Advanced File I/O (
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If you're reading to take your understanding of Linux I/O file system calls to the next level, keep reading. This seven-part article series will show you everything from system calls to performance considerations. It is excerpted from chapter four of the book Linux System Programming: Talking Directly to the Kernel and C Library, written by Robert Love (O'Reilly, 2007; ISBN: 0596009585). Copyright © 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.
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 the
poll()
and
select()
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.
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