Linux Memory Management<br />Case Study :                            Operating System<br />Muhammad  Ather  Rasool    IS/1...
Introduction:<br />Linux is a UNIX like system that has gained popularity in recent years.<br />Development begins in 1991...
Components of Linux Memory<br />Two Components<br />Physical memory:<br />kernel is loaded in memory<br />rest of memory i...
Paging<br />Page Size: Intel 4 KB, Alpha 8 KB<br />Linux kernel is the page allocator<br />Each process has its own page t...
Fetch Policy<br />Linux use a demand paging<br /> With no preparing <br />And no working set concept<br />There is a syste...
Linux Memory Management<br />Three level page table structure<br />Page directory<br />Page middle directory<br />Page tab...
7<br />Fig 1<br />
Placement Policy<br />All page frames are grouped into 10 lists of blocks that contain groups of 1, 2, 4, 8, 16, 32,64, 12...
Buddy Algorithm problem<br />Linux manages memory using the buddy algorithm. The algorithm leads to the internal fragmenta...
Fig 2:Buddy Algorithm<br />
Replacement Policy<br />Paging system  can divided into two sections <br />First policy algorithm decides which pages to w...
Clock Algorithm – Another Implementation of Second Chance<br />Order pages in circular list<br />“Hand” of the clock point...
Fig 3: Clock Algorithm for replacement<br />
Replacement Scope<br />Local replacement policy – chooses only among the resident pages of the process that generated page...
References<br />Picture:<br />Fig 1:operating System 4th Edition by William Stalling<br />Fig 2:Modern  operating Systems ...
G4 Group<br />
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  1. 1. Linux Memory Management<br />Case Study : Operating System<br />Muhammad Ather Rasool IS/17115/Aut-07/M<br />Muhammad Muzammil IS/17104/Aut-07/M<br />Muhammad Zahid Majeed IS/17105/Aut-07/M<br />Federal Urdu university of Arts, Science & Technology,. <br />
  2. 2. Introduction:<br />Linux is a UNIX like system that has gained popularity in recent years.<br />Development begins in 1991.<br />Two different GUI (GNOME & KDE) for Linux.<br />In early days, Linux development revolved around central OS kernel.<br />Linux kernel:<br /> Is an entirely original piece of SW developed from scratch by the Linux community.<br />
  3. 3. Components of Linux Memory<br />Two Components<br />Physical memory:<br />kernel is loaded in memory<br />rest of memory is available for user pages <br />Allocate and free physical memory pages<br />Virtual Memory:<br /> Each Linux process on a 32 bit get 3 GB for virtual addressing<br />Remaining 1 GB reserve for its Page Table and kernel data<br />Frequently accessed pages will be on the &quot;hot&quot; list<br />Free pages will be on the &quot;cold&quot; or &quot;free&quot; list<br />
  4. 4. Paging<br />Page Size: Intel 4 KB, Alpha 8 KB<br />Linux kernel is the page allocator<br />Each process has its own page table<br />The entire page table may take up too much main memory<br />Page tables are also stored in virtual memory<br />When a process is running, part of its page table is in main memory<br />
  5. 5. Fetch Policy<br />Linux use a demand paging<br /> With no preparing <br />And no working set concept<br />There is a system call in which user can give a hint that a certain page may be needed soon<br />Demand paging<br />
  6. 6. Linux Memory Management<br />Three level page table structure<br />Page directory<br />Page middle directory<br />Page table<br />6<br />
  7. 7. 7<br />Fig 1<br />
  8. 8. Placement Policy<br />All page frames are grouped into 10 lists of blocks that contain groups of 1, 2, 4, 8, 16, 32,64, 128, 256, and 512 contiguous page frames respectively<br /> If a small area is needed and only a larger area is available, the larger area is split into two Halves (buddies), possibly repeatedly<br />When a block is released, the kernel attempts to merge together pairs of free buddy<br />Buddy Algorithm<br />
  9. 9. Buddy Algorithm problem<br />Linux manages memory using the buddy algorithm. The algorithm leads to the internal fragmentation because if we want 65 page chunks w required 128 page chunks<br />For this problem to solve Linux has a second memory allocation which manages smaller units separately<br />
  10. 10. Fig 2:Buddy Algorithm<br />
  11. 11. Replacement Policy<br />Paging system can divided into two sections <br />First policy algorithm decides which pages to write out to disk, and when to write them.<br />Second the paging mechanism carries out the transfer and pages data back into physical memory when they are needed again.<br />Under Linux a clock is used, and every page has an age that is adjusted on each pass of the clock<br />Age is measure of youthfulness of pages<br />
  12. 12. Clock Algorithm – Another Implementation of Second Chance<br />Order pages in circular list<br />“Hand” of the clock points to the page to be replaced currently<br />When required to evict a page<br />If page pointed to has R=0, then evict it<br />If R=1, then reset R and move hand forward<br />Clock algorithm can be used with NRU (decision based on both R and M bits)<br />
  13. 13. Fig 3: Clock Algorithm for replacement<br />
  14. 14. Replacement Scope<br />Local replacement policy – chooses only among the resident pages of the process that generated page fault in selecting a page to replace<br />Global replacement policy – considers all unlocked pages in main memory as candidates for replacement, regardless of which process owns a particular page<br />Global policies are more attractive because of the simplicity of implementation and minimal overhead<br />
  15. 15. References<br />Picture:<br />Fig 1:operating System 4th Edition by William Stalling<br />Fig 2:Modern operating Systems 2nd Edition by A Tanenbaum.<br />Fig 3:operating System 4th Edition by William Stalling<br />Text:<br />Modern operating Systems 2nd Edition by A Tanenbaum.<br />operating System 4th Edition by William Stalling<br />
  16. 16. G4 Group<br />
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