nginx sendfile parameter explanation

sendfile
Nowadays, popular web servers provide the sendfile option to improve server performance. So what exactly is sendfile and how does it affect performance? sendfile is actually a system call launched after Linux 2.0+. The web server can decide whether to use the sendfile system call by adjusting its own configuration. Let’s first look at the traditional network transmission process without sendfile:
read(file,tmp_buf, len);
write(socket,tmp_buf, len);
Hard disk >> kernel buffer >> user buffer>> kernel socket buffer >>Protocol stack
Generally speaking, a network application completes network transmission by reading data from the hard disk and then writing the data to the socket. The above two lines of code explain this point, but the above two simple lines of code cover up many underlying operations. Let’s take a look at how the bottom layer executes the above two lines of code:
1. The system call read() generates a context switch: switching from user mode to kernel mode, and then DMA performs a copy to read the file data from the hard disk into a kernel buffer.
2. The data is copied from the kernel buffer to the user buffer, and then the system call read() returns. At this time, a context switch occurs: switching from kernel mode to user mode.
3. The system calls write() to generate a context switch: switch from user mode to kernel mode, and then copy the data read from the user buffer in step 2 to the kernel buffer (the second time the data is copied to the kernel buffer), but this time it is different. Kernel buffer, this buffer is associated with the socket.
4. The system call write() returns, resulting in a context switch: switching from kernel mode to user mode (the fourth time), and then DMA copies the data from the kernel buffer to the protocol stack (the fourth time).
The above 4 steps have 4 context switches and 4 copies. We found that if we can reduce the number of switches and copies, the performance will be effectively improved. In the kernel2.0+ version, the system call sendfile() is used to simplify the above steps and improve performance. sendfile() can not only reduce the number of switches but also the number of copies.
Let’s take a look at the process of using sendfile() for network transmission:
sendfile(socket,file,len);
Hard disk >> kernel buffer (quick copy to kernelsocket buffer) >>Protocol stack
1. The system calls sendfile() to copy the hard disk data to the kernel buffer through DMA, and then the data is directly copied by the kernel to another kernel buffer related to the socket. There is no switching between user mode and kernel mode. Copying from one buffer to another is completed directly in the kernel.
2. DMA copies data from the kernelbuffer directly to the protocol stack. There is no switching, and there is no need to copy data from user mode to kernel mode, because the data is in the kernel.

Article reference: http://www.th7.cn/system/lin/201306/41314.shtml

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