virtual memory 4 1 Zoom logistics recommend: exit full screen - PowerPoint PPT Presentation

least recently used (the good case) 3 choose to replace A D last accessed in 2 time units ago B last accessed in 1 time unit ago A last accessed in 3 time units ago choose to replace C C last accessed 3 time units ago B last accessed 1 time unit ago A last accessed 2 time units ago time referenced (virtual) pages: page# phys. D C B A 2 C A 1 B C B D A D B A C B 23

least recently used (the worst case) A C B A phys. page# time 1 B C 2 B C 3 C D 8 replacements with LRU versus 3 replacements with MIN: 3 D A A B C D A B C D B A C 1 A D C B 2 B 24

least recently used (exercise) [intro] A B A D C B D B C D A 1 2 3 25

least recently used (exercise) A D 3 B B B 2 A A A 1 A A D C B D B C D A B 26

least recently used (exercise) (2) A D D 3 C B B B 2 A A A A A 1 A D C B D B C D A B 28

least recently used (exercise) (3) C B B B C C C C C B 3 D D D D D D D 2 B A D B A D C B D B C A B 1 A A A A A B B 30

least recently used (exercise) (4) C B B B C C C C C C A 3 D D D D D D D D 2 B A D B A D C B D B C A B 1 A A A A A B B B 32

pure LRU implementation implementing LRU in software maintain doubly-linked list of all physical pages whenever a page is accessed: remove page from linked list, then add page to head of list whenever a page needs to replaced: remove a page from the tail of the linked list, then evict that page from all page tables (and anything else) and use that page for whatever needs to be loaded need to run code on every access probably 100+x slowdown? 33

pure LRU implementation implementing LRU in software maintain doubly-linked list of all physical pages remove page from linked list, then add page to head of list whenever a page needs to replaced: remove a page from the tail of the linked list, then evict that page from all page tables (and anything else) and use that page for whatever needs to be loaded need to run code on every access probably 100+x slowdown? 33 whenever a page is accessed:

so, what’s practical probably won’t implement LRU — too slow what can we practically do? 34

tools for tracking accesses approximating LRU = “was this accessed recently”? don’t need to detect all accesses, only one recent one “was this accessed since we started looking a few seconds ago?” ways to detect accesses AKA references: mark page invalid, if page fault happens make valid and record ‘accessed/referenced’ ‘accessed’ or ‘referenced’ bit set by HW 35

recording accesses goal: “check is this physical page still being used?” software support: temporarily mark page table invalid use resulting page fault to detect “yes” hardware support: accessed bits in page tables hardware sets to 1 when accessed 36

temporarily invalid PTE (software support) … … … … page table for program 1 PPN last known access? … … … … 0x04442 (never) … … … … OS page info processor does lookup oops! page fault update page info + mark present processor does lookup no page fault, not recorded in OS info OS clears present bit to check for next access processor does lookup oops! page fault update page info + mark present … 0x4442 mov 0x123 456, %ecx … mov 0x123 789, %ecx … … mov 0x123 300, %ecx program 1 … (OS exception’s handler) … the kernel VPN present? PPN 0x00000 0 --- … … 0 0 0x00123 … … … … --- --- … --- 0 0x00001 37 writable? …

temporarily invalid PTE (software support) … … … … page table for program 1 PPN last known access? … … … … 0x04442 at time X … … … … OS page info processor does lookup oops! page fault update page info + mark present processor does lookup no page fault, not recorded in OS info OS clears present bit to check for next access processor does lookup oops! page fault update page info + mark present … 0x4442 mov 0x123 456, %ecx … mov 0x123 789, %ecx … … mov 0x123 300, %ecx program 1 … (OS exception’s handler) … the kernel VPN present? PPN 0x00000 0 --- … … 0 1 0x00123 … … … … --- --- … --- 0 0x00001 37 writable? …

