ICS 143
PRINCIPLES OF OPERATING SYSTEMS
Discussion Section Week 10
Today
2
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Announcements
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Lecture Review
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Pop Quiz #3
6/1/2015
Announcements
3
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Please fill out your EEE instructor evaluation.
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Schedule switch-up in Week 10:
Regular class on Wed, 3 June 2015
 Two classes on Fri, 5 June 2015 at 11:00am and 3:00pm in
the usual rooms
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Homework #4:
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Programming Assignment
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Due Thu, 4 June 2015, 11:55pm via EEE Dropbox
Due Wed, 10 June 2015, 11:55pm via EEE Dropbox
Final
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Tue, 9 June 2015, 1:30-3:30pm in EH 1200
6/1/2015
Memory Segmentation
4
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Separate user’s view of memory and
the actual physical memory
Division of a computer's primary
memory into segments (or sections)
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subroutine
segment 3
A reference to a memory location
includes a value that identifies a
segment and an offset within that
segment
A logical address space is a collection
of segments
A segment is a logical unit

segment 0
e.g. main program, procedure, function,
local variables, global variables,
common block
<segment-number, offset>
stack
Sqrt
segment 1
segment 2
main
program
logical address space
6/1/2015
Segment Table
5
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Maps two-dimensional user-defined addresses into
one-dimensional physical addresses.
Each table entry has:
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Segment-table base register (STBR):
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Base - contains the starting physical address where the
segments reside in memory.
Limit - specifies the length of the segment.
points to the segment table’s location in memory.
Segment-table length register (STLR):

indicates the number of segments used by a program;
segment number is legal if s < STLR.
6/1/2015
Segment Table (2)
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limit
offset
segment 0
segment 2
segment number
base
0 1000
1400
1 400
6300
2 500
3600
<segment,offset>
A: <2,300>
B: <0,1200>
points to?
segment 1
A: 3600+300=3900
B: invalid
6/1/2015
Segmentation Architecture
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Relocation is dynamic - by segment table
Sharing
 Code
sharing occurs at the segment level.
 Shared segments must have same segment number.
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Allocation - dynamic storage allocation problem
 use
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best fit/first fit, may cause external fragmentation
Protection
 protection
bits associated with segments
 read/write/execute
privileges
 array in a separate segment - hardware can check for
illegal array indexes.
Virtual Memory
8
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Goal of memory management:
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Virtual Memory:
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keep many processes in memory
simultaneously to allow multiprogramming
Problem: require entire process to be in
memory before process can execute
Only part of the program needs to be in
memory for execution
Programs can be larger than physical memory
Separates user logical memory from physical
memory
Allows easy sharing of files and address
spaces
Virtual Memory can be implemented via
demand paging or demand segmentation
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Demand Paging
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Similar to a paging system
with swapping
Bring a page into memory
only when it is needed.
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Less I/O needed
Less Memory needed
Faster response
More users
The first reference to a
page will trap to OS with a
page fault.
OS looks at another table to
decide

