What is the difference between concurrency and parallelism?

What's the difference between concurrency and parallelism ？
Concurrency is in a period of time , Multiple tasks will be processed ; But at some point , There is only one task in progress . A single core processor can achieve concurrency . For example, there are two processes A and B,A After running a time slice , Switch to B,B After running a time slice, switch to A. Because the switching speed is fast enough , So the macro performance is that it can run multiple programs at the same time in a period of time .

Parallel is at the same time , There are multiple tasks in progress . This requires a multi-core processor to complete , At the micro level, multiple instructions can be executed at the same time , Different programs are run on different processors , It's a physical process that's going on at the same time .

What's the difference between a big kernel and a microkernel ？
Big kernel , It is to put all the functions of the operating system into the kernel , Including scheduling 、 file system 、 The Internet 、 Device driver 、 Storage management and so on , Form a tightly connected whole . The advantage of a large kernel is high efficiency , But it's hard to locate bug, Poor expansibility , Every time you need to add new features , You have to recompile the new code and the original kernel code .
Microkernels are different from monolithic kernels , Microkernel just adds the core function of operation to the kernel , Include IPC、 Address space allocation and basic scheduling , These things are all running in kernel mode , Other functions are called by the kernel as modules , And it's running in user space . Microkernel is easier to maintain and expand , But efficiency may not be high , Because you need to switch between kernel mode and user mode frequently .
What's the difference between a time-sharing system and a real-time system ？
Time sharing system (Sharing time system) It's the system that puts CPU Time is divided into very short time slices , Assign to multiple assignments in turn . Its advantage is that it can guarantee fast enough response time for multiple jobs of multiple users , And effectively improve the utilization of resources .
Real time systems (Real-time system) It's information that the system inputs from the outside , Be able to （ deadline ） Deal with it and react . It has the advantage of being able to process and respond in a centralized and timely manner , high reliability , Security .
Usually computers use time sharing , It's multiple processes ／ Sharing between users CPU, Realize multi task from the situation . Individual users ／ Scheduling between processes is not particularly accurate , If a process is locked , You can give it more time . The real-time operating system is different , Software and hardware must comply with strict time limits , Processes that exceed the time limit may be terminated directly . In such an operating system , Every time you lock, you need to think about it carefully .
What's the difference between static linking and dynamic linking ？
Static linking is during compilation , Static libraries are integrated into applications by compilers and connectors , And make it into target files and executable files that can operate independently . A static library is a collection of external functions and variables .
Static libraries are very convenient , But if we just want to use a function in the library , And still have to link everything in . A more modern approach is to use shared libraries , Avoid a lot of duplication of static libraries in files .
Dynamic links can be executed on the first load , It can also be done at the beginning of the program . This is done by the dynamic linker , Like standard C library (libc.so) It's usually dynamically linked , So all programs can share the same library , It doesn't have to be packaged separately .
What are the stages of compilation ？
Pretreatment stage ： Deal with # The preprocessing command at the beginning ;
Compile phase ： Translate into assembly file ;
Assembly stage ： Translate assembly files into relocatable target files ;
Link phase ： The target file and... Will be relocatable printf.o Wait for separate precompiled target files to merge , Get the final executable target file .
What are the states of the process ？

In the five state model , There is a total of 5 Medium state , They are to create 、 be ready 、 function 、 End 、 Blocking .
The running state is that the process is CPU Up operation . In a single processor environment , At most one process is running at any one time .
Ready state means that the process is ready to run , That is, the process obtains the division of CPU All the resources you need , Once you get it CPU You can run .
A blocking state is when a process is waiting for an event to stop running , For example, waiting for a resource to be available or waiting for I/O complete . Even if CPU Free , The process can't run either .
Running state → Blocking state ： It's often due to waiting for peripherals , Waiting for the allocation of resources such as main memory or waiting for manual intervention .

Blocking state → The ready state ： The condition of waiting has been met , Just assign it to the processor and run .

Running state → The ready state ： It's not for your own reasons , Instead, the process in running state gives up the processor for external reasons , Now it's ready . For example, the time slice runs out , Or a higher priority process to preempt the processor .

The ready state → Running state ： The system selects a process in the ready queue to occupy the processor according to a certain policy , At this point, it becomes the running state .