No Slide Title
Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th Edition,
Virtual Memory
*
Virtual Memory
Background
Demand Paging
Copy-on-Write
Page Replacement
Allocation of Frames
Thrashing
Memory-Mapped Files
Allocating Kernel Memory
Other Considerations
Operating-System Examples
*
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 principle of the working-set model
*
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
*
Virtual Memory That is Larger Than Physical Memory
*
Virtual-address Space
*
Shared Library Using Virtual Memory
*
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
*
Transfer of a Paged Memory to Contiguous Disk Space
*
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:
During address translation, if valid–invalid bit in page table entry
is I page fault
v
v
v
v
i
i
i
….
Frame #
valid-invalid bit
page table
*
Page Table When Some Pages Are Not in Main Memory
*
Page Fault
If there is a reference to a page, first reference to that page will trap to operating system:
page fault
Operating system looks at another table to decide:
Invalid reference abort
Just not in memory
Get empty frame
Swap page into frame
Reset tables
Set validation bit = v
Restart the instruction that caused the page fault
*
Steps in Handling a Page Fault
*
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)
= (1 – p) * memory access + p (page fault overhead
+ swap page out
+ swap page in
+ restart overhead)
*
Demand Paging Example
Memory access time = 200 ns
Average page-fault service time = 8 ms
EAT = (1 – p) * 200 + p (8 ms)
= (1 – p) * 200 + p(8,000,000) ns
≈ 200 + (8*106)p for p << 1
If one access out of 1,000 causes a page fault, i.e., p = 0.001
EAT ≈ 8200 ns.
This is a slowdown by a factor of 40!!
*
Process Creation
Virtual memory allows other benefits during process creation:
- Copy-on-Write
- Memory-Mapped Files (later)
*
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
*
Before Process 1 Modifies Page C
*
thread0
thread1
Dirty COW exploit:
Thread t0 – write()
Thread t1 – madvise(args->thp_map, MAP_SIZE, MADV_DONTNEED);
// discard and don’t update page table
Objective: Write to original page before page table is updated.
1. Allocate temp memory (cached) for new COW page. ( t0 : write( ) )
2. Read original page to temp cache.
3. Write to cached page (to be written to new COW page)
4. (don’t need) (page table unchanged) (t1 :madvise(..))
5. Suppose to write temp page to new COW page, but page table still points to original page.
passwd file copy
passwd file
*
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
*
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
*
Need For Page Replacement
*
Basic Page Replacement
Find the location of the desired page on disk
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
Bring the desired page into the (newly) free frame; update the page and frame tables
Restart the process
*
Page Replacement
*
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
*
Graph of Page Faults Versus The Number of Frames
*
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)
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
4
4
3
*
FIFO Page Replacement
*
FIFO Illustrating Belady’s Anomaly
*
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
*
Optimal Page Replacement
*
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
*
LRU Page Replacement
*
LRU Algorithm (Cont.)
Stack implementation – keep a stack of page numbers in a double link form:
Page referenced:
move it to the top
No search for replacement
*
Use Of A Stack to Record The Most Recent Page References
*
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
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
*
Second-Chance (clock) Page-Replacement Algorithm
*
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
*
LRU Hardware Implementation
Matrix L of size (n x n) for n pages.
Initially all entries of L are set to zero.
When page i is referenced:
① set rows of i to 1
② set columns of i to 0
③ page i is LRU if it has the lowest binary value
LRU Hardware Implementation
Reference string: 2, 1, 0, 3
0 1 2 3
0 0 1 1 0
1 0 0 1 0
2 0 0 0 0
3 1 1 1 0
Buddy System
Eliminates external fragmentation
uses the way numbers are stored – in binary.
memory is only allocated in units that are powers of two.
If 3 bytes are requested you get 4, and
if 129 bytes are requested you get 256.
leads to wasted space (internal fragmentation).
Buddy System
Buddy System
a list of lists of free space is maintained.
first list: 1 byte blocks,
second: 2 byte blocks,
next: 4 byte blocks, etc.
When a request is made
① the size is rounded up
② a search made of the appropriate list.
③ if available it is allocated.
④ o.w., a search is made of the next largest, and so on until a
block is found that can be used.
Buddy System
A block that is too large is split into two.
Each part is known as the “buddy” of the other.
When it is split, it is taken off the free list for its size.
One buddy is placed on the free list for the next size down, and the other is used, splitting it again if needed.
Buddy System
For example:
① request is made for a 3 byte piece of memory
② smallest free block is 32 bytes.
③ it is split into two 16 byte buddies,
– one is placed on the 16 byte free list.
– other is split into two 8 byte buddies,
one of which is placed on the 8 byte list.
– the other is split into two 4 byte buddies,
– one of which is placed on the 4 byte free list, and
④ the other – finally – is used.
Buddy System
Buddy System
➊ When memory is released,
– it is placed back on the appropriate free list.
➋ Trick:
– two free blocks can only be combined if they are buddies,
– buddies have addresses that differ only in 1 bit.
– two 1 byte blocks are buddies iff they differ in the last bit,
– two 2 byte blocks are buddies iff they differ in the 2nd bit,etc.,
can find out if two blocks can be combined real fast.
➌ advantage fast granting and returning memory.
disadvantage internal fragmentation.
Buddy System
Buddy System
Buddy System Example
Buddy System Example
Buddy System Example
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
*
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
*
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
*
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
*
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
*
Thrashing (Cont.)
*
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
*
Locality In A Memory-Reference Pattern
*
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
*
Working-set model
*
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
*
Working Sets and Page Fault Rates
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
*
Memory Mapped Files
*
Memory-Mapped Shared Memory in Windows
*
Allocating Kernel Memory
Treated differently from user memory
Often allocated from a free-memory pool
Kernel requests memory for structures of varying sizes
Some kernel memory needs to be contiguous
*
Slab Allocator
Alternate strategy
Slab is one or more physically contiguous pages
Cache consists of one or more slabs
Single cache for each unique kernel data structure
Each cache filled with objects – instantiations of the data structure
When cache created, filled with objects marked as free
When structures stored, objects marked as used
If slab is full of used objects, next object allocated from empty slab
If no empty slabs, new slab allocated
Benefits include no fragmentation, fast memory request satisfaction
*
Slab Allocation
*
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
s * α > s * (1- α) or
s * α < s * (1- α)
α near zero prepaging loses
*
Other Issues – Page Size
Page size selection must take into consideration:
fragmentation
table size
I/O overhead
locality
*
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
*
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
*
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
*
Reason Why Frames Used For I/O Must Be In Memory
*
Operating System Examples
Windows XP
Solaris
*
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
*
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 start 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
*
Solaris 2 Page Scanner
*
Barriers to Critical Thinking
Five Powerful Barriers to Critical Thinking:
Egocentrism
Unwarranted
Assumptions
Sociocentrism
Relativistic
Thinking
Wishful
Thinking
Self-centered thinking
self-interested thinking
self-serving bias
Group-centered thinking
Group bias
Conformism
Beliefs that are presumed to be true without adequate evidence or justification
Assumption
Stereotyping
Believing that something is true because one wishes it were true.
The truth is “just a matter of opinion”
Relativism
Subjectivism
Cultural relativism
*
m
S
s
p
a
m
s
S
p
s
i
i
i
i
i
i
´
=
=
=
å
=
=
for
allocation
frames
of
number
total
process
of
size
59
64
137
127
5
64
137
10
127
10
64
2
1
2
»
´
=
»
´
=
=
=
=
a
a
s
s
m
i