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10. Problem 1: Disk Thrashing

Disk Thrashing is a problem that may occur when virtual memory is being used. As main memory fills up, then more and more pages need to be swapped in and out of virtual memory. This swapping leads to a very high rate of hard disk access.

Compared to RAM, moving a hard disk head takes thousands of times longer. And so eventually more time is spent swapping the pages than on processing the data. If disk thrashing continues then most likely program will lock-up or not run.

It doesn't do the hard disk any good either as it is being accessed rapidly (thousands of times per second) by the memory manager. Sometimes you can hear the disk whirring away when disk thrashing occurs.

Usually the operating system will warn the user of 'Low virtual memory' to encourage them to shut down a few applications before everything grinds to a halt.