Chapter 9: Virtual Memory

Chapter 9: Virtual Memory

 Background
 Demand Paging
 Process Creation
 Page Replacement
 Allocation of Frames
 Thrashing
 Demand Segmentation
 Operating System Examples

Operating System Concepts 9.2 Silberschatz, Galvin and Gagne

 Background
 Virtual memory – 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 9.3 Silberschatz, Galvin and Gagne

 Virtual Memory That is Larger Than Physical
Memory

Operating System Concepts 9.4 Silberschatz, Galvin and Gagne

 Virtual-address Space

Operating System Concepts 9.5 Silberschatz, Galvin and Gagne

 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

Operating System Concepts 9.6 Silberschatz, Galvin and Gagne

 Transfer of a Paged Memory to Contiguous Disk
Space

Operating System Concepts 9.7 Silberschatz, Galvin and Gagne

 Valid-Invalid Bit
 With each page table entry a valid–invalid bit is associated
(1  in-memory, 0  not-in-memory)
 Initially valid–invalid but is set to 0 on all entries
 Example of a page table snapshot:

Frame # valid-invalid bit

1
1
1
1
0


0
0
page table

 During address translation, if valid–invalid bit in page table

entry is 0  page fault

Operating System Concepts 9.8 Silberschatz, Galvin and Gagne

 Page Table When Some Pages Are Not in Main
Memory

Operating System Concepts 9.9 Silberschatz, Galvin and Gagne

 Page Fault
 If there is ever a reference to a page, first reference
will trap to
OS  page fault
 OS looks at another table to decide:
 Invalid reference  abort.
 Just not in memory.
 Get empty frame.
 Swap page into frame.
 Reset tables, validation bit = 1.
 Restart instruction:

Operating System Concepts 9.10 Silberschatz, Galvin and Gagne

 Steps in Handling a Page Fault

Operating System Concepts 9.11 Silberschatz, Galvin and Gagne

 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 9.12 Silberschatz, Galvin and Gagne

 Process Creation
 Virtual memory allows other benefits during process
creation:

- Copy-on-Write

- Memory-Mapped Files (later)

Operating System Concepts 9.13 Silberschatz, Galvin and Gagne

 Copy-on-Write
 Copy-on-Write (COW) allows both parent and child
processes to initially share the same pages in
memory

If either process modifies a shared page, only then is

the page copied

 COW allows more efficient process creation as only

modified pages are copied

 Free pages are allocated from a pool of zeroed-out

pages

Operating System Concepts 9.14 Silberschatz, Galvin and Gagne

 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

Operating System Concepts 9.15 Silberschatz, Galvin and Gagne

 Page Replacement

Operating System Concepts 9.16 Silberschatz, Galvin and Gagne

 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 9.17 Silberschatz, Galvin and Gagne

 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

 FIFO Replacement – Belady’s Anomaly

 more frames  more page faults
Operating System Concepts 9.18 Silberschatz, Galvin and Gagne
 FIFO Page Replacement

Operating System Concepts 9.19 Silberschatz, Galvin and Gagne

 Optimal Algorithm
 Replace 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 9.20 Silberschatz, Galvin and Gagne

 Optimal Page Replacement

Operating System Concepts 9.21 Silberschatz, Galvin and Gagne

 Least Recently Used (LRU)
Algorithm
 Reference string: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5

1 5

3 5 4
4 3
 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

Operating System Concepts 9.22 Silberschatz, Galvin and Gagne

 LRU Page Replacement

Operating System Concepts 9.23 Silberschatz, Galvin and Gagne

 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 9.24 Silberschatz, Galvin and Gagne

 Use Of A Stack to Record The Most Recent Page
References

Operating System Concepts 9.25 Silberschatz, Galvin and Gagne

 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

Operating System Concepts 9.26 Silberschatz, Galvin and Gagne

 Second-Chance (clock) Page-Replacement
Algorithm

Operating System Concepts 9.27 Silberschatz, Galvin and Gagne

 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

Operating System Concepts 9.28 Silberschatz, Galvin and Gagne

 Allocation of Frames

 Each process needs minimum number of pages

 Example: IBM 370 – 6 pages to handle SS 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 9.29 Silberschatz, Galvin and Gagne

 Fixed Allocation

 Equal allocation – For example, if there are 100 frames

and 5 processes, give each process 20 frames.
 Proportional allocation – Allocate according to the size
of sprocess
i  size of process pi
S   si
m  total number of frames
s
ai  allocation for pi  i  m
S
m  64
si  10
s2  127
10
a1   64  5
137
127
a2   64  59
137

Operating System Concepts 9.30 Silberschatz, Galvin and Gagne

 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 9.31 Silberschatz, Galvin and Gagne

 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

Operating System Concepts 9.32 Silberschatz, Galvin and Gagne

 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

Operating System Concepts 9.33 Silberschatz, Galvin and Gagne

 Thrashing (Cont.)

Operating System Concepts 9.34 Silberschatz, Galvin and Gagne

 Demand Paging and Thrashing

 Why does demand paging work?

