When the operating system of a computer that has a
virtual memory system is too busy paging to be useful,
the system is said to be thrashing. If a computer is thrashing, it needs more memory added to it. One way to tell if a machine is thrashing is this: if the computer only has one hard disk, almost every attempt to use the machine is met with a long pause and loud crunching noises from the disk as the VM tries to page in the memory you currently need.