Module 9 - Virtual memory. Virtual memory can be a very interesting subject since it has so many different aspects: page faults, managing the backing store, page replacement, frame allocation, thrashing, page size. The objectives of this chapter are to explain these concepts and show how paging works. | Lecture Operating system concepts Fifth edition Module 9 - Avi Silberschatz Peter Galvin Module 9 Virtual Memory Background Demand Paging Performance of Demand Paging Page Replacement Page-Replacement Algorithms Allocation of Frames Thrashing Other Considerations Demand Segmenation Silberschatz and Galvin 1999 Background Virtual memory separation of user logical memory from physical memory. Only part of the program needs to be in memory for execution. Logical address space can therefore be much larger than physical address space. Need to allow pages to be swapped in and out. Virtual memory can be implemented via Demand paging Demand segmentation Silberschatz and Galvin 1999 Demand Paging Bring a page into memory only when it is needed. Less I O needed Less memory needed Faster response More users Page is needed reference to it invalid reference abort not-in-memory bring to memory Silberschatz and Galvin 1999 Valid-Invalid Bit With each page table entry a valid invalid bit is associated 1 in-memory 0 not-in-memory Initially valid invalid but is set to 0 on all entries. Example of a page table snapshot. Frame valid-invalid bit 1 1 1 1 0 0 0 page table During address translation if valid invalid bit in page table entry is 0 page fault. Silberschatz and Galvin 1999 Page Fault If there is ever a reference to a page first reference will trap to OS page fault OS looks at another table to decide Invalid reference abort. Just not in memory. Get empty frame. Swap page into frame. Reset tables validation bit 1. Restart instruction Least Recently Used block move auto increment decrement location Silberschatz and Galvin 1999 What happens if there is no free frame Page replacement find some page in memory but not really in use swap it out. algorithm performance want an algorithm which will result in minimum number of page faults. Same page may be brought into memory several times. Silberschatz and Galvin 1999 Performance of Demand Paging Page Fault Rate
