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Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Virtual Memory
9.2 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
What to Learn?
Chapter 8 in the text book OSCE. Chapter 9 in OSC.
The benefits of a virtual memory system
The concepts of
demand paging
page-replacement algorithms
and allocation of physical page frames
Other related techniques
Copy-on-write. Memory mapped files
9.3 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Background
Virtual memory
Only part of the program needs to be in memory for execution
Logical address space can be much larger than physical address space
Allows address spaces to be shared by several processes
9.4 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Virtual Memory That is Larger Than Physical Memory
9.5 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Transfer of a Paged Memory to Contiguous Disk Space
9.6 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Demand Paging for Virtual Memory
Bring a page into memory ONLY when it is needed Less I/O needed Less memory needed Faster response More users supported
Check a page table entry when a page is needed not-in-memory bring to memory
9.7 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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 Not in memory page fault
v
v
v
v
i
i
i
….
Frame # valid-invalid bit
page table
9.8 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Page Table When Some Pages Are Not in Main Memory
9.9 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Handling a Page Fault
1.Get an empty physical frame
2.Swap page from the disk into frame
3.Reset the page table
Set validation bit = v
4.Restart the instruction that caused the page fault
9.10 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Page Fault (Cont.)
Restart instruction Special handling
block move
Auto increment/decrement
9.11 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Steps in Handling a Page Fault
9.12 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Performance of Demand Paging
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
)
9.13 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Demand Paging Performance 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!!
9.14 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Other Benefits
Virtual memory allows other benefits
- Shared pages
- Copy-on-Write during process creation
- Memory-Mapped Files
9.15 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Shared Pages with Virtual Memory
9.16 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Copy-on-Write: Lazy copy during process creation
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, this page is copied
COW allows more efficient process creation as only modified pages are copied
9.17 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Before Process 1 Modifies Page C
9.18 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Copy on Write: After Process 1 Modifies Page C
9.19 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Memory-Mapped Files
Allow file I/O to be treated as routine memory access by mapping a disk block to a page in memory
Simplifies file access by treating file I/O through direct memory access rather than read() write() system calls
File access is managed with demand paging.
Several processes may map the same file into memory as shared data
9.20 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Memory Mapped Files
9.21 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Example of Linux mmap
#define NUMINTS 1000
#define FILESIZE NUMINTS * sizeof(int)
fd = open(FILEPATH, O_RDWR | O_CREAT | O_TRUNC, 0600);
result = lseek(fd, FILESIZE-1, SEEK_SET);
result = write(fd, "", 1);
map = mmap(0, FILESIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
for (i = 1; i <=NUMINTS; ++i) map[i] = 2 * i;
munmap(map, FILESIZE);
close(fd);
9.22 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Demand paging when 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
9.23 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Need For Page Replacement
9.24 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Page Replacement
9.25 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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. Swap out: Use modify (dirty) bit to reduce overhead of page transfers – only modified pages are written to disk
4. Bring the desired page into the free frame.
Update the page and frame tables
9.26 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Expected behavior: # of Page Faults vs. # of Physical Frames
9.27 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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). Maintain a queue and select based on FIFO order.
4 frames
Belady’s Anomaly: more frames more 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
9.28 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
FIFO Page Replacement
9.29 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
FIFO Illustrating Belady’s Anomaly
9.30 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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
How do you know this?
Used for measuring how well your algorithm performs
1
2
3
4
6 page faults
4 5
9.31 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Optimal Page Replacement
9.32 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Least Recently Used (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
5
2
4
3
1
2
3
4
1
2
5
4
1
2
5
3
1
2
4
3
9.33 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
LRU Page Replacement
9.34 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
LRU Algorithm Implementation
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
9.35 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Use Of A Stack to Record The Most Recent Page References
9.36 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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
If page has reference bit = 1 then:
Leave this page in memory, but set reference bit 0.
Visit next page (in clock order)
If reference bit is 0, replace it.
9.37 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Second-Chance Page-Replacement Algorithm
9.38 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Issues with Page Allocation and Replacement
Initial Allocation. Each process needs minimum number of pages
Two schemes: Fixed allocation vs. priority allocation
Where to find frames
• Global replacement – find a frame from all processes.
• Local replacement – find only from its own allocated frames
9.39 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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 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
9.40 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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
9.41 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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
9.42 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Thrashing (Cont.)
9.43 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Relationship of Demand Paging and Thrashing
Why does demand paging work?Locality model
Process migrates from one locality to another
Localities may overlap
Need a minimum working set to be effective
Why does thrashing occur? working-sets > total memory size
9.44 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Locality In A Memory-Reference Pattern
9.45 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Working Sets and Page Fault Rates
9.46 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Prefetching Pages (Prepaging)
Optimization to reduce page faults (e.g. during process startup)
Preload some pages before they are referenced
But if prefetched pages are unused, I/O and memory was wasted
Assume s pages are prepaged and α % of the pages is used
Page fault cost of s * α >
cost of prepaging s * (1- α) unnecessary pages
9.47 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Tradeoffs: Page Size
Impact of page size selection:
Internal fragmentation
Page table size
I/O overhead
locality
9.48 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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
Increase the Page Size
Increase TLB reach
But may increase internal fragmentation as not all applications require a large page size
9.49 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Other Issues – Impact of Program Structure on Page Faults
<128 physical memory pages for each process. Each page is of 128x4 bytes
Program structure Int[128,128] data; Each row is stored in one page Page faults of Program 1?
for (j = 0; j <128; j++) for (i = 0; i < 128; i++) data[i,j] = 0;
Page faults of Program 2 ?
for (i = 0; i < 128; i++) for (j = 0; j < 128; j++) data[i,j] = 0;
9.50 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
Other Issues – Impact of Program Structure
<128 physical pages for each process. Each page is of 128x4 bytes 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
9.51 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition
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
