What Is: Swapping? Advantages of Swapping

Swapping serves two main purposes:417 To expand the address space that is effectively usable by a process " To expand the amount of dynamic RAM (what is left of the RAM once the kernel code and static data structures have been initialized) to load processes Let's give a few examples of how swapping benefits the user. The simplest is when a program's data structures take up more space than the size of the available RAM. A swap area will allow this program to be loaded without any problem, thus to run correctly. A more subtle example involves users who issue several commands trying to simultaneously run large applications that require a lot of memory. If no swap area is active, the system might reject requests to launch a new application. In contrast, a swap area allows the kernel to launch it, since some memory can be freed at the expense of some of the already existing processes These two examples illustrate the benefits, but also the drawbacks, of swapping. Simulation of RAM is not like RAM in terms of performance. Every access by a process to a page that is currently swapped-out increases the process execution time by several orders of magnitude. In Understanding the Linux Kernel 418 short, if performance is of great importance, swapping should be used only as a last resort; adding RAM chips still remains the best solution to cope with increasing computing needs. It is fair to say, however, that, in some cases, swapping may be beneficial to the system as a whole. Long-running processes typically access only half of the page frames obtained. Even when some RAM is available, swapping unused pages out and using the RAM for disk cache can improve overall system performance. Swapping has been around for many years. The first Unix system kernels monitored the amount of free memory constantly. When it became less than a fixed threshold, they performed some swapping-out. This activity consisted of copying the entire address space of a process to disk. Conversely, when the scheduling algorithm selected a swapped-out process, the whole process was swapped in from disk. This approach has been abandoned by modern Unix kernels, including Linux, mainly because context switches are quite expensive when they involve swapping in swapped-out processes. To compensate for the burden of such swapping activity, the scheduling algorithm must be very sophisticated: it must favor in-RAM processes without completely shutting out the swapped-out ones. In Linux, swapping is currently performed at the page level rather than at the process address space level. This finer level of granularity has been reached thanks to the inclusion of a hardware paging unit in the CPU. We recall from Section 2.4.1 in Chapter 2, that each Page Table entry includes a Present flag: the kernel can take advantage of this flag to signal to the hardware that a page belonging to a process address space has been swapped out. Besides that flag, Linux also takes advantage of the remaining bits of the Page Table entry to store the location of the swapped-out page on disk. When a "Page fault" exception occurs, the corresponding exception handler can detect that the page is not present in RAM and invoke the function that swaps the missing page in from the disk. Much of the algorithm's complexity is thus related to swapping-out. In particular, four main issues must be considered: " Which kind of page to swap out " How to distribute pages in the swap areas " How to select the page to be swapped out " When to perform page swap-out Let us give a short preview of how Linux handles these four issues before describing the main data structures and functions related to swapping.