- Advantages of Segmentation
- Different protection for different segments read-only status for code
- Enables sharing of selected segments (see Fig. 4.17)
- Easier to relocate segments than entire address space
- Enables sparse allocation of address space
- Disadvantages of Segmentation
- Still expensive/difficult to allocate contiguous memory to segments
- External fragmentation: Wasted memory
- Paging; Allocation easier, Reduces fragmentation
- Advantages of Paging
- Fast to allocate and free;
- Alloc: Keep free list of free pages and grab first page in list, no searching by first-fit, best-fit
- Free: Add page to free list, no inserting by address or size
- Easy to swap-out memory to disk
- Page size matches disk block size
- Can swap-out only necessary pages
- Easy to swap-in pages back from disk
- Disadvantages of Paging
- Additional memory reference: Inefficient. Page table too large to store as registers in MMU. Page tables kept in main memory. MMU stores only base address of page table.
- Storage for page tables may be substantial
- Simple page table: Require entry for all pages in address space. Even if actual pages are not allocated
- Partial solution: Base and bounds (limits) for page table. Only processes with large address spaces need large page tables. Does not help processes with stack at top and heap at bottom.
- Internal fragmentation: Page size does not match allocation size
- How much memory is wasted (on average) per process?
- Wasted memory grows with larger pages
- Combine Paging and Segmentation
- Structure
- Segments correspond to logical units: code, data, stack. Segments vary in size and are often large
- Each segment contains one or more (fixed-size) pages
- Two levels of mapping to make tables manageable (2 look-ups!)
- Page table for each segment
- Base (real address) and bound (size) for each page table
- Segments
Pages Advantages
- Advantages of Segments
- Supports sparse address spaces. If segment is not used, no need for page table. Decreases memory required for page tables.
- Advantages of Paging
- Eliminate external fragmentation
- Segments to grow without any reshuffling
- Advantages of Both. Increases flexibility of sharing. Share at two levels: Page or segment (entire page table)
- Segments + Pages Disadvantages
- Internal fragmentation increases. Last page of every segment in every process
- Increases overhead of accessing memory
- Translation tables in main memory
- 1 or 2 overhead references for every real reference
- Large page tables
- Do not want to allocate page tables contiguously
- More problematic with more logical address bits
- Two potential solutions: Page the user page tables (multilevel page table), Inverted page table
2004-05-25
