Sendfile
Now the popular Web server has the Sendfile option to improve server performance, what exactly is sendfile, how does it affect performance? Sendfile is actually a system call after linux2.0+, and the Web server can decide whether to take advantage of the Sendfile system call by adjusting its configuration. Let's take a look at the traditional network transfer process without sendfile:
Read (file,tmp_buf, Len);
Write (Socket,tmp_buf, Len);
HDD >> kernel buffer >> user buffer>> kernel socket buffer >> protocol stack
In general, a network application is to complete the network transmission by reading the hard disk data and then writing the data to the socket. The above 2 lines explain this in code, but the above 2 lines of simple code mask Many of the underlying operations. Let's see how the bottom line executes the above 2 lines of code:
1. The system calls read () to produce a context switch: switch from user mode to kernel mode, then perform a copy of the DMA and read the file data from the hard disk into a kernel buffer.
2. The data is copied from kernel buffer to user buffer, and then the system calls read () to return, then a context switch is generated: switch from kernel mode to user mode.
3. The system call write () produces a context switch: switch from user mode to kernel mode, and then read step 2 to the data copy of user buffer to kernel buffer (the 2nd copy of the data to kernel buffer), but this time A different kernel buffer, this buffer is associated with the socket.
4. The system calls write () to return a context switch: switch from kernel mode to user mode (the 4th switch), and then DMA copies the data from kernel buffer to the protocol stack (4th copy).
The above 4 steps have 4 context switches and 4 copies, and we find that reducing the number of switches and the number of copies will effectively improve performance. In the kernel2.0+ version, System call Sendfile () is used to simplify the above steps to improve performance. Sendfile () Not only reduces the number of transitions but also reduces the number of copies.
Take a look at the process of using sendfile () for network transmission:
Sendfile (Socket,file, Len);
HDD >> kernel buffer (fast copy to Kernelsocket buffer) >> protocol stack
1, the system call Sendfile () through the DMA to copy the hard disk data to kernel buffer, and then the data is kernel directly copied to another socket-related kernel buffer. There is no switch between user mode and kernel mode, and a copy from buffer to buffer is completed directly in the kernel.
2. DMA copies the data directly from the Kernelbuffer to the protocol stack, does not switch, and does not require the data to be copied 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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The above describes the Nginx sendfile parameter interpretation, including the aspects of the content, I hope that the PHP tutorial interested in a friend helpful.
