Achieving Efficient Data Transfer with Zero‑Copy Techniques

During data transmission, each crossing between user space and kernel space normally incurs a memory copy, consuming CPU cycles and memory bandwidth. Zero‑copy technology allows an application to move data directly from a disk file to a socket without passing through user‑level buffers, dramatically improving performance and reducing context‑switch overhead.

Traditional data transfer involves four copies and multiple context switches: the application reads data into a kernel buffer, copies it to a user buffer, then copies it back to a kernel buffer for sending, and finally the kernel moves it to the protocol engine. Diagrams (Fig.1 and Fig.2) illustrate these steps.

Code example of the traditional approach:

File.read(fileDesc, buf, len);
Socket.send(socket, buf, len);

Because the data passes through an intermediate kernel buffer, performance can suffer when the data size exceeds the buffer capacity, leading to additional copies.

Zero‑copy method removes the unnecessary second and third copies by transferring data directly from the read buffer to the socket buffer. In Java, the transferTo() method of FileChannel achieves this:

public void transferTo(long position, long count, WritableByteChannel target);

The method relies on OS support for zero‑copy; on Unix/Linux it is routed to the sendfile() system call.

Data flow when using transferTo() is shown in Fig.3 and Fig.4.

In this approach, the DMA engine copies file contents to the read buffer, then the kernel moves the data directly into the socket’s kernel buffer, eliminating user‑space copies and reducing context switches.

Summary

Zero‑copy techniques such as Linux sendfile and splice , or Java’s FileChannel.transferTo , enable the kernel to transfer data from source to destination without intermediate user‑level copies. This improves network throughput, reduces CPU usage, and benefits many scenarios including file I/O, multimedia processing, and high‑performance networking, though developers must evaluate suitability for each specific use case.

Translated from: https://developer.ibm.com/articles/j-zerocopy/