basic unit of independent scheduling .
   6） A process contains multiple threads
   7） Processes are independent , But threads are often coupled to each other .
   8） Low thread execution cost , But it is not conducive to the management and protection of resources ; Process execution cost is high , Conducive to the management and protection of resources

2.2 What are the states of a process ?

Five states of process ：

1. Create a state of ： The process is being created , The operating system allocates resources to the process , initialization PCB
2. Ready state ： CPU ： × Other resource requirements ：
3. Running state ： CPU ： Other resource requirements ： ×
4. Blocked state ： CPU ： × Other resource requirements ： × Pausing while waiting for an event / wait for IO operation
5. End state ： The process is being undone from the system , The operating system will recycle the resources owned by the process , revoke PCB

2.3 State transitions between processes ？

1. The ready state -> Running state ： The process is scheduled
2. Running state -> The ready state ： It's time to , or CPU Preempted by other high priority processes .
3. Running state -> Blocking state ： Wait for system resource allocation or wait for an event to occur （ Take the initiative ）
4. Blocking state -> The ready state ： Resources are allocated in place , Wait for the event to happen （ Passive behavior ）
5. Create state -> The ready state ： The system completes the work related to the creation process
6. Running state -> Termination state ： End of process running , Or encountered an irreparable error in the process

2.4 Communication between processes （7 Kind of ）

1. Shared memory ： Multiple processes can access the same memory space mutually exclusive .（ Most useful , Synchronous operation can be realized through mutex lock and semaphore ）
2. Anonymous pipeline (Pipes) ： Communication between parent-child processes or brother processes with kinship . Files that exist only in memory .
3. Famous pipeline (Names Pipes) : Strictly follow the "first in, first out" principle (first in first out), Exist as a disk file .
4. The signal (Signal)： Notifies the receiving process that an event has occurred
5. Semaphore (Semaphores： A semaphore is a counter , The intention is to synchronize between processes .
6. Message queue ： Deliver structured messages . The message queue is a linked list of messages , Have a specific format , Stored in memory and identified by the message queue identifier .
7. Sockets： The client and server communicate through the network . It is an agreement between the two parties of communication , Use the relevant functions in the socket to complete the communication process .
8. Message queue has obvious advantages ：
   1） It overcomes the problem that the signal carries less information
   2） Pipes can only carry unformatted
   3） The size of byte stream buffer is limited .

2.5 The difference between process synchronization and process asynchrony ？

1. Process synchronization ：
   Concept ： The concurrent execution of processes brings asynchrony , But the process is sometimes sequential , Solve the asynchrony problem through process synchronization .
2. Process synchronization method ：
   1） Software
   （1） Single sign method ： Common shaping variable record . The two processes alternately enter the critical zone . A process is accessing the critical area , Will give access to another process .
   （2） Double sign check first ： Check the object status first , Then put your own logo .
   （3） Double sign post check ： Put your own logo first , Check the object status again . Disadvantages of the double marker method ： It is possible to access and not access at the same time , Because modification and judgment cannot be completed at the same time .
   （4） Pearson algorithm ： Combination of single sign and double sign post inspection , Avoid waiting indefinitely .
   2） Hardware
   （1）TestAndSet Use instructions for atomic operations
   Interrupt mask
3. Asynchronous process ： Multiple processes cannot access critical resources at the same time , Mutual exclusive access required , At the same time , There is a visit .

2.6 What are the ways to synchronize threads ?

1. Why do you need synchronization between threads ？
   Because threads share the resources of this process , If multiple threads execute concurrently , It is necessary to avoid the use conflict of key shared resource variables .

2. How to synchronize threads ：
   1） A critical region ： Only one thread is allowed to access shared resource variables at any time , If more than one thread attempts to access a common resource , Latecomers will be suspended .
   2） The mutex (Mutex)： Adopt mutually exclusive object mechanism , Only threads with mutexes have access to public resources .（ There is only one mutex ）. Example ： such as Java Medium synchronized Key words and all kinds of Lock It's all this mechanism . A lock is its mutually exclusive object .
   3） Semaphore (Semphares)： Allow multiple threads to access the same resource at the same time , Control the maximum number of threads accessing this resource at the same time .
   4） event (Event)： Keep multi-threaded synchronization through notification operations .

2.7 What are the scheduling algorithms of processes in the operating system ?

1. First come, first serve (FCFS) Scheduling algorithm （ Unfriendly to short processes ） :
   First come first serve , Until the operation ends or is blocked , Like a bank party .

