The disk caches examined in previous chapters used RAM as an extension of the disk; the goal was to improve system response time and the solution was to reduce the number of disk accesses. In this chapter, we introduce an opposite approach called swapping, in which the kernel uses some space on disk as an extension of RAM. Swapping is transparent to the programmer: once the swapping areas are properly installed and activated, the processes may run under the assumption that they have all the physical memory available that they can address, never knowing that some of their pages are stored away and retrieved again as needed.
Disk caches enhance system performance at the expense of free RAM, while swapping extends the amount of addressable memory at the expense of access speed. Thus, disk caches are "good" and desirable, while swapping should be regarded as some sort of last resort to be used whenever the amount of free RAM becomes too scarce.
We start by defining swapping in Section 16.1. Then we describe in Section 16.2 the main data structures introduced by Linux to implement swapping. We discuss the swap cache and the low-level functions that transfer pages between RAM and swap areas, and vice versa. The two crucial sections are Section 16.5, where we describe the procedure used to select a page to be swapped out to disk, and Section 16.6, where we explain how a page stored in a swap area is read back into RAM when the need occurs.
This chapter effectively concludes our discussion of memory management. Just one topic remains to be covered—namely, page frame reclaiming; this is done in the last section, which is related only in part to swapping. With so many disk caches around, including the swap cache, all the available RAM could eventually end up in these caches and no more free RAM would be left. We shall see how the kernel prevents this by monitoring the amount of free RAM and by freeing pages from the caches or from the process address spaces, as the need occurs.