Chapter 9: Virtual Memory
Operating System Concepts – 8th Edition, Silberschatz, Galvin and Gagne ©2009
Chapter 9: Virtual Memory
Background
Demand Paging
Page Replacement
Thrashing
Operating System Concepts – 8th Edition 9.2 Silberschatz, Galvin and Gagne ©2009
Objectives
To describe the benefits of a virtual memory system
To explain the concepts of demand paging, page-replacement algorithms,
and allocation of page frames
To discuss the thrashing problem.
Operating System Concepts – 8th Edition 9.3 Silberschatz, Galvin and Gagne ©2009
Virtual Memory
Virtual memory is a technique that allows the execution of processes that are
not completely in memory.
One major advantage of this scheme is that programs can be larger than
physical memory.
Further, virtual memory abstracts main memory into an extremely large,
uniform array of storage, separating logical memory as viewed by the user
from physical memory.
Operating System Concepts – 8th Edition 9.4 Silberschatz, Galvin and Gagne ©2009
Background
Virtual memory involves the separation of user logical memory from
physical memory.
Only part of the program needs to be in memory for execution
Logical address space can therefore be much larger than physical
address space
Allows address spaces to be shared by several processes
Allows for more efficient process creation
Virtual memory can be implemented via:
Demand paging
Demand segmentation
Operating System Concepts – 8th Edition 9.5 Silberschatz, Galvin and Gagne ©2009
Virtual Memory That is Larger Than Physical Memory
Operating System Concepts – 8th Edition 9.6 Silberschatz, Galvin and Gagne ©2009
Virtual-address Space
Operating System Concepts – 8th Edition 9.7 Silberschatz, Galvin and Gagne ©2009
Shared Library Using Virtual Memory
Operating System Concepts – 8th Edition 9.8 Silberschatz, Galvin and Gagne ©2009
Demand Paging
Bring a page into memory only when it is needed
Less I/O needed
Less memory needed
Faster response
More users
Page is needed reference to it
invalid reference abort
not-in-memory bring to memory
Lazy swapper – never swaps a page into memory unless page will be
needed
Swapper that deals with pages is a pager
Operating System Concepts – 8th Edition 9.9 Silberschatz, Galvin and Gagne ©2009
Transfer of a Paged Memory to Contiguous Disk Space
Operating System Concepts – 8th Edition 9.10 Silberschatz, Galvin and Gagne ©2009
Valid-Invalid Bit
With each page table entry a valid–invalid bit is associated
(v in-memory, i not-in-memory)
Initially valid–invalid bit is set to i on all entries
Example of a page table snapshot:
Frame # valid-invalid bit
v
v
v
v
i
….
i
i
page table
During address translation, if valid–invalid bit in page table entry
is I page fault
Operating System Concepts – 8th Edition 9.11 Silberschatz, Galvin and Gagne ©2009
Page Table When Some Pages Are Not in Main Memory
Operating System Concepts – 8th Edition 9.12 Silberschatz, Galvin and Gagne ©2009
Page Fault
If there is a reference to a page, first reference to that page will
trap to operating system:
page fault
1. Operating system looks at another table to decide:
Invalid reference abort
Just not in memory
2. Get empty frame
3. Swap page into frame
4. Reset tables
5. Set validation bit = v
6. Restart the instruction that caused the page fault
Operating System Concepts – 8th Edition 9.13 Silberschatz, Galvin and Gagne ©2009
Steps in Handling a Page Fault
Operating System Concepts – 8th Edition 9.14 Silberschatz, Galvin and Gagne ©2009
Virtual to Physical Address Translation (Mapping)
Algorithm In The Paging Scheme
The machine uses TLB (translation look-aside buffer) in the
cache and PT (page tables) in the main memory.
Given the page size (= frame size), the virtual addresses
generated by the CPU which consists of: Page #, offset (p,d)
and the access type(AT): Read-only(R), Read-Write(RW), or
Execute-only (E), use the following algorithm:
Operating System Concepts – 8th Edition 9.15 Silberschatz, Galvin and Gagne ©2009
IF p >= PTLR THEN trap ("Invalid page number")
//* page# >= # of pages for this process *//
IF p in TLB THEN
IF NOT AT in protection THEN trap ("memory-protection violation")
ELSE
"Cache Hit"
Physical add. = frame# * page(frame) size + d
ENDIF
ELSE
IF in PT presence/absence bit (valid/invalid bit) = present (valid) THEN
// * this page is currently loaded in a frame of the main memory *//
IF NOT AT in protection THEN trap ("memory-protection violation")
ELSE
"Cache Miss"
Physical add. = frame# * page(frame) size + d
ENDIF
ELSE
// * this page is not in the main memory, it's in the disk. must be swapped in according to the paging
replacement algorithm *//
Trap ("page fault") ENDIF
Operating System Concepts – 8th Edition 9.16 Silberschatz, Galvin and Gagne ©2009
Performance of Demand Paging
Demand paging can significantly affect the performance of a computer system. Let's
compute the effective access time for a demand-paged memory.
