How does thrashing occur in virtual memory?

Thrashing in virtual memory occurs when a system spends more time swapping pages than executing processes.

Thrashing is a phenomenon that occurs in a virtual memory system when the system is excessively busy swapping pages of memory in and out of physical memory, rather than executing application programs. This situation arises when a computer's physical memory is overcommitted and it constantly needs to swap data in and out to accommodate the needs of the running processes.

Virtual memory allows a computer to use more memory than it physically has, by using a portion of the hard drive as if it were additional RAM. This is achieved through a process called paging, where data is transferred to and from the hard drive in fixed-sized blocks, or pages. However, when the system does not have enough physical memory to keep the necessary pages in memory, it has to constantly swap pages between RAM and the hard drive. This is known as page swapping.

When a system is thrashing, it is spending more time managing memory than doing useful work. This is because the operating system is spending a significant amount of processing power on deciding which pages to swap in and out, and then performing the actual swapping. This results in a significant decrease in system performance, as the system is effectively doing more administrative work than productive work.

The main cause of thrashing is poor memory management. If too many processes are running concurrently, or if a single process requires more memory than is available, the system can become overwhelmed and start thrashing. To prevent thrashing, operating systems use various memory management techniques. One common technique is the Least Recently Used (LRU) algorithm, which swaps out the pages that have not been used for the longest time. Another technique is to use a working set model, which keeps a certain number of pages in memory for each process, based on its recent memory usage.

In conclusion, thrashing is a situation that can severely degrade a system's performance. It is caused by overcommitment of physical memory, leading to excessive page swapping. Proper memory management techniques can help prevent thrashing and keep a system running smoothly.