thrashing is caused by an excessive amount of page swapping between the main memory and secondary storage.
but how does the OS detect the problem(thrashing)?
and how to fix it?

and

what's the purpose of modified bit serve in demand paging?
is it want it to be loaded faster???... it seems that demand paging introduce the concept of loading partial of program into the memory and when begins the pages'll be loaded into the memory as they'r e needed.