What are the problems with traditional IO? Why is zero copy introduced?

Conventional IO What are the problems ？ Why zero copy ？

If the server wants to provide the function of file transfer , The simplest way we can think of is ： Read out the files on the disk , And then send it to the client through the network protocol .

Tradition  I/O  The way we work is , Data read and write are copied back and forth from user space to kernel space , And the kernel space data is through the operating system level  I/O  Interface to read or write from disk .

The code is usually as follows , Generally, two system calls are required ：

read(file, tmp_buf, len);
write(socket, tmp_buf, len);


The code is simple , Although only two lines of code , But there's a lot going on here .

First ,

A total of  4  Context switch between secondary user mode and kernel mode

, Because there were two system calls , Once it was  read() , Once it was  write(), Every system call must first switch from user mode to kernel mode , After the kernel completes its task , Then switch from kernel mode to user mode .

The cost of context switching is not small , A switch takes tens of nanoseconds to microseconds , Although the time seems short , But in a high concurrency scenario , This kind of time is easy to accumulate and amplify , This will affect the performance of the system .

secondly , It also happened  

4  Secondary data copy

, among

Twice  DMA  A copy of the

, in addition

Twice through  CPU  Copy

Of , Let's talk about the process ：

• The first copy
  , Copy the data on the disk into the buffer of the operating system kernel , The process of copying is through  DMA  Carrying .

• Second copy
  , Copy the data from the kernel buffer to the user's buffer , So our application can use this data , This copy to process is done by  CPU  Accomplished .

• The third copy
  , Just copy the data to the user's buffer , And then copy it to the kernel  socket  In the buffer of , The process is still by  CPU  Carrying .

• The fourth copy
  , Put the kernel of  socket  Data in the buffer , Copy to the network card buffer , This process is again caused by  DMA  Carrying .

Let's go back to the file transfer process , We're just carrying a piece of data , It turned out to be carrying  4  Time , Too many copies of data will no doubt consume  CPU  resources , Greatly reduces system performance .

This simple and traditional way of file transfer , There are redundant handoffs and data copies , It's very bad in a highly concurrent system , A lot of unnecessary expenses , Will seriously affect system performance .

therefore ,

To improve the performance of file transfer , You need to reduce 「 Context switch between user mode and kernel mode 」 and 「 Memory copy 」 The number of times

.

If you reprint , Please indicate the source ： Open source byte    https://sourcebyte.cn/article/169.html