Scatter/Gather I/O

Chapter 4
CHAPTER 4
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 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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