Question

Compare the segmented paging scheme with the hashed page table scheme for handling large address spaces. Under what circumstances is one scheme preferable to the other?

Answer 1

Answer:

Segmented Paging Scheme: It's a scheme that implements the combination of segmentation and paging. In this, the process is first divided into segments and then these segments are divided into pages and stored in the frames of the main memory. There exist a Page Table that keeps track of frames containing pages of a particular segment.

Advantages:

1. Only one entry for a segment in the Segment Table.
2. Memory usage is low.
3. Page Table size is limited by the size of the segment.
4. The external fragmentation problem is solved by this scheme.

Disadvantages:

1. It suffers from internal fragmentation.
2. Higher complexity level.

Hashed Page Table Scheme: Hashed Page Table Scheme is used when address space is greater than 32 bits. A virtual page number is hashed into a page table that contains an element hashing chain to the same location.

Advantages:

1. Used for larger address spaces.
2. No internal fragmentation.
3. Save memory

Disadvantages:

1. Traversing problems arise if many pages have the same page table entry.

Comparison between segmented paging scheme and hashed page table scheme:

When a small portion of large address space is occupied by the program then the use of the Hashed Page Table Scheme is preferred due to its problem arises. The problem arises in using Hashed Page Table Scheme when multiple pages onto the same page entry and traversing a list corresponding to that entry increases overhead, such overheads are minimal in the Segmented Paging Scheme where each page table entry maintains information of only one page.