03/28/2007csci 315 operating systems design1 page replacement algorithms (virtual memory)
TRANSCRIPT
![Page 1: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/1.jpg)
03/28/2007 CSCI 315 Operating Systems Design 1
Page Replacement Algorithms(Virtual Memory)
![Page 2: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/2.jpg)
03/28/2007 CSCI 315 Operating Systems Design 2
Servicing a Page Fault
i
free frame
operatingsystem
1
6
3
2
45
physicalmemory
disk
page tablerunningprocess
![Page 3: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/3.jpg)
03/28/2007 CSCI 315 Operating Systems Design 3
No free frame: now what?
• Page replacement: Are all those pages in memory being referenced? Choose one to swap back out to disk and make room to load a new page.– Algorithm: How you choose a victim.– Performance: Want an algorithm which will result in
minimum number of page faults.
• Side effect: The same page may be brought in and out of memory several times.
![Page 4: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/4.jpg)
03/28/2007 CSCI 315 Operating Systems Design 4
Page Replacement
• Prevent over-allocation of memory by modifying page-fault service routine to include page replacement.
• Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk.
• Page replacement completes separation between logical memory and physical memory – large virtual memory can be provided on a smaller physical memory.
![Page 5: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/5.jpg)
03/28/2007 CSCI 315 Operating Systems Design 5
Basic Page Replacement
1. Find the location of the desired page on disk.
2. Find a free frame:- If there is a free frame, use it.- If there is no free frame, use a page
replacement algorithm to select a victim frame.
3. Read the desired page into the (newly) free frame. Update the page and frame tables.
4. Restart the process.
![Page 6: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/6.jpg)
03/28/2007 CSCI 315 Operating Systems Design 6
Page Replacement Algorithms• Goal: Produce a low page-fault rate.• Evaluate algorithm by running it on a particular
string of memory references (reference string) and computing the number of page faults on that string.
• The reference string is produced by tracing a real program or by some stochastic model. We look at every address produced and strip off the page offset, leaving only the page number. For instance:
1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
![Page 7: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/7.jpg)
03/28/2007 CSCI 315 Operating Systems Design 7
Graph of Page Faults Versus The Number of Frames
![Page 8: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/8.jpg)
03/28/2007 CSCI 315 Operating Systems Design 8
FIFO Page Replacement• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5.• 3 frames (3 pages can be in memory at a time per process)
• 4 frames
• FIFO Replacement Belady’s Anomaly: more frames less page faults.
1
2
3
1
2
3
4
1
2
5
3
4
9 page faults
1
2
3
1
2
3
5
1
2
4
5 10 page faults
44 3
![Page 9: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/9.jpg)
03/28/2007 CSCI 315 Operating Systems Design 9
FIFO Page Replacement
![Page 10: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/10.jpg)
03/28/2007 CSCI 315 Operating Systems Design 10
FIFO (Belady’s Anomaly)
![Page 11: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/11.jpg)
03/28/2007 CSCI 315 Operating Systems Design 11
Optimal Algorithm• Replace the page that will not be used for longest period
of time. (How can you know what the future references will be?)
• 4 frames example: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• Used for measuring how well your algorithm performs.
1
2
3
4
6 page faults
4 5
![Page 12: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/12.jpg)
03/28/2007 CSCI 315 Operating Systems Design 12
Optimal Page Replacement
![Page 13: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/13.jpg)
03/28/2007 CSCI 315 Operating Systems Design 13
LRU Algorithm• Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
• Counter implementation:– Every page entry has a counter; every time page is referenced through
this entry, copy the clock into the counter.
– When a page needs to be changed, look at the counters to determine which are to change.
1
2
3
5
4
4 3
5
![Page 14: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/14.jpg)
03/28/2007 CSCI 315 Operating Systems Design 14
LRU Page Replacement
![Page 15: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/15.jpg)
03/28/2007 CSCI 315 Operating Systems Design 15
LRU Algorithm (Cont.)
• Stack implementation – keep a stack of page numbers in a double link form:– Page referenced:
• move it to the top• requires 6 pointers to be changed
– No search for replacement.
![Page 16: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/16.jpg)
03/28/2007 CSCI 315 Operating Systems Design 16
Use Of A Stack to Record The Most Recent Page References
![Page 17: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/17.jpg)
03/28/2007 CSCI 315 Operating Systems Design 17
LRU Approximation Algorithms
• Reference bit– With each page associate a bit, initially = 0
– When page is referenced bit set to 1.
