Demand Paging

1
Demand Paging

• Virtual memory indirect addressing demand
  paging
• Without demand paging, indirect addressing by
  itself is valuable because it reduces external
  fragmentation
• Demand paging attributes
• Process may run with less than all pages in
  memory
• Unused portion of process address space resides
  on backing store (on disk)
• Pages are loaded into memory from backing store,
  and visa versa upon page eviction
• Pages loaded into memory as they are referenced
  (or demanded) less useful pages are evicted to
  make room

2
Demand Paging Solves

• Insufficient memory for single process
• Insufficient memory for several processes during
  multiprogramming
• Relocation during execution via paging
• Efficient use of memory only active subset of
  process pages are in memory (unavoidable waste
  is internal to pages)
• Ease of programming no need to be concerned
  with partition starting address and limits
• Protection separate address spaces enforced by
  translation of hardware
• Sharing map same physical page into more than 1
  address space
• Less disk activity than with swapping alone
• Fast process start-up process can run with as
  little as 1 page
• Higher CPU utilization since many partially
  loaded processes can be in memory simultaneously

3
Data Structures for Demand Paging

• Page tables location of pages that are in
  memory
• Backing store map location of pages that are
  not in memory
• Physical memory map (frame map)
• Fast physical to virtual mapping (to invalidate
  translations for page that is no longer in
  memory)
• Allocation/use of page frames in use or free
• Number of page frames are allotted to each
  process
• Cache of page tables Translation Lookaside
  Buffer (TLB)

4
Hardware Influence on Demand Paging

• TLB is special hardware
• Base/limit registers describe kernels and
  current process page table area
• Size of PTE and meaning of certain bits in PTE
  (present/absent, protection, referenced, modified
  bits)

5
Virtual Memory Policies

• Fetch policy which pages to load and when?
• Page replacement policy which pages to
  remove/overwrite and when in order to free up
  memory space?

6
Fetch Policy

• Demand paging pages are loaded on demand, not
  in advance
• Process starts up with none of its pages loaded
  in memory page faults load pages into memory
• Initially many page faults
• Context switching may clear pages and will cause
  faults when process runs again

7
Fetch Policy

• Pre-paging load pages before process runs
• Need a working set of pages to load on context
  switches
• Few systems use pure demand paging, but instead,
  does pre-paging
• Thrashing occurs when program causes page fault
  every few instructions
• What can be done to alleviate problem?

8
Page Replacement Policy

• Question of which page to evict when memory is
  full and a new page is demanded
• Goal reduce the number of page faults
• Why? Page fault is very expensive 10 msec. to
  fetch from disk on a 100 MIPS machine, 10 msec.
  is equal to 1 Million instructions

9
Page Replacement Policy

• Demand paging is likely to cause a large number
  of page faults, especially when a process starts
  up
• Locality of reference saves demand paging
• Locality of reference next reference more
  likely to be near last one
• Reasons
• Next instruction in stream is close to the last
• Small loops
• Common subroutines
• Locality in data layout (arrays, matrices of
  data, fields of structure are contiguous)
• Sequential processing of files and data
• With locality of reference, a page that is
  brought in by one instruction is likely to
  contain the data required by the next instruction

10
Page Replacement Policy

• Policy can be global (inter-process) or local
  (per-process)
• Policies
• Optimal replace page that will be used farthest
  in the future (or never used again)
• FIFO variants might throw out important pages
• Second chance, clock
• LRU variants difficult to implement exactly
• NFU crude approximation to LRU
• Aging efficient and good approximation to LRU
• NRU crude, simplistic version of LRU
• Working set expensive to implement
• Working set clock efficient and most widely
  used in practice

11
Page Replacement Policy NRU

• Not recently used a poor mans LRU
• Uses 2 bits (can be implemented in HW)
• R page has been referenced
• M page has been modified
• OS clears R-bit periodically
• R0 means pages are cold
• R1 means pages are hot or recently referenced
• When free pages are needed, sweep through all of
  memory and reclaim pages based on R and M
  classes
• 00 not referenced, not modified
• 01 not referenced, modified
• 10 referenced, not modified
• 11 referenced and modified
• Pages should be removed in what order?
• How can class 01 exist if it was modified,
  shouldnt the page have been referenced?

