Replacement of Page in O/S

The number of frames allocated to a process can also dynamically change depending on whether you have used global replacement or local replacement for replacing page in case of page fault.

1. Local replacement :-

when a process needs a page which is not in the memory, it can bring in the new page and allocate it a frame from its own set of allocated frames only.

Advantage :-

The pages in the memory for a particular process and the page fault ratio nis affacted by the paging behaviour of only that process.

Disadvantage :-

In local replacement the number of frames allocated to a process does not change so a low priority process may hender a high priority process by not making available to the high priority process its frames.

2. Global replacement :-

When a process needs a page which is not in the memory, it can bring in the new frame and allocate it a frame from the set of all frames, even if that frame is currently allocated to some other processes. i.e one process can take a frame from other.

Advantage :-

Does not hinder the performance of process and hence results in greater system throughput.

Disadvantage :-

The page fault ratio of a process can not be sololy controlled by the process itself. The page in the memory for a process depends on the paging behaviour of other processes as well.

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