– Replace the one which is 0 (if one exists). We do not know the order, however.
• Second chance– Need reference bit.
– Clock replacement.
– If page to be replaced (in clock order) has reference bit = 1. then:
• set reference bit 0.• leave page in memory.• replace next page (in clock order), subject to same rules.
![Page 18: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/18.jpg)
03/28/2007 CSCI 315 Operating Systems Design 18
Second-Chance (clock) Page-Replacement Algorithm
![Page 19: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/19.jpg)
03/28/2007 CSCI 315 Operating Systems Design 19
Counting Algorithms
• Keep a counter of the number of references that have been made to each page.
• LFU Algorithm: replaces page with smallest count.
• MFU Algorithm: based on the argument that the page with the smallest count was probably just brought in and has yet to be used.
![Page 20: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/20.jpg)
03/28/2007 CSCI 315 Operating Systems Design 20
Allocation of Frames
• Each process needs a minimum number of pages.
• There are two major allocation schemes:– fixed allocation– priority allocation
![Page 21: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/21.jpg)
03/28/2007 CSCI 315 Operating Systems Design 21
Fixed Allocation• Equal allocation – e.g., if 100 frames and 5 processes, give
each 20 pages.• Proportional allocation – Allocate according to the size of
process.
mSs
pa
m
sS
ps
iii
i
ii
for allocation
frames of number total
process of size
5964137127
56413710
127
10
64
2
1
2
a
a
s
s
m
i
![Page 22: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/22.jpg)
03/28/2007 CSCI 315 Operating Systems Design 22
Priority Allocation
• Use a proportional allocation scheme using priorities rather than size.
• If process Pi generates a page fault,
– select for replacement one of its frames.– select for replacement a frame from a process
with lower priority number.
![Page 23: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/23.jpg)
03/28/2007 CSCI 315 Operating Systems Design 23
Global vs. Local Allocation
• Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another.
• Local replacement – each process selects from only its own set of allocated frames.
![Page 24: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/24.jpg)
03/28/2007 CSCI 315 Operating Systems Design 24
Thrashing
• If a process does not have “enough” pages, the page-fault rate is very high. This leads to:– Low CPU utilization.– Operating system thinks that it needs to increase the
degree of multiprogramming.– Another process added to the system.
• Thrashing a process is busy swapping pages in and out.
![Page 25: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/25.jpg)
03/28/2007 CSCI 315 Operating Systems Design 25
Thrashing
• Why does paging work?Locality model– Process migrates from one locality to another.– Localities may overlap.
• Why does thrashing occur? size of locality > total memory size
![Page 26: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/26.jpg)
03/28/2007 CSCI 315 Operating Systems Design 26
Locality in Memory-Reference Pattern
![Page 27: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/27.jpg)
03/28/2007 CSCI 315 Operating Systems Design 27
Working-Set Model working-set window a fixed number of page
references.
• WSSi (working set of Process Pi) =total number of pages referenced in the most recent (varies in time)– if too small will not encompass entire locality.– if too large will encompass several localities.– if = will encompass entire program.
• D = WSSi total demand frames
• if D > m Thrashing• Policy if D > m, then suspend one of the processes.
![Page 28: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/28.jpg)
03/28/2007 CSCI 315 Operating Systems Design 28
Working-set model
![Page 29: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/29.jpg)
03/28/2007 CSCI 315 Operating Systems Design 29
Keeping Track of the Working Set
• Approximate with interval timer + a reference bit• Example: = 10,000
– Timer interrupts after every 5000 time units.– Keep in memory 2 bits for each page.– Whenever a timer interrupts copy and sets the values of all
reference bits to 0.– If one of the bits in memory = 1 page in working set.
• Why is this not completely accurate?• Improvement = 10 bits and interrupt every 1000 time
units.
![Page 30: 03/28/2007CSCI 315 Operating Systems Design1 Page Replacement Algorithms (Virtual Memory)](https://reader036.vdocument.in/reader036/viewer/2022062417/5519c817550346695e8b4999/html5/thumbnails/30.jpg)
03/28/2007 CSCI 315 Operating Systems Design 30
Page-Fault Frequency Scheme
Establish “acceptable” page-fault rate.– If actual rate too low, process loses frame.– If actual rate too high, process gains frame.