For most computer systems, the memory-access time (ma) ranges from 10 to 200
nanoseconds.
As long as we have no page faults, the effective access time is equal to the memory
access time.
If, however a page fault occurs, we must first read the relevant page from disk and
then access the desired word.
Page Fault Rate 0 p 1.0
if p = 0 no page faults
if p = 1, every reference is a fault
Effective Access Time (EAT)
EAT = (1 – p) x memory access+ p (page fault overhead
+ swap page out + swap page in + restart overhead)
Operating System Concepts – 8th Edition 9.17 Silberschatz, Galvin and Gagne ©2009
Demand Paging Example
Memory access time = 200 nanoseconds
Average page-fault service time = 8 milliseconds
EAT = (1 – p) x 200 + p (8 milliseconds)
= (1 – p ) x 200 + p x 8,000,000
= 200 + p x 7,999,800
If one access out of 1,000 causes a page fault, then
EAT = 8.2 microseconds.
This is a slowdown by a factor of 40!!
Operating System Concepts – 8th Edition 9.18 Silberschatz, Galvin and Gagne ©2009
What happens if there is no free frame?
Page replacement – find some page in memory, but not
really in use, swap it out
algorithm
performance – want an algorithm which will result in
minimum number of page faults
Same page may be brought into memory several times
Operating System Concepts – 8th Edition 9.19 Silberschatz, Galvin and Gagne ©2009
Page Replacement
If we increase the degree of multiprogramming we are
over-allocating memory.
Prevent over-allocation of memory by modifying page-
fault service routine to include page replacement
Operating System Concepts – 8th Edition 9.20 Silberschatz, Galvin and Gagne ©2009
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
But if we use demand paging we must solve Two major problems:
Develop Frame-allocation algorithms, if we have multiple processes in
memory, we must decide how many frames to allocate to each process.
Develop Page-replacement algorithms, When page replacement is required,
we must select the frames that are to be replaced.
Operating System Concepts – 8th Edition 9.21 Silberschatz, Galvin and Gagne ©2009
Need For Page Replacement
Operating System Concepts – 8th Edition 9.22 Silberschatz, Galvin and Gagne ©2009
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. Bring the desired page into the (newly) free frame;
update the page and frame tables
4. Restart the process
Operating System Concepts – 8th Edition 9.23 Silberschatz, Galvin and Gagne ©2009
Page Replacement
Operating System Concepts – 8th Edition 9.24 Silberschatz, Galvin and Gagne ©2009
Page Replacement Algorithms
Want lowest 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
In all our examples, the reference string is
1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
Operating System Concepts – 8th Edition 9.25 Silberschatz, Galvin and Gagne ©2009
Graph of Page Faults Versus The Number of Frames
Operating System Concepts – 8th Edition 9.26 Silberschatz, Galvin and Gagne ©2009
FIFO Page Replacement
The simplest page-replacement algorithm is a first-
in, first-out (FIFO) algorithm.
A FIFO replacement algorithm associates with each
page the time when that page was brought into
memory (FIFO queue).
When a page must be replaced, the oldest page is
chosen.
The FIFO page-replacement algorithm is easy to
understand and program. However, its performance
is not always good.
Operating System Concepts – 8th Edition 9.27 Silberschatz, Galvin and Gagne ©2009
FIFO Page Replacement
there are 15 faults altogether
Operating System Concepts – 8th Edition 9.28 Silberschatz, Galvin and Gagne ©2009
First-In-First-Out (FIFO) Algorithm
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)
1 1 4 5
2 2 1 3 9 page faults
3 3 2 4
4 frames
1 1 5 4
2 2 1 5 10 page faults
3 3 2
4 4 3
Belady’s Anomaly: more frames more page faults
Operating System Concepts – 8th Edition 9.29 Silberschatz, Galvin and Gagne ©2009
FIFO Illustrating Belady’s Anomaly
Operating System Concepts – 8th Edition 9.30 Silberschatz, Galvin and Gagne ©2009
Optimal Algorithm
An optimal page-replacement algorithm has the lowest page-fault rate of all
algorithms (called OPT or MIN). It is simply this:
Replace the page that will not be used for longest period of time
4 frames example : 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1 4
2 6 page faults
3
4 5
How do you know this?