12
Page Replacement Policy FIFO

• First-in, first-out
• Easy to implement, but does not consider page
  usage pages that are frequently referenced
  might be removed
• Keep linked list representing order in which
  pages have been brought into memory

13
Page Replacement Policy Second Chance

• Variant of FIFO
• When free page frames are needed, examine pages
  in FIFO order starting from the beginning
• If R0, reclaim page
• If R1, set R0 and place at the end of FIFO list
  (hence, the second chance)
• If not enough reclaims on first pass, revert to
  pure FIFO on second pass

14
Page Replacement Policy Clock

• Variant of FIFO, better implementation of Second
  Chance
• Pages are arranged in circular list pages never
  moved around the list
• When free page frames are needed, examine pages
  in clock-wise order from current position
• If R0, reclaim page
• If R1, set R0 and advance hand

15
Page Replacement Policy Clock

• 2-hand clock variant
• one hand changes R from 1 to 0
• second hand reclaims pages
• How is this different from 1-hand clock?
• With 1 hand, time between cycles of the hand is
  proportional to memory size
• With 2 hands, time between changing R and
  reclaiming pages can be varied dynamically
  depending on the momentary need to reclaim page
  frames

16
Page Replacement Policy LRU

• Keep linked list representing order in which
  pages have been used
• On potentially every reference, find referenced
  page in the list and bring it to the front very
  expensive!
• Can do LRU faster with special hardware
• On reference store time (or counter) in PTE find
  page with oldest time (or lowest counter) to
  evict
• NxN Matrix algorithm
• Initially set NxN bits to 0
• When page frame K is referenced,
• Set all bits of row K to 1
• Set all bits of column K to 0
• Row with lowest binary value is LRU
• But, hardware is often not available!

17
Page Replacement Policy NFU and Aging

• Simulating LRU in software
• NFU
• At each clock interrupt, add R bit (0 or 1) to
  counter associated with page
• Reclaim page with lowest counter
• Disadvantage no aging of pages
• Aging
• Shift counter to the right, then add R to the
  left recent R bit is added as most significant
  bit, thereby dominating the counter value
• Reclaim page with lowest counter
• Acceptable disadvantages
• Cannot distinguish between references early in
  clock interval from latter references since shift
  and add is done to all counters at once
• With 8-bit counter, have only a memory of 8 clock
  ticks back

18
Page Replacement Policy Working Set

• Working set set of pages process is currently
  using
• As a function of k most recent memory references,
  w(k,t) is working set at any time t
• As a function of past e msec. of execution time,
  w(e,t) is set of pages process referenced in the
  last e msec. of process execution time
• Replacement policy find page not in working set
  and evict it

19
Page Replacement Policy Working Set
Implementation

• If R0, page is candidate for removal
• Calculate age current time time of last use
• If age threshold, page is reclaimed
• If age be removed if it is oldest page in working set
• If R1, set time of last use current time
• Page was recently referenced, so in working set
• If no page has R0, choose random page for
  removal (one that requires no writeback)

20
Page Replacement Policy Working Set Clock

• Use circular list of page frames
• If R0, age threshold, and
• Page is clean (M0), reclaim
• Page is dirty (M1), schedule write but advance
  hand to check other pages
• If R1, set R0 and advance hand
• At end of 1st pass, if no page has been
  reclaimed
• If write has been scheduled, keep moving hand
  until write is done and page is clean. Evict 1st
  clean page
• If no writes scheduled, claim any clean page even
  though it is in the working set

21
Page Replacement Policy Summary

• FIFO easy to implement, but does not account for
  page usage
• Because of locality of reference, LRU is a good
  approximation to optimal
• Naïve LRU has high overhead update some data
  structure on every reference!
• Practical algorithms
• Approximate LRU
• Maintain list of free page frames for quick
  allocation
• When number of page frames in free list falls
  below a low water mark (threshold), invoke page
  replacement policy until number of free page
  frames goes above high water mark

22
Page Replacement Policy Evaluation Metrics

• Results page fault rate over some workload
• Speed work that must be done on each reference
• Speed work that must be done when page is
  reclaimed
• Overhead storage required by algorithms
  bookkeeping and special hardware required