temporarily invalid PTE (software support) … … … … page table for program 1 PPN last known access? … … … … 0x04442 at time X … … … … OS page info processor does lookup oops! page fault update page info + mark present processor does lookup no page fault, not recorded in OS info OS clears present bit to check for next access processor does lookup oops! page fault update page info + mark present … 0x4442 mov 0x123 456, %ecx … mov 0x123 789, %ecx … … mov 0x123 300, %ecx program 1 … (OS exception’s handler) … the kernel VPN present? PPN 0x00000 0 --- … … 0 0 0x00123 … … … … --- --- … --- 0 0x00001 37 writable? …

temporarily invalid PTE (software support) … … … … page table for program 1 PPN last known access? … … … … 0x04442 at time Y … … … … OS page info processor does lookup oops! page fault update page info + mark present processor does lookup no page fault, not recorded in OS info OS clears present bit to check for next access processor does lookup oops! page fault update page info + mark present … 0x4442 mov 0x123 456, %ecx … mov 0x123 789, %ecx … … mov 0x123 300, %ecx program 1 … (OS exception’s handler) … the kernel VPN present? PPN 0x00000 0 --- … … 0 1 0x00123 … … … … --- --- … --- 0 0x00001 37 writable? …

accessed bit usage (hardware support) … … … … 0x00123 1 0 0 … 0x4442 … … … … … … page table for program 1 processor does lookup sets accessed bit to 1 processor does lookup keeps access bit set to 1 OS reads + records + clears access bit processor does lookup sets accessed bit to 1 (again) … … mov 0x123 456, %ecx present? mov 0x123 789, %ecx … … mov 0x123 300, %ecx program 1 … (OS exception’s handler) … the kernel VPN PPN --- 0x00000 0 --- --- … --- 0x00001 0 --- --- … 38 accessed? writable? …

accessed bits: multiple processes --- … … … --- … --- 0 … 0x00001 --- … --- --- 0 … … PPN … for the physical page all accessed bits OS needs to clear+check page table for program 2 … … … 0x00483 … … 0x4442 … 0 1 1 0x00000 present? VPN --- --- … --- --- 0 0x00001 … … --- --- 0 0x00000 PPN present? … … … … VPN page table for program 1 … … … … … 0x4442 … 0 0 1 0x00123 … … 39 accessed? writable? … accessed? writable? …

dirty bits “was this part of the mmap’d fjle changed?” “is the old swapped copy still up to date?” software support: temporarily mark read-only hardware support: dirty bit set by hardware same idea as accessed bit, but only changed on writes 40

x86-32 accessed and dirty bit A: acccessed — processor sets to 1 when PTE used used = for read or write or execute likely implementation: part of loading PTE into TLB D: dirty — processor sets to 1 when PTE is used for write 41

approximating LRU: second chance ordered list of physical pages ‘referenced’ bit set? “new” pages start at top of list yes, reset referenced bit and put back on list no, evict this page page made it to the bottom was it referenced in that time? yes — give a second chance page made it to the bottom was it referenced in that time? no — good choice to evict 42

second chance example (0) becomes referenced 2R 2R 1R 1R place A in physical page 1 accessed right after place B in physical page 2 1R accessed right after becomes referenced future slides: going to skip writing these intermediate steps (just for space) 1R 43 A C B C 1 A 2 B 3 page list — *1R 2NR *2R 3NR *3R last added 3NR 1NR 2NR 3NR 3NR end of list 1NR 2NR 2NR 3NR 3NR

second chance example (0) A (just for space) these intermediate steps going to skip writing future slides: becomes referenced accessed right after place B in physical page 2 place A in physical page 1 1R 1R 2R 2R 1R 1R 43 — 3 B C 1 A 2 B C *3R page list *1R 2NR *2R 3NR accessed right after → becomes referenced last added 3NR 1NR 2NR 3NR 3NR end of list 1NR 2NR 2NR 3NR 3NR

second chance example (0) A (just for space) these intermediate steps going to skip writing future slides: place B in physical page 2 becomes referenced accessed right after place A in physical page 1 1R 1R 2R 2R 1R 1R 43 — 3 B C 1 A 2 B C *3R page list *1R 2NR *2R 3NR accessed right after → becomes referenced last added 3NR 1NR 2NR 3NR 3NR end of list 1NR 2NR 2NR 3NR 3NR

