How does operating system detect thrashing?

Answer 1

if your computer has a slow disk drive and you are doing a lot of paging (using virtual memory) to switch from one program to another rapidly, then your disk drive will become a performance bottleneck and your computer will seem to have trouble keeping up with your commands. The computer, here, is "thrashing", spending all of it's time trying to keep up. Imagine a person drowning. They are thrashing because they are spending all of their energy doing one thing to stay alive.