About i/o -- the relationship between memory and CPU and disk

About I/O

One sentence sums up what is I/O: Simple data Copy. Between memory and external devices copy Namely I/O(input/output)

I/O And CPU
Usually CPU Speed ： Nanosecond level ( Command execution speed )
Usually I/O Speed ： Millisecond level ( Once on a disk seek)
Thousands of times the speed difference , How to match... Needs to be solved I/O and CPU Speed 、 Dispatch CPU etc. .

Suppose you're an acute (CPU), Need to wait for an important file , Unfortunately, this document can only be sent by express (I/O), Then you choose not to do anything , Staring at the door affectionately, just like Hani who is looking forward to you, he is concentrating on waiting for the express ？ Let's leave the express service alone for the time being , Play a game, watch a movie, brush a short video and wait for the express to come ？ Obviously , A better way is to do something else first , When the express comes .

Therefore, the key point here is that the matter at hand can be suspended first , Switch to other tasks , Switch back when the express arrives .

perform I/O What's going on at the bottom

1. About Block：

In the above figure, there are two processes in memory , process A And processes B, The current process A Running , process A There is a code to read the file in , No matter what language it is in, it is common to customize a for loading data buffer, And then call read Functions like that , like this ：

read(buff);

This is a typical I/O operation , When CPU When this code is executed, a read request is sent to the disk , Pay attention to and CPU It's faster to execute instructions than ,I/O The operation is very slow , Therefore, it is impossible for the operating system to waste valuable computing data on unnecessary waiting , At this time, the external equipment performs I/O The operation is quite slow , So in I/O The process cannot move forward until the operation is completed , That's what's called Blocking , That is what is usually said block. The operating system has detected that the process is running towards I/O After the device initiates the request, it pauses the process , You need to record the running status of the current process and put CPU Of PC Registers point to instructions from other processes .

2. About I/O Blocking queue and ready queue ：

When a process is paused, it will have Carry on , Therefore, the operating system must save the suspended process for subsequent execution , Obviously we can use queues to hold processes that are suspended , As shown in the figure , process A Is suspended and put in the blocking queue ( Be careful , Different operating systems have different implementations , Maybe each I/O Every device has a corresponding Blocking queues , But this difference in implementation details does not affect our discussion ).

The disk is equivalent to reading asynchronously , And the number of times will put the read end callback into I/O queue ,CPU I went to do other things .

In fact, in addition to blocking queues, there are also Ready queue , The so-called ready queue means that the processes in the queue are ready to be CPU Yes , You may ask why you don't have to have a ready queue to execute directly ？ The answer is simple , That's it cannot meet the needs of the people , The only 2 Thousands of processes can be created on a core machine ,CPU It's impossible to execute so many processes at the same time , So there must be such a process , Even if it's all ready, it can't be allocated to computing resources , Such a process is put on the ready queue . Process now B Right in the ready queue , Everything is ready CPU, As shown in the figure ：

Pay attention to the picture above , here process B In being CPU perform , The disk is running to the process A In memory space of copy data , data copy And instruction execution at the same time , Under operating system scheduling CPU、 The disk is fully utilized , That's the wisdom of programmers .

3. interrupt

After that, the disk finally got all the data copy In the process A The memory of the , At this time, the disk informs the operating system that the task is completed , This is it. interrupt .

The operating system finds the data after receiving the disk interrupt copy complete , process A Re qualify for continued operation , At this time, the operating system carefully put the process A From the blocking queue to the ready queue , As shown in the figure ：

Be careful , The operating system does not run processes directly A Of , process A Must be put in the ready queue to wait , It's fair to everyone . After that, the process B Carry on , process A Continue to wait for , process B After a while, the operating system thinks that the process B It took long enough to execute , So put the process B Put it in the ready queue , And put the process A Remove to continue . Note that the operating system puts the process B Put it in the ready queue , So the process B It's suspended just because the time slot is up, not because it's started I/O Request blocked , As shown in the figure ：

process A Carry on , here buff It's full of the data you want , process A It's going on happily , It's like it's never been suspended , The process knows nothing about being suspended , That's the magic of the operating system .

Zero copy ：
Bypass the operating system , Direct data copying is called zero copy .

In fact, in general I/O Data is the first thing copy Inside the operating system , And then the operating system copy Into process space

Secrets from the bottom of the computer —— breeze , official account ： Manon's island for survival

This book is easy to read , A style similar to the big talk design pattern , Sense of humor , illustrated , simple , It is much better than the crumpled text description and dry code in the textbook .