Understanding the Linux Kernel, 3rd Edition

The Page Frame Reclaiming Algorithm

One of the fascinating aspects of Linux is that the checks performed before allocating dynamic memory to User Mode processes or to the kernel are somewhat perfunctory.

No rigorous check is made, for instance, on the total amount of RAM assigned to the processes created by a single user (the limits mentioned in the section "Process Resource Limits" in Chapter 3 mostly affect single processes). Similarly, no limit is placed on the size of the many disk caches and memory caches used by the kernel.

This lack of controls is a design choice that allows the kernel to use the available RAM in the best possible way. When the system load is low, the RAM is filled mostly by the disk caches and the few running processes can benefit from the information stored in them. However, when the system load increases, the RAM is filled mostly by pages of the processes and the caches are shrunken to make room for additional processes.

As we saw in previous chapters, both memory and disk caches grab more and more page frames but never release any of them. This is reasonable because cache systems don’t know if and when processes will reuse some of the cached data and are therefore unable to identify the portions of cache that should be released. Moreover, thanks to the demand paging mechanism described in Chapter 9, User Mode processes get page frames as long as they proceed with their execution; however, demand paging has no way to force processes to release the page frames ...