2. Short job preferred (SJF) Scheduling algorithm of （ Take care of the short process , Ignore the long process , Will be hungry ）：
   Select a process from the ready queue with the shortest estimated run time to allocate resources to it , Until the operation ends or is blocked .

3. Highest response ratio ：
   Response ratio = response time / Service time

4. Priority scheduling ：
   Schedule according to the priority of the process , Until the scheduling is finished .

5. Time slice rotation scheduling algorithm :
   Each process runs the time slice , Perform traversal and execute .

6. Multilevel feedback queue scheduling algorithm ：
   Recognized as the best scheduling algorithm ： It can make high priority jobs respond , It can also make short homework complete quickly .
   Multi level priority alignment , High priority low time slice , Then the high priority consumption time slice is processed first , Then process the sub priority time slice .

2.8 What is a deadlock ？

1. A deadlock describes one of the following situations ：
   The processes wait for each other's resources , Cause the process to jam , Unable to move forward .
   The allocation of inalienable resources is unreasonable , May cause deadlock .
2. Deadlock , What's the difference between hunger and death cycle ？
   1） Deadlock ： At least two processes are blocked at the same time — Operating system solution
   2） hunger ： Only one process can be blocked ----- Operating system solution
   3) Dead cycle ： There may be only one . ------ appear bug, Programmers solve
3. Necessary conditions for deadlock ：
   1) Mutually exclusive ： Resources must be unshared , Only one process can use... At a time .
   2) non-preemptive ： Resources cannot be preempted
   3) Take possession and wait ： The process occupies at least one resource , And wait for another resource
   4) Loop waiting for ： Resource waiting forms a closed loop
   5) Four conditions are met at the same time , Before a deadlock can occur .

2.9 How to solve the deadlock problem ？

Four methods to solve deadlock problem ：

1. The prevention of ： No deadlock allowed ; static state ： Deadlock prevention ; Four necessary conditions for breaking deadlock
2. avoid ： No deadlock allowed ; dynamic ： Avoid deadlock ; Prevent the system from entering an unsafe state （ Banker Algorithm ）
3. testing ： Allow deadlock to occur ; When a deadlock occurs , Accurately detect the location and related resources of deadlock
4. relieve ： Allow deadlock to occur ; Cooperate with detection , Release the process from deadlock

1. Prevention is to avoid four conditions , Avoidance is the use of four conditions , Find the most reasonable ;
2. Prevention and avoidance are deadlocks that don't happen ; Detection and release are operations after deadlock occurs .
3. Avoid the banker's algorithm in ： Allocate resources for the process by probing , Judge whether the system is in a safe state after allocation through the security algorithm .

3. Operating system memory management foundation

3.1 What is the main memory management of the operating system ？

Mainly completed Memory allocation and recycling ; Address translation ; Memory space expand and Storage protection .

1. Memory allocation , Recycling ：malloc function , Application memory ;free function , Free memory
2. address translation ： Convert logical address to physical address .
3. Expansion of memory space ： Virtual storage technology or automatic coverage technology
4. Storage protection ： Each job runs in its own storage space , Mutual interference .

3.2 Common memory management mechanisms

Memory management falls into two broad categories , Four sub categories ：
1) Continuous distribution management scheme ： Single continuous , Fixed partition and dynamic partition
2) Discontinuous distribution management scheme ： Basic paging management ; Basic segment management and segment page management ;

1. Block management ： Divide the memory into fixed blocks , process / Job continuous storage ;
2. Basic paging management ： Divide the memory into several blocks , The process is also divided into blocks of corresponding size . Improve memory utilization ; Page table for logical address and physical address
3. Basic segment management ： Paging management , Each page has no specific meaning , Segmented management gives each paragraph specific meaning （ Main program section , Subroutine segments, etc ）, Corresponding logical address and physical address through segment table .
4. Basic segment page management ： Segment main memory first , Page again , Combine the advantages of both

3.3 What are fast tables and multi-level page tables ？

3.4 The common points and differences between paging mechanism and segmentation mechanism ？

3.5 Logic ( fictitious ) Difference between address and physical address ？

3.6 CPU Addressing understand ? Why do I need a virtual address space ?

4. Virtual memory

4.1 What is virtual memory ？

4.2 What is the principle of locality

4.3 What is virtual storage

4.4 Implementation technology of virtual memory

4.5 Page replacement algorithm