Virtual Memory: Page Replacement

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Virtual Memory Page Replacement
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Realizing Virtual Memory

• Hardware support
• Memory Management Unit (MMU) address
  translation, bits, interrupts
• Operating system support
• Page replacement policy
• Resident set management
• Load control
• degree of multiprogramming

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Page Replacement Policy

• Resident set maintenance
• Fixed or variable allocation
• Per-process or global replacement
• Page replacement problem
• A fixed number of frames, M, is used to map the
  process virtual memory pages
• Which page should be replaced when a page fault
  occurs and all M frames are occupied?

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Requirements and Metrics

• Workload a sequence of virtual memory references
  (page numbers)
• Page fault rate
• page faults/memory references
• Minimize the page fault rate for workloads
  obeying the principle of locality
• Keep hardware/software overhead as small as
  possible

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Algorithms

• Optimal (OPT)
• Least Recently Used (LRU)
• First-In-First-Out (FIFO)
• Clock

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Optimal Policy (OPT)

• Replace the page which will be referenced again
  in the most remote future
• Impossible to implement
• Why?
• Serves as a baseline for other algorithms

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Least Recently Used (LRU)

• Replace the page that has not been referenced for
  the longest time
• The best approximation of OPT for the locality
  constrained workloads
• Possible to implement
• Infeasible as the overhead is high
• Why?

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First-In-First-Out (FIFO)

• Page frames are organized in a circular buffer
  with a roving pointer
• Pages are replaced in round-robin style
• When page fault occur, replace the page to which
  the pointer points to
• Simple to implement, low overhead
• High page fault rate, prone to anomalous behavior

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Clock (second chance)

• Similar to FIFO but takes page usage into account
• Circular buffer page use bit
• When a page is referenced set use_bit1
• When a page fault occur For each page
• if use_bit1 give page a second chance
  use_bit0 continue scan
• if use_bit0 replace the page

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Example Page 727 is needed
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After replacement
0
n
Page 19 use 1
1
Page 9 use 1
Page 1 use 0
.
.
2
Page 45 use 0
.
next frame pointer
Page 191 use 0
Page 222 use 0
3
8
Page 727 use 0
Page 33 use 1
4
Page 13 use 0
Page 67 use 1
7
5
6
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Example of all algorithms
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LRU and non-local workloads

• Workload 1 2 3 4 5 1 2 3 4 5
• Typical for array based applications
• What is the page fault rate for M1,,5?
• A possible alternative is to use a Most Recently
  Use (MRU) replacement policy

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Beladys Anomaly

• It is reasonable to expect that regardless of a
  workload, the number of page faults should not
  increase if we add more frames not true for the
  FIFO policy

1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1
1
5
4
1
1
4
5
2
2
1
5
2
2
1
3
3
3
2
3
3
2
4
4
4
3
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Algorithm comparison
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Clock algorithm with 2 bits

• Use modified bit to evict unmodified (clean)
  pages in preference over modified (dirty) pages
• Four classes
• u0 m0 not recently used, clean
• u0 m1 not recently used, dirty
• u1 m0 recently used, clean
• u1 m1 recently used, dirty

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Clock algorithm with 2 bits

• First scan look for (0,0) frame, do not change
  the use bit
• If (0,0) frame is found, replace it
• Second scan look for (0,1) frame, set use bit to
  0 in each frame bypassed
• If (0,1) frame is found, replace it
• If all failed, repeat the above procedure
• this time we will certainly find something

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Page buffering

• Evicted pages are kept on two lists
• free and modified page lists
• Pages are read into the frames on the free page
  list
• Pages are written to disk in large chunks from
  the modified page list
• If an evicted page is referenced, and it is still
  on one of the lists, it is made valid at a very
  low cost

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Page Buffering
B
B
B
B
B
36 N
21 N
3 N
78 N
2 N
47 N
22 N
39 N
4 N
8 N
55 N
B
B
B
B
36 N
21 N
3 N
78 N
2 N
47 N
22 B
39 N
4 N
8 N
Buffered frames (B)
Page fault 55 is needed 22 is evicted
Normal frames (N)
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Resident set management

• With multiprogramming, a fixed number of memory
  frames are shared among multiple processes
• How should the frames be partitioned among the
  active processes?
• Resident set is the set of process pages
  currently allocated to the memory frames

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Global page replacement

• All memory frames are candidates for page
  eviction
• A faulting process may evict a page of other
  process
• Automatically adjusts process sizes to their
  current needs
• Problem can steal frames from wrong processes
• Leads to thrashing

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Local page replacement

• Only the memory frames of a faulting process are
  candidates for replacement
• Dynamically adjust the process allocation
• Working set model
• Page-Fault Frequency (PFF) algorithm

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The working set model Denning68

• Working set is the set of pages in the most
  recent page references

• Working set is an approximation of the program
  locality

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The working set strategy

• Monitor the working set for each currently active
  process
• Adjust the number of pages assigned to each
  process according to its working set size
• Monitoring working set is impractical
• The optimal value of is unknown and would vary

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Page-Fault Frequency (PFF)

• Approximate the page-fault frequency
• Count all memory references for each active
  process
• When a page fault occurs, compare the current
  counter value with the previous page fault
  counter value for the faulting process
• If lt F, expand the WS Otherwise, shrink the WS
  by discarding pages with use_bit0

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Swapping

• If a faulting process cannot expand its working
  set (all frames are occupied), some process
  should be swapped out
• The decision to swap processes in/out is the
  responsibility of the long/medium term scheduler
• Another reason not enough memory to run a new
  process

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Long (medium) term scheduling

• Controls multiprogramming level
• Decision of which processes to swap out/in is
  based on
• CPU usage (I/O bound vs. CPU bound)
• Page fault rate
• Priority
• Size
• Blocked vs. running

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UNIX process states
running user
schedule
sys. call interrupt
return
zombie
ready user
interrupt
running kernel
terminated
preempt
wait for event
schedule
ready kernel
event done
blocked
created
Swap out
Swap in
Swap out
ready swapped
blocked swapped
event done
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Next File system, disks, etc