While swapping (writing modified pages out to the system swap space) is a normal part of a system's operation, it is possible to experience too much swapping. The reason to be wary of excessive swapping is that the following situation can easily occur, over and over again:
Pages from a process are swapped
The process becomes runnable and attempts to access a swapped page
The page is faulted back into memory (most likely forcing some other processes' pages to be swapped out)
A short time later, the page is swapped out again
If this sequence of events is widespread, it is known as thrashing and is indicative of insufficient RAM for the present workload. Thrashing is extremely detrimental to system performance, as the CPU and I/O loads that can be generated in such a situation quickly outweigh the load imposed by a system's real work. In extreme cases, the system may actually do no useful work, spending all its resources moving pages to and from memory.