Thrashing occurs when a computer spends too much time loading data into random access memory (RAM) and not enough time computing. In extreme cases, a computer's time may be virtually monopolized by thrashing.

What causes thrashing? Suppose one wishes to run a program too large to fit into available RAM. In principle, it is easy to do so: one simply loads and runs fragments of the program in sequence, as needed (perhaps repeatedly), until the whole program has been run. Suppose further that the program in question tends to alternate between two subroutines (chunks of code) for long stretches of computing time. If both subroutines can fit into RAM at once, all is well; otherwise, every time the computer finishes one routine it must load the other. In the latter case, since loading code from a hard drive or other archival memory device is time-consuming by computer standards...