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Thrashing (computer science)
In computer science, thrashing occurs when a computer's virtual memory subsystem is in a constant state of paging, rapidly exchanging data in memory for data on disk, to the exclusion of most application-level processing. This causes the performance of the computer to degrade or collapse. The situation may continue indefinitely until the underlying cause is addressed. The term is also used for various similar phenomena, particularly movement between other levels of the memory hierarchy, where a process progresses slowly because significant time is being spent acquiring resources.
==Overview==
If a process does not have enough pages, thrashing is a high paging activity, and the page fault rate is high. This leads to low CPU utilization. In modern computers, thrashing may occur in the paging system (if there is not sufficient physical memory or the disk access time is overly long), or in the communications system (especially in conflicts over internal bus access), etc. Depending on the configuration and algorithms involved, the throughput and latency of a system may degrade by multiple orders of magnitude. Thrashing is a state in which the CPU performs 'productive' work less and 'swapping' more. The CPU is busy in swapping pages, so much that it can not respond to users' programs as much as required. Thrashing occurs when there are too many pages in memory, and each page refers to another page. The real memory shortens in capacity to have all the pages in it, so it uses 'virtual memory'. When each page in execution demands that page that is not currently in real memory (RAM) it places some pages on virtual memory and adjusts the required page on RAM. If the CPU is too busy in doing this task, thrashing occurs.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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