After completing initialization, most programs operate on a small number of code and data pages compared to the total memory the program requires. The pages most frequently accessed are called the working set. When the working set is a small percentage of the system's total number of pages, virtual memory systems work most efficiently and an insignificant amount of computing is spent resolving page faults. As the working set grows, resolving page faults remains manageable until the growth reaches a critical point. Then faults go up dramatically and the time spent resolving them overwhelms time spent on the computing the program was written to do. This condition is referred to as thrashing. Thrashing occurs on a program that works with huge data structures, as its large working set causes continual page faults that drastically slow down the system. Satisfying page faults may require ing pages that will soon have to be re-read from disk. "Thrashing" is also used in contexts other than virtual memory systems; for example, to describe cache issues in computing or silly window syndrome in networking.
A worst case can be imagined on IBM System/370 and comparable mainframes. An execute instruction crossing a page boundary could point to a move instruction that also crosses a page boundary, which is set to move data from a source that crosses a page boundary to a target that crosses a page boundary. This single instruction references eight pages; if not all are in RAM, it will cause a page fault. If the operating system could not allocate eight pages to this program, then remedying the page fault would discard another page the instruction needs, and any restart of the instruction would fault again. To decrease excessive paging and resolve thrashing problems, a user can increase the number of pages available per program, either by running fewer programs concurrently or increasing the amount of RAM in the computer.
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