Locality model
 Process migrates from one locality to another
 Localities may overlap

 Why does thrashing occur?

 size of locality > total memory size

Operating System Concepts 9.35 Silberschatz, Galvin and Gagne

 Locality In A Memory-Reference Pattern

Operating System Concepts 9.36 Silberschatz, Galvin and Gagne

 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 9.37 Silberschatz, Galvin and Gagne

 Working-set model

Operating System Concepts 9.38 Silberschatz, Galvin and Gagne

 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

Operating System Concepts 9.39 Silberschatz, Galvin and Gagne

 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 9.40 Silberschatz, Galvin and Gagne

 Memory-Mapped Files
 Memory-mapped file I/O allows file I/O to be treated as
routine memory access by mapping a disk block to a
page in memory

 A file is initially read using demand paging. A page-

sized portion of the file is read from the file system
into a physical page. Subsequent reads/writes to/from
the file are treated as ordinary memory accesses.

 Simplifies file access by treating file I/O through

memory rather than read() write() system calls

 Also allows several processes to map the same file

allowing the pages in memory to be shared

Operating System Concepts 9.41 Silberschatz, Galvin and Gagne

 Memory Mapped Files

Operating System Concepts 9.42 Silberschatz, Galvin and Gagne

 Memory-Mapped Files in Java

import java.io.*;
import java.nio.*;
import java.nio.channels.*;
public class MemoryMapReadOnly
{
// Assume the page size is 4 KB
public static final int PAGE SIZE = 4096;
public static void main(String args[]) throws IOException {
RandomAccessFile inFile = new
RandomAccessFile(args[0],"r");
FileChannel in = inFile.getChannel();
MappedByteBuffer mappedBuffer =
in.map(FileChannel.MapMode.READ ONLY, 0,
in.size());
long numPages = in.size() / (long)PAGE SIZE;
if (in.size() % PAGE SIZE > 0)
++numPages;

Operating System Concepts 9.43 Silberschatz, Galvin and Gagne

 Memory-Mapped Files in Java
(cont)

// we will "touch" the first byte of every page

int position = 0;
for (long i = 0; i < numPages; i++) {
byte item = mappedBuffer.get(position);
position += PAGE SIZE;
}
in.close();
inFile.close();
}
}
 The API for the map() method is as follows:
map(mode, position, size)

Operating System Concepts 9.44 Silberschatz, Galvin and Gagne

 Other Issues -- Prepaging
 Prepaging
 To reduce the large number of page faults that
occurs at process startup
 Prepage all or some of the pages a process will
need, before they are referenced
 But if prepaged pages are unused, I/O and memory
was wasted
 Assume s pages are prepaged and α of the pages is
used
 Is cost of s * α save pages faults > or < than the
cost of prepaging
s * (1- α) unnecessary pages?
 α near zero  prepaging loses

Operating System Concepts 9.45 Silberschatz, Galvin and Gagne

 Other Issues – Page Size

 Page size selection must take into

consideration:
 fragmentation
 table size
 I/O overhead
 locality

Operating System Concepts 9.46 Silberschatz, Galvin and Gagne

 Other Issues – TLB Reach

 TLB Reach - The amount of memory accessible

from the TLB
 TLB Reach = (TLB Size) X (Page Size)
 Ideally, the working set of each process is stored
in the TLB. Otherwise there is a high degree of
page faults.
 Increase the Page Size. This may lead to an
increase in fragmentation as not all applications
require a large page size
 Provide Multiple Page Sizes. This allows
applications that require larger page sizes the
opportunity to use them without an increase in
fragmentation.

Operating System Concepts 9.47 Silberschatz, Galvin and Gagne

 Other Issues – Program Structure
 Program structure
 Int[128,128] data;
 Each row is stored in one page
 Program 1
for (j = 0; j <128; j++)
for (i = 0; i < 128; i++)
data[i,j] = 0;

128 x 128 = 16,384 page faults

 Program 2
for (i = 0; i < 128; i++)
for (j = 0; j < 128; j++)
data[i,j] = 0;

128 page faults

Operating System Concepts 9.48 Silberschatz, Galvin and Gagne

 Other Issues – I/O interlock

 I/O Interlock – Pages must sometimes be

locked into memory

 Consider I/O. Pages that are used for copying

a file from a device must be locked from
being selected for eviction by a page
replacement algorithm.

Operating System Concepts 9.49 Silberschatz, Galvin and Gagne

 Reason Why Frames Used For I/O Must Be In
Memory

Operating System Concepts 9.50 Silberschatz, Galvin and Gagne

 Operating System Examples
 Windows XP

 Solaris

Operating System Concepts 9.51 Silberschatz, Galvin and Gagne

 Windows XP
 Uses demand paging with clustering. Clustering
brings in pages surrounding the faulting page.
 Processes are assigned working set minimum and
working set maximum
 Working set minimum is the minimum number of
pages the process is guaranteed to have in memory
 A process may be assigned as many pages up to its
working set maximum
 When the amount of free memory in the system falls
below a threshold, automatic working set trimming is
performed to restore the amount of free memory
 Working set trimming removes pages from processes
that have pages in excess of their working set
minimum

Operating System Concepts 9.52 Silberschatz, Galvin and Gagne

 Solaris
 Maintains a list of free pages to assign faulting
processes
 Lotsfree – threshold parameter (amount of free
memory) to begin paging
 Desfree – threshold parameter to increasing paging
 Minfree – threshold parameter to being swapping
 Paging is performed by pageout process
 Pageout scans pages using modified clock algorithm
 Scanrate is the rate at which pages are scanned. This
ranges from slowscan to fastscan
 Pageout is called more frequently depending upon the
amount of free memory available

Operating System Concepts 9.53 Silberschatz, Galvin and Gagne

 Solaris 2 Page Scanner

Operating System Concepts 9.54 Silberschatz, Galvin and Gagne

End of Chapter 9