Editing Memory paging (section)

==Thrashing==
{{Main|Thrashing (computer science)}}

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 (computer science)|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 freeing pages that will soon have to be re-read from secondary storage.{{efn|name=secondary}} "Thrashing" is also used in contexts other than virtual memory systems; for example, to describe [[cache (computing)|cache]] issues in computing or [[silly window syndrome]] in networking.

A worst case might occur on [[VAX]] processors. A single MOVL crossing a page boundary could have a source operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and a destination operand using a displacement deferred addressing mode, where the longword containing the operand address crosses a page boundary, and the source and destination could both cross page boundaries. This single instruction references ten pages; if not all are in RAM, each will cause a page fault. As each fault occurs the operating system needs to go through the extensive memory management routines perhaps causing multiple I/Os which might include writing other process pages to disk and reading pages of the active process from disk.  If the operating system could not allocate ten 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.