Used for measuring how well your algorithm performs
Operating System Concepts – 8th Edition 9.31 Silberschatz, Galvin and Gagne ©2009
Optimal Page Replacement
9 page faults
Operating System Concepts – 8th Edition 9.32 Silberschatz, Galvin and Gagne ©2009
Least Recently Used (LRU) Algorithm
replace the page that has not been used for the longest period of time
Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
1 1 1 1 5
2 2 2 2 2
3 5 5 4 4
4 4 3 3 3
Counter implementation
Every page entry has a time-of-use field; every time page is
referenced, copy the CPU clock/counter into the time-of-use field
When a page needs to be replaced, look at the time-of-use field
values to determine which page should be replaced
Operating System Concepts – 8th Edition 9.33 Silberschatz, Galvin and Gagne ©2009
LRU Page Replacement
12 page faults
Operating System Concepts – 8th Edition 9.34 Silberschatz, Galvin and Gagne ©2009
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
Operating System Concepts – 8th Edition 9.35 Silberschatz, Galvin and Gagne ©2009
Use Of A Stack to Record The Most Recent Page
References
Operating System Concepts – 8th Edition 9.36 Silberschatz, Galvin and Gagne ©2009
Counting Algorithms
Keep a counter of the number of references that have been made to
each page
The least frequently used (LFU) Algorithm: replaces page with
smallest count
The most frequently used (MFU) Algorithm: based on the argument
that the page with the smallest count was probably just brought in and
has yet to be used
Operating System Concepts – 8th Edition 9.37 Silberschatz, Galvin and Gagne ©2009
Allocation of Frames
How do we allocate the fixed amount of free memory among the
various processes?
Each process needs minimum number of pages
Example: IBM 370 – 6 pages to handle Storage location to storage
location MOVE instruction:
instruction is 6 bytes, might span 2 pages
2 pages to handle from
2 pages to handle to
Two major allocation schemes
fixed allocation
priority allocation
Operating System Concepts – 8th Edition 9.38 Silberschatz, Galvin and Gagne ©2009
Fixed Allocation
The easiest way to split m frames among n processes is
to give everyone an equal share, m/n frames. This
scheme is called Equal allocation
For example, if there are 100 frames and 5 processes,
give each process 20 frames.
Operating System Concepts – 8th Edition 9.39 Silberschatz, Galvin and Gagne ©2009
Proportional allocation
we allocate available memory to each process according to its size.
si size of process pi
S si
m total number of frames
s
ai allocation for pi i m
S
m 64
s 1 10
s 2 127
10
a 1 64 5
137
127
a 2 64 59
137
Operating System Concepts – 8th Edition 9.40 Silberschatz, Galvin and Gagne ©2009
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
Operating System Concepts – 8th Edition 9.41 Silberschatz, Galvin and Gagne ©2009
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
With a local replacement strategy, the number of frames allocated to a
process does not change.
With global replacement, a process may happen to select only frames
allocated to other processes, thus increasing the number of frames allocated
to it (assuming that other processes do not choose its frames for
replacement).
Global replacement generally results in greater system throughput and is
therefore the more common method.
Operating System Concepts – 8th Edition 9.42 Silberschatz, Galvin and Gagne ©2009
Thrashing
If a process does not have “enough” frames, the page-fault rate is
very high.
If the process does not have the number of frames it needs to
support pages in active use, it will quickly page-fault.
At this point, it must replace some page.
However, since all its pages are in active use, it must replace a
page that will be needed again right away.
Consequently, it quickly faults again, and again, and again,
replacing pages that it must bring back in immediately.
Thrashing a process is busy swapping pages in and out, it is
spending more time paging than executing.
Operating System Concepts – 8th Edition 9.43 Silberschatz, Galvin and Gagne ©2009
Cause of thrashing
This leads to:
low CPU utilization
operating system thinks that it needs to increase the degree of
multiprogramming
another process added to the system
Operating System Concepts – 8th Edition 9.44 Silberschatz, Galvin and Gagne ©2009
Thrashing (Cont.)
Operating System Concepts – 8th Edition 9.45 Silberschatz, Galvin and Gagne ©2009
Demand Paging and Thrashing
Why does demand paging work?
Locality model
A process is composed of several different localities. Process migrates from
one locality to another.
A locality is a set of pages that are actively used together.
Localities may overlap
Why does thrashing occur?
size of locality > total memory size
Operating System Concepts – 8th Edition 9.46 Silberschatz, Galvin and Gagne ©2009
Thrashing
Working-Set Model.
Page-Fault Frequency Scheme.
Operating System Concepts – 8th Edition 9.47 Silberschatz, Galvin and Gagne ©2009
Working-Set Model
working-set window a fixed number of page references
Example: 10,000 instruction
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
Operating System Concepts – 8th Edition 9.48 Silberschatz, Galvin and Gagne ©2009
Working-set model
Operating System Concepts – 8th Edition 9.49 Silberschatz, Galvin and Gagne ©2009
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
Operating System Concepts – 8th Edition 9.50 Silberschatz, Galvin and Gagne ©2009
Conclusion
Virtual memory is commonly implemented by demand
paging.
Demand paging is used to reduce the number of frames
allocated to a process.
We need both page-replacement and frame-allocation
algorithms.
Operating System Concepts – 8th Edition 9.51 Silberschatz, Galvin and Gagne ©2009
End of Chapter 9
Operating System Concepts – 8th Edition, Silberschatz, Galvin and Gagne ©2009