second chance example (0) becomes referenced 2R 2R 1R 1R place A in physical page 1 accessed right after place B in physical page 2 1R accessed right after becomes referenced future slides: going to skip writing these intermediate steps (just for space) 1R 43 A C B C 1 A 2 B 3 page list — *1R 2NR *2R 3NR *3R last added 3NR 1NR 2NR 3NR 3NR end of list 1NR 2NR 2NR 3NR 3NR

second chance example (1) not referenced on return from page fault handler 1R 2R 3R 1R 2R 3R 1NR 2NR *2R place A in page 1 immediately referenced by program when page fault handler returns A page 2 was at bottom of list is not referenced okay to use page 1 was at bottom of list reference — give second chance moves to top of list clear referenced bit eventually page 1 gets to bottom of list again but now not referenced — use B referenced — fmips referenced bit 3NR — 1R 2 B C D — — — B 1 A D B *1R 3 C C page list last added *1R *2R *3R 44 1NR 2NR 3NR 1NR 2NR 3NR 3NR end of list 2NR 3NR

second chance example: exercise (1) 2R *3R *1R 1R — 3NR 1R 3R A 1R 2R 3R 1NR 2NR *2R exercise: What does this access to A replace? (D, B, or C?) what is at end of list after? (PP 1, 2, or 3?) *2R *1R last added page list B C D — — — B A 1 A D 2 B 3 C C 45 1NR 2NR 3NR 1NR 2NR 3NR 3NR end of list 2NR 3NR

second chance example: exercise (2) 3R *3R *1R 1R 2NR *3R — 3NR 1R 2R 1R A 2NR 1R 2R 3R 1NR 2NR *2R 3NR 1R exercise: What does this access to C replace? (D, B, or A?) what is at end of list after? (PP 1, 2, or 3?) *2R *1R last added page list B C D — — — B A — C 1 46 A D ? 2 B ? 3 C C A ? 1NR 2NR 3NR 1NR 2NR 3NR 3NR end of list 2NR 3NR

second chance example (2) 1R 1R 2NR *3R 1NR *2R — 3NR 1R 2R 3R 2NR A 3R 1NR 1R 2R 3R 1NR 2NR *2R 3NR 1R 2NR 3R *1R 48 *3R 1 B C D — — — B A — C — A *2R D 2 B C 3 C C A page list last added *1R 1NR 2NR 3NR 1NR 2NR 3NR 3NR end of list 2NR 3NR

second chance cons performs poorly with big memories… may need to scan through lots of pages to fjnd unaccessed likely to count accesses from a long time ago want some variation to tune its sensitivity one idea: smaller list of pages to scan for accesses 49

approximating LRU: SEQ know: not referenced ‘recently’ extra details needed: how big is the inactive list? this is current Linux algorithm for non-fjle pages or mark invalid + get fault scan reference bits detecting references? “new” pages start in active list evict page at bottom of inactive list active list move to active list not really inactive inactive page referenced? is really inactive page guess: oldest active page inactive list 50

tracking usage: CLOCK (view 1) ordered list add to top of list clear reference bit for next pass record current referenced bit take page from bottom of list periodically: of physical pages page #3: last referenced bits: Y Y N… page #4: last referenced bits: Y Y Y… page #2: last referenced bits: N N N… page #1: last referenced bits: Y Y Y… page #8: last referenced bits: Y Y N… page #7: last referenced bits: Y N Y… page #6: last referenced bits: N Y Y… page #5: last referenced bits: N N N… 51

tracking usage: CLOCK (view 2) last ref. bits: Y Y N… last ref. bits: Y Y N… page #8: last ref. bits: N N N… page #7: last ref. bits: Y Y Y… page #6: page #5: page #1: last ref. bits: Y N Y… page #4: last ref. bits: N Y Y… page #3: last ref. bits: N N N… page #2: last ref. bits: Y Y Y… 52

virtual memory 4 1 Zoom logistics recommend: exit full screen - PowerPoint PPT Presentation

virtual memory 4 1 Zoom logistics recommend: exit full screen open chat + participants window participants window has non-verbal feedback features I will try to monitor the chat window I can take questions via raise hand + turn on your

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