Swap-space Management
- 2. Swap-Space Management • Swap-space — virtual memory uses disk space as an extension of main memory. • Main goal for the design and implementation of swap space is to provide the best throughput for VM system • Swap-space use –Swapping –use swap space to hold entire process image – Paging –store pages that have been pushed out of memory
- 3. Swap-Space Management(con..) • Some OS may support multiple swap- space –Put on separate disks to balance the load • Better to overestimate than underestimate –If out of swap-space, some processes must be aborted or system crashed
- 4. Swap-Space Location • Swap-space can be carved out of the normal file system, or in a separate disk partition. • A large file within the file system: simple but inefficient –Navigating the directory structure and the disk-allocation data structure takes time and potentially extra disk accesses
- 5. Swap-Space Location (con..) –External fragmentation can greatly increase swapping times by forcing multiple seeks during reading or writing of a process image –Improvement • Caching block location information in main memory • Contiguous allocation for the swap file –But, the cost of traversing FS data structure still remains
- 6. Swap-Space Location (con..) • In a separate partition: raw partition –Create a swap space during disk partitioning –A separate swap-space storage manager is used to allocate and de-allocate blocks –Use algorithms optimized for speed, rather than storage efficiency –Internal fragment may increase •Linux supports both approaches
- 7. Swap-space Management: Example • Solaris 1 –Text-segment pages are brought in from the file system and are thrown away if selected for paged out • More efficient to re-read from FS than write it to the swap space –Swap space: only used as a backing store for pages of anonymous memory
- 8. Example • Stack, heap, and uninitialized data • Solaris 2 –Allocates swap space only when a page is forced out of physical memory • Not when the virtual memory page is first created.
- 9. Data Structures for Swapping on Linux Systems
- 10. Data Structures for Swapping on Linux Systems