VIRTUAL MEMORY

VIRTUAL MEMORY

1. Tecnológico de Costa Rica, Carrera: Ingeniería en Computación, Sede: Cartago, Curso: Principios de sistemas operativos, Profesor: Esteban Arias Méndez, Estudiante: Jason David Espinoza Siles, Carné: 201223941, II semestre, 2016, Libro: Operating System Concepts: capítulo 9. Reporte #9

2. BACKGROUND

   • Code needs to be in memory to execute, but entire program rarely used  Entire program code not needed at same time  Consider ability to execute partially-loaded program
   • Virtual memory – separation of user logical memory from physical memory
   • Virtual address space – logical view of how process is stored in memory
     • Usually design logical address space for stack to start at Max logical address and grow “down” while heap grows “up”
     • System libraries shared via mapping into virtual address space

3. Demand Paging

   • Could bring entire process into memory at load time  Or bring a page into memory only when it is needed
   • Lazy swapper – never swaps a page into memory unless page will be needed

4. Valid-Invalid Bit

   • With each page table entry a valid–invalid bit is associated (v ⇒ in-memory – memory resident, i ⇒ not-in-memory)

5. PAGE FAULT

6. Performance of Demand Paging

   • OPTIMIZACIONES
     • Swap space I/O faster than file system I/O even if on the same device
     • Copy entire process image to swap space at process load time
     • Demand page in from program binary on disk, but discard rather than paging out when freeing frame

7. Copy-on-Write (COW) allows both parent and child processes to initially share the same pages in memory

8. Page Replacement

   • Prevent over-allocation of memory by modifying page- fault service routine to include page replacement  Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk  Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory

9. FIFO

10. OPTIMAL ALGORITHM

11. Least Recently Used (LRU) Algorithm

12. Enhanced Second-Chance Algorithm

13. Counting Algorithms

14. Page-Buffering Algorithms

15. Global vs. Local Allocation

    • Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another
    • Local replacement – each process selects from only its own set of allocated frames

16. Non-Uniform Memory Access

    • Optimal performance comes from allocating memory “close to” the CPU on which the thread is scheduled

17. TRASHING

    • Thrashing ≡ a process is busy swapping pages in and out

18. Page-Fault Frequency

    • Establish “acceptable” page-fault frequency (PFF) rate and use local replacement policy
    • Working Sets and Page Fault Rates

19. Memory-Mapped Files

    • Memory-mapped file I/O allows file I/O to be treated as routine memory access by mapping a disk block to a page in memory

20. Allocating Kernel Memory

21. Buddy System

    • BUDDY SYSTEM ALLOCATOR

22. Slab Allocator

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