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Invalid reference - abort
Just not in memory
paged
main memory
large secondary
storage
Valid-Invalid Bit
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With each page table
entry a valid-invalid bit
is associated (1  inmemory, 0  not in
memory).
Initially, valid-invalid bit
is set to 0 on all entries.
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During address
translation, if validinvalid bit in page table
entry is 0 --- page fault
occurs.
Page Replacement
11
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What happens if there is no free frame?
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Page replacement - find some page in memory that is not really
in use and swap it.
Handling a page fault:
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Find the location of the desired page on disk.
Find a free frame:
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If there is a free frame, use it.
If there is no free frame, use a page-replacement algorithm to select
a victim frame.
Write the victim page to disk; change the page and frame tables
accordingly
Read the desired page into the (newly) free frame; change the
page and frame tables
Restart the user process
6/1/2015
Dirty Bit
12
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Note: If no frames are
free, two page transfers
(in and out) are required
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doubles the page-fault
service time!
Reduce overhead with
dirty bit:
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set by hardware whenever
any word or byte in the
page is written into
if bit is set, page has been
modified and we need to
write it to disk
if bit is not set, no writing
to disk required
6/1/2015
Page Replacement Strategies
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The Principle of Optimality
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Replace the page that will not be used again the farthest time
into the future.
Random Page Replacement
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Choose a page randomly
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Replace the page that has been in memory the longest.
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Replace the page that has not been used for the longest time.
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Replace the page that is used least often.
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An approximation to LRU
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Keep in memory those pages that the process is actively using
FIFO - First In First Out
LRU - Least Recently Used
LFU - Least Frequently Used
NUR - Not Used Recently
Working Set
First In First Out (FIFO) Replacement
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Associate with each page the time when that page was
brought into memory
When a page must be replaced, choose the oldest
page
Bélády's anomaly: increasing the number of page
frames results in more page faults
6/1/2015
Optimal Page Replacement
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Replace the page that will not be used for the longest
period of time
How do you know this???
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You don’t. Generally used to measure how well an algorithm
performs; guarantees lowest possible page-fault rate
6/1/2015
LRU Page Replacement
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Replace the page that has not been used for the
longest period of time (least recently used; LRU)
Approximation to optimal algorithm: assume recent past
indicates the future
Problem: Needs sufficient hardware support, often hard
to implement!
?
6/1/2015
Second-Chance Replacement
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FIFO replacement
algorithm
With each page, associate
a reference bit:
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hardware sets the bit
whenever page is
referenced (either read or
write to any byte in the
page)
When a page is selected,
inspect reference bit
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If reference bit = 0,
replace the page
If reference bit = 1, set bit
=0, continue
6/1/2015
Enhanced Second-Chance Replacement
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Need a reference bit and a modify bit as an ordered pair.
4 situations are possible:
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(0,0) - neither recently used nor modified - best page to replace.
(0,1) - not recently used, but modified - not quite as good,
because the page will need to be written out before
replacement.
(1,0) - recently used but clean - probably will be used again
soon.
(1,1) - probably will be used again, will need to write out before
replacement.
Used in the Macintosh virtual memory management scheme
6/1/2015
Allocation of Frames
19

How do we allocate 𝑚 free frames among 𝑛
processes?
 Equal
allocation:
 each
process gets 𝑚/𝑛 frames
 Proportional
 number
 Priority
allocation:
of frames is proportional to process size
allocation:
 number
of frames is proportional to process priority
6/1/2015
Global vs. Local Allocation
20
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Global allocation
Process selects a replacement frame from the set of all
frames
 Process can take a frame from another
 Process may not be able to control its page fault rate
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Local allocation
Each process selects from only its own set of allocated
frames
 Process slowed down even if other less used pages of
memory are available
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Global replacement has better throughput
6/1/2015
Thrashing
21
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If a process does not have
enough pages, the page-fault
rate is very high. This leads to:
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low CPU utilization.
OS thinks that it needs to
increase the degree of
multiprogramming
Another process is added to the
system.
System throughput plunges...
Thrashing:
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High paging activity: Process is
spending more time paging
than executing
6/1/2015
Working-Set Model
22
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Locality:
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Δ = working-set window
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set of pages that are actively used together
process migrates from one locality to another
examine the most recent Δ page references
working set: set of pages in the most recent Δ page references
If a page is in active use, it will be in the working set
If a page is no longer being used, it will drop from the working
set Δ time units after its last reference
𝐷 = working set size of a process
𝑚 = number of available frames
If 𝐷 > 𝑚: thrashing => suspend one of the processes
6/1/2015
Page-Fault Frequency
23
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Thrashing has a high
page-fault rate
Idea: Control thrashing
by establishing an
acceptable page-fault
rate
rate is too low,
process loses frame
 If rate is too high,
process needs and gains
a frame
increase number of frames
 If
decrease number of frames
6/1/2015
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Pop Quiz #3
6/1/2015