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4.4%20Page%20replacement%20algorithms

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Recently used at front; oldest at rear. ... Set all bits in row k of M to 1. Set all bits in column k of M to 0. Row with lowest binary value is least recently ... – PowerPoint PPT presentation

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Title: 4.4%20Page%20replacement%20algorithms


1
4.4 Page replacement algorithms
2
Page replacement algorithms
  • Also seen in
  • CPU cache
  • Web server cache of web pages
  • Buffered I/O (file) caches

3
Optimal page replacement
  • Page fault occurs
  • Scan all pages currently in memory
  • Determine which page wont be needed (referenced)
    until furthest in the future
  • Replace that page
  • Not really possible. (But useful as a
    benchmark.)
  • Depends on code as well as data.

4
Algorithms we will discuss
  1. Optimal
  2. NRU
  3. FIFO
  4. Second chance
  5. Clock
  6. LRU
  7. NFU
  8. Aging
  9. Working set
  10. WSClock

5
NRU (not recently used)
  • Bits set by hardware after every memory reference
  • Cleared only by software (OS)
  • R bits set when page is referenced (read or
    write)
  • M bits set when page is modified (written)
  • Periodically (after k clock interrupts), R bits
    are cleared

6
NRU page categories
  • Not referenced, not modified
  • Not referenced, modified
  • Occurs when 4s R bit is cleared during clock
    interrupt
  • Referenced, not modified
  • Referenced, modified
  • NRU algorithm remove random page from lowest
    numbered non empty class

7
NRU algorithm evaluation
  • Simple
  • Efficient
  • Not optimal but adequate

8
FIFO page replacement
  • Queue pages as they are requested.
  • Remove page at head (front) of queue.
  • Oldest page is removed first
  • simple/efficient
  • - might remove a heavily used page

9
Second chance page replacement
  • Inspect R bit of oldest page
  • If R0 then
  • page is old unused
  • Replace it
  • Else
  • Clear R bit
  • Move page to from head to tail of FIFO
  • Treating it as a newly loaded page
  • Try another page

10
Second chance page replacement
load time
page
11
Clock page replacement
  • Circular list instead of queue
  • Clock hand points to oldest page
  • If (R0) then
  • Page is unused
  • Replace it
  • Else
  • Clear R
  • Advance clock hand

12
Clock page replacement
13
LRU (least recently used) page replacement
  • Page recently used is likely to be used in the
    near future page not used in ages is not likely
    to be used in the near future.
  • Algorithm
  • age the pages
  • Maintain a queue of pages in memory.
  • Recently used at front oldest at rear.
  • Every time a page is referenced, it is removed
    from the queue and placed at the front of the
    queue.
  • This is slow!

14
LRU in hardware
  • implementation 1
  • 64 bit counter, C, incremented after every
    instruction
  • Each page also has a 64 bit counter
  • When a page is referenced, C is copied to its
    counter.
  • Page with lowest counter is oldest.

15
LRU in hardware
  • implementation 2
  • Given n page frames, let M be a nxn matrix of
    bits initially all 0.
  • Reference to page frame k occurs.
  • Set all bits in row k of M to 1.
  • Set all bits in column k of M to 0.
  • Row with lowest binary value is least recently
    used.

16
LRU in hardware implementation 2 example
oldest
17
NFU (Not Frequently Used)
  • Hardware doesnt often support LRU
  • Software counter associated w/ each page
    initially set to 0.
  • At each clock interrupt
  • Add R bit (either 0 or 1) to the counter for each
    page.
  • Page with lowest counter is NFU

18
NFU problem
  • It never forgets!
  • So pages that were frequently referenced (during
    initialization for example) but are no longer
    needed appear to be FU.
  • Solution (called aging)
  • Shift all counters to right 1 bit before R bit is
    added in.
  • Then R bit is added to MSb (leftmost bit) instead
    of LSb (rightmost bit).
  • Page w/ lowest value is chosen for removal.

19
NFU w/ aging
  1. Shift to right
  2. MSb R

00010000
20
Differences between LRU and NFU
  • LRU updated after every instruction so its
    resolution is very fine.
  • NFU is coarse (updated after n instructions
    execute between clock interrupts).
  • A given page referenced by n-1 instruction is
    given equal weight to a page referenced by only 1
    instruction (between clock interrupts).
  • n/2 references to a given page at the beginning
    of the interval are given equal weight with n/2
    references to another page at the end of the
    interval.

21
Working set page replacement algorithm
  • Demand paging start up processes with 0 pages
    and only load whats needed.
  • Locality of reference during any phase of
    execution, the process references only a
    relatively small fraction of its pages.
  • Working set set of pages that a process is
    currently using.
  • Thrashing causing a page fault every few
    instructions.

22
Working sets
  • Working set model make sure a page is in memory
    before the process needs it.
  • a.k.a. prepaging
  • w.s. set of pages used in the k most recent
    memory references.

23
Working set algorithm
  • Uses current virtual time amount of CPU time a
    process has actually used since it started.
  • T is a threshold on CVT
  • R and M bits as before clock interrupt

24
Working set algorithm
age current virtual time time of last use
? (greatest age/least virtual time) and choose
that one if no better candidate exists.
If no suitable candidate exists, pick one at
random.
25
WSClock page replacement
  • Previous WS algorithm requires entire page table
    be scanned at each page fault.
  • WSClock
  • Simple, efficient, widely used.
  • Uses circular list of page frames.

26
WSClock page replacement
  • At each page fault
  • Loop once through page table
  • Examine PTE pointed to by clock hand.
  • If r bit 1 then
  • clear r bit advance clock hand goto loop
  • else
  • If agegtt
  • If page is clean then use this page!
  • Else
  • write dirty page to disk advance clock hand
  • goto loop
  • If write scheduled, wait for completion and used
    that page.
  • Else pick a victim at random.

27
WSClock page replacement
clear r bit and advance clock hand.
Replace old and advance.
28
Summary of page replacement algorithms
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