As long as our programs have good temporal locality,virtual memory systems work quite well. But of
course, not all programs exhibit good temporal locality .If the working set size exceeds the size of physical memory.then the program can produce an unfortunate situation known as thrashing, where pages are
swapped in and out continuously. Although virtual memory is usually efficient, if a program’s performance
slows to a crawl, the wise programmer will consider the possibility that it is thrashing