MMAP zero copy knowledge point notes

In the traditional way IO Send local files to the network

Co occurring 4 Second copy ,4 Context switching between subkernel state and user state .

• call read() Method , The program sinks from user mode to kernel mode .

• Copy the files in the disk to the kernel buffer （DMA copy）.

• Copy the data in the kernel buffer to the user buffer （CPU copy）, The program changes from kernel mode to user mode ,read() Method returns .

• call write() Method , The program sinks from user mode to kernel mode again , And copy the data in the user buffer to socket buffer （CPU copy）.

• take socket The data in the buffer is copied to the network card and sent （DMA copy）, The program changes from kernel mode to user mode ,write() Method returns .

DMA： Direct memory access （ For sharing CPU The pressure of copying ）

With mmap Send local files to the network

Co occurring 3 Second copy ,4 Context switching between subkernel state and user state .

• call mmap() Method , The program sinks from user mode to kernel mode .

• Copy the files in the disk to the kernel buffer （DMA copy）.

• The program changes from kernel mode to user mode ,mmap() Method returns . At the same time, the kernel buffer and user mode buffer establish a mapping relationship , Kernel buffer and user buffer realize memory sharing .

• call write() Method , The program sinks from user mode to kernel mode again . And copy the data in the kernel buffer directly to socket buffer （CPU copy）.

• take socket The data in the buffer is copied to the network card and sent （DMA copy）, The program changes from kernel mode to user mode ,write() Method returns .