Advanced File I/O

In Chapter 2, we looked at the basic I/O system calls in Linux. These calls form not only the basis of file I/O, but also the foundation of virtually all communication on Linux. In Chapter 3, we looked at how user-space buffering is often needed on top of the basic I/O system calls, and we studied a specific user-space buffering solution, C’s standard I/O library. In this chapter, we’ll look at the more advanced I/O system calls that Linux provides:

Scatter/gather I/O

Allows a single call to read or write data to and from many buffers at once; useful for bunching together fields of different data structures to form one I/O transaction.

Epoll

Improves on thepoll()andselect() system calls described in Chapter 2; useful when hundreds of file descriptors have to be polled in a single program.

Memory-mapped I/O

Maps a file into memory, allowing file I/O to occur via simple memory manipulation; useful for certain patterns of I/O.

File advice

Allows a process to provide hints to the kernel on its usage scenarios; can result in improved I/O performance.

Asynchronous I/O

Allows a process to issue I/O requests without waiting for them to complete; useful for juggling heavy I/O workloads without the use of threads.

The chapter will conclude with a discussion of performance considerations and the kernel’s I/O subsystems.