After studying this chapter, you should be able to: Discuss basic concepts related to concurrency, such as race conditions, OS concerns, and mutual exclusion requirements; understand hardware approaches to supporting mutual exclusion; define and explain semaphores; define and explain monitors. | Module 9: Virtual Memory Background Demand Paging Performance of Demand Paging Page Replacement Page-Replacement Algorithms Allocation of Frames Thrashing Other Considerations Demand Segmenation Operating System Concepts 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 Operating System Concepts 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 Operating System Concepts 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 . | Module 9: Virtual Memory Background Demand Paging Performance of Demand Paging Page Replacement Page-Replacement Algorithms Allocation of Frames Thrashing Other Considerations Demand Segmenation Operating System Concepts 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 Operating System Concepts 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 Operating System Concepts 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. During address translation, if valid–invalid bit in page table entry is 0 page fault. 1 1 1 1 0 0 0 Frame # valid-invalid bit page table Operating System Concepts 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 Operating System Concepts 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. Operating System Concepts Performance of Demand Paging Page Fault Rate 0 p if p = 0 no page faults if p = 1, every reference is a fault .
