ThrashingThe system spends most of its time shuttling pages between main memory and secondary memory due to frequent page faults. This behavior is known as thrashing.

A process is thrashing if it is spending more time paging than executing. This leads to:

low CPU utilization and the operating system thinks that it needs to increase the degree of multiprogramming.

Global Versus Local Allocation

We can classify page-replacement algorithms into two broad categories: global replacement and local replacement.

Global replacement allows a process to select a replacement frame from the set of all frames, even if that frame is currently allocated to some other process; one process can take a frame from another.

Local replacement requires that each process selected from only its own set of allocated frames.

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