Editing Thrashing (computer science) (section)

===Causes===
In [[virtual memory]] systems, thrashing may be caused by programs or workloads that present insufficient [[locality of reference]]: if the [[working set]] of a program or a workload cannot be effectively held within physical memory, then constant data swapping, ''i.e.,'' thrashing, may occur. The term was first used during the tape operating system days to describe the sound the tapes made when data was being rapidly written to and read.
A worst case might occur on [[VAX]] processors. A single <code>MOVL</code> 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. The total number of pages thus involved in this particular instruction is ten, and all ten pages must be simultaneously present in memory.  If any one of the ten pages cannot be [[Paging|swapped in]] (for example to make room for any of the other pages), the instruction will fault, and every attempt to restart it will fail until all ten pages can be swapped in.

A system thrashing is often a result of a sudden spike in page demand from a small number of running programs. Swap-token<ref>{{Cite conference |author= Song Jiang, and Xiaodong Zhang | title="Token-ordered LRU: an effective page replacement policy and its implementation in Linux systems" |conference=Performance Evaluation| year=2005 |pages = 5–29|doi=10.1016/j.peva.2004.10.002}}</ref> is a lightweight and dynamic thrashing protection mechanism. The basic idea is to set a token in the system, which is randomly given to a process that has page faults when thrashing happens. The process that has the token is given a privilege to allocate more physical memory pages to build its working set, which is expected to quickly finish its execution and release the memory pages to other processes. A timestamp is used to hand over the tokens one by one. The first version of swap-token is implemented in Linux[http://fxr.watson.org/fxr/source/mm/thrash.c?v=linux-2.6 .] The second version is called preempt swap-token[https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7602bdf2fd14a40dd9b104e516fdc05e1bd17952 .] In this updated swap-token implementation, a priority counter is set for each process to track the number of swap-out pages. The token is always given to the process with a high priority, which has a high number of swap-out pages. The length of the time stamp is not a constant but is determined by the priority: the higher the number of swap-out pages of a process, the longer the time stamp for it will be.