Zero-copy

"Zero-copy" describes computer operations in which the CPU does not perform the task of copying data from one memory area to another. This is frequently used to save CPU cycles and memory bandwidth when transmitting a file over a network.

Principle

Zero-copy versions of operating system elements, such as device drivers, file systems, and network protocol stacks, greatly increase the performance of certain application programs and more efficiently utilize system resources. Performance is enhanced by allowing the CPU to move on to other tasks while data copies proceed in parallel in another part of the machine. Also, zero-copy operations reduce the number of time-consuming mode switches between user space and kernel space. System resources are utilized more efficiently since using a sophisticated CPU to perform extensive copy operations, which is a relatively simple task, is wasteful if other simpler system components can do the copying.

As an example, reading a file and then sending it over a network the traditional way requires two data copies and two context switches per read/write cycle. One of those data copies use the CPU. Sending the same file via zero copy reduces the context switches to two and eliminates all CPU data copies.