Thrashing

Before closing, we address one final question: what should the OS do when memory is simply oversubscribed, and the memory demands of the set of running processes simply exceed the available physical memory? In this case, the system will constantly be paging, a condition sometimes referred to as thrashing“Virtual Memory” by Peter J. Denning. Computing Surveys, Vol. 2, No. 3, September 1970. Denning’s early and famous survey on virtual memory systems..

Admission control

Some earlier operating systems had a fairly sophisticated set of mechanisms to both detect and cope with thrashing when it took place. For example, given a set of processes, a system could decide not to run a subset of processes, with the hope that the reduced set of processes’ working sets (the pages that they are using actively) fit in memory and thus can make progress. This approach, generally known as admission control, states that it is sometimes better to do less work well than to try to do everything at once poorly, a situation we often encounter in real life as well as in modern computer systems (sadly).

Some current systems take a more draconian approach to memory overload. For example, some versions of Linux run an out-of-memory killer when memory is oversubscribed; this daemon chooses a memory-intensive process and kills it, thus reducing memory in a none-too-subtle manner. While successful at reducing memory pressure, this approach can have problems, if, for example, it kills the X server and thus renders any applications requiring the display unusable.