The Operating System Perspective

1. The Operating System Perspective

2. RTE's memory management services •  memory management mechanism, as viewed from the OS perspective •  opposed to the programming language perspective

3. Virtual Memory (VM) • scheme enables each process to have its own independent view of memory • OS (with help from the hardware) maps process memory into real memory • illusion that more virtual memory exists than physical memory

4. Virtual Memory (VM) • process is presented with a virtual address space, 0-232 • process thinks is running alone • CPU has no notion of virtual memory • MMU (memory management unit) – translates virtual address into physical and sends it to CPU

5. Virtual Memory (VM) • there is less RAM installed than 232 • we assume most processes will actually use less RAM than full virtual space. • when not enough memory in RAM, use disk and swap between RAM and disk • heavy penalty to go to disk to fetch data

6. Virtual Memory (VM) • locality of reference: processes cluster memory references to small subsets of VM • virtual memory (caching) is efficient

7. Paging - VM implementation • physical memory is broken down into pieces of small size 4K • physical memory can now be seen as an array of physical pages •  virtual address space is broken into pages of same size 4K • virtual page is mapped to physical page

9. Finding the Page of an Address • an address addrresides: • page number addr >> log(page size) addr/ page size • offsetaddr % page size

10. how to break down a virtual address into virtual page number and offset when using pages of 4k on a 32 bit CPU:

11. Page Tables • translation table for each process • maps virtual address space into the physical memory • OS a translation table for each process:Page Table

12. Page Tables • each row represents a virtual page and • column holds the physical page address in which the virtual page resides • register stores a pointer to table. • in context switch, osupdates register with current process' page table address

13. Address lifecycle • process executes code involving memory address, addr • MMU finds the virtual page number, vpn, in which addrresides • MMU locates vpnentry in page table - retrieves physical page numberppn • MMU replaces addrby paddrpaddr = (addr % page_size) + (ppn << log(page_size)

14. example • assume page size = 4096bytes (212 1<<12), • virtual address 0x80403040 • vpn = 0x8040 • by page table vpn->ppn=0x1020 • paddr= 0x10203040

15. TLB and Context Switches • TLB – translation lookasidebuffer • caches recent translation made by MMU. • when MMU needs to locate a physical page for a virtual one - first checks TLB • in a context switch, TLB is irrelevant

16. Demand Paging and Swap • process has memory size of 232 • what if each process access so much memory? • a process does not need all of its memory all of the time • swap unneeded data from memory, and restore it when needed

17. Implementation • metadata attribute for each page: • Access rights: either read, write, execute or a combination thereof • Present bit: is the page mapped to a physical page at all? • Dirty bit: has the page been written to since loading it into physical memory?

18. Present bit • OSevicts P from main memory - writes P  • OS sets present bit of P to 0 • if in MMU translate page is not present, MMU throws page fault interrupt - calls OS page fault handler • OS reads saved page from disk into physical page, and update page table

19. Present bit •  lazy page allocation: •  pretend to allocate the memory: OS updates the page tables of the process • the present bit is cleared from each of these pages

20. Dirty bit • a page needs to be evicted, but the page is not dirty. • if the page was already written to disk earlier, no need to re-copy to disk

21. Page Replacement Policy •  The operating system must decide which page to swap out • page replacement policy: usually relies on some kind of LRU (recently used list)