Chapter 9: Virtual Memory

Operating System Concepts – 9th Silberschatz, Galvin and Gagne

 Chapter 9: Virtual Memory
● Background
● Demand Paging
● Copy-on-Write
● Page Replacement
● Allocation of Frames
● Thrashing

Operating System Concepts – 9th 9.2 Silberschatz, Galvin and Gagne

 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
● To examine the relationship between shared memory
and memory-mapped files
● To explore how kernel memory is managed

Operating System Concepts – 9th 9.3 Silberschatz, Galvin and Gagne

 Background
● Code needs to be in memory to execute, but entire
program rarely used
● Error code, unusual routines, large data structures
● Entire program code not needed at same time
● Consider ability to execute partially-loaded program
● Program no longer constrained by limits of physical
memory
● Each program takes less memory while running ->
more programs run at the same time
4 Increased CPU utilization and throughput with no
increase in response time or turnaround time
● Less I/O needed to load or swap programs into
memory -> each user program runs faster

Operating System Concepts – 9th 9.4 Silberschatz, Galvin and Gagne

 Background (Cont.)

● 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
● More programs running concurrently
● Less I/O needed to load or swap processes

Operating System Concepts – 9th 9.5 Silberschatz, Galvin and Gagne

 Background (Cont.)
● Virtual address space – logical view of how process
is stored in memory
● Usually start at address 0, contiguous addresses until end
of space
● Meanwhile, physical memory organized in page frames
● MMU must map logical to physical
● Virtual memory can be implemented via:
● Demand paging
● Demand segmentation

Operating System Concepts – 9th 9.6 Silberschatz, Galvin and Gagne

 Virtual Memory That is Larger Than Physical Memory

Operating System Concepts – 9th 9.7 Silberschatz, Galvin and Gagne

 Virtual-address Space
● Usually design logical address space
for stack to start at Max logical
address and grow “down” while
heap grows “up”
● Maximizes address space use
● Unused address space
between the two is hole
4 No physical memory
needed until heap or
stack grows to a given
new page
● Enables sparse address spaces with
holes left for growth, dynamically
linked libraries, etc
● System libraries shared via mapping
into virtual address space
● Shared memory by mapping pages
read-write into virtual address space
● Pages can be shared during fork(),
speeding process creation
Operating System Concepts – 9th 9.8 Silberschatz, Galvin and Gagne
 Shared Library Using Virtual Memory

Operating System Concepts – 9th 9.9 Silberschatz, Galvin and Gagne

 Demand Paging
● Could bring entire process into
memory at load time
● Or bring a page into memory only
when it is needed
● Less I/O needed, no
unnecessary I/O
● Less memory needed
● Faster response
● More users
● Similar to paging system with
swapping (diagram on right)
● 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 – 9th 9. Silberschatz, Galvin and Gagne

 Basic Concepts
● With swapping, pager guesses which pages will be used
before swapping out again
● Instead, pager brings in only those pages into memory
● How to determine that set of pages?
● Need new MMU functionality to implement demand
paging
● If pages needed are already memory resident
● No difference from non demand-paging
● If page needed and not memory resident
● Need to detect and load the page into memory from
storage
4 Without changing program behavior
4 Without programmer needing to change code

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Valid-Invalid Bit
● With each page table entry a valid–invalid bit is associated
(v ⇒ in-memory – memory resident, i ⇒ not-in-memory)
● Initially valid–invalid bit is set to i on all entries
● Example of a page table snapshot:

● During MMU address translation, if valid–invalid bit in page

table entry is i ⇒ page fault

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Page Table When Some Pages Are Not in Main Memory

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Steps in Handling a Page Fault

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Aspects of Demand Paging
● Extreme case – start process with no pages in memory
● OS sets instruction pointer to first instruction of process,
non-memory-resident -> page fault
● And for every other process pages on first access
● Pure demand paging
● Actually, a given instruction could access multiple pages ->
multiple page faults
● Consider fetch and decode of instruction which adds 2
numbers from memory and stores result back to memory
● Pain decreased because of locality of reference
● Hardware support needed for demand paging
● Page table with valid / invalid bit
● Secondary memory (swap device with swap space)
● Instruction restart

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 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. Find free frame
3. Swap page into frame via scheduled disk operation
4. Reset tables to indicate page now in memory
Set validation bit = v
5. Restart the instruction that caused the page fault

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Instruction Restart
● Consider an instruction that could access several different
locations
● block move

● auto increment/decrement location

● Restart the whole operation?
4 What if source and destination overlap?

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Performance of Demand Paging
● Stages in Demand Paging (worse case)
1. Trap to the operating system
2. Save the user registers and process state
3. Determine that the interrupt was a page fault
4. Check that the page reference was legal and determine the location of the page
on the disk
5. Issue a read from the disk to a free frame:
1. Wait in a queue for this device until the read request is serviced
2. Wait for the device seek and/or latency time
3. Begin the transfer of the page to a free frame
6. While waiting, allocate the CPU to some other user
7. Receive an interrupt from the disk I/O subsystem (I/O completed)
8. Save the registers and process state for the other user
9. Determine that the interrupt was from the disk
10. Correct the page table and other tables to show page is now in memory
11. Wait for the CPU to be allocated to this process again
12. Restore the user registers, process state, and new page table, and then resume
the interrupted instruction

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Performance of Demand Paging (Cont.)
● Three major activities
● Service the interrupt – careful coding means just several
hundred instructions needed
● Read the page – lots of time
● Restart the process – again just a small amount of time
● Page Fault Rate 0 ≤ p ≤ 1
● 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 )

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 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!!
● If want performance degradation < 10 percent
● 220 > 200 + 7,999,800 x p
20 > 7,999,800 x p
● p < .0000025
● < one page fault in every 400,000 memory accesses

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Demand Paging Optimizations
● Swap space I/O faster than file system I/O even if on the same
device
● Swap allocated in larger chunks, less management needed than
file system
● Copy entire process image to swap space at process load time
● Then page in and out of swap space
● Used in older BSD Unix
● Demand page in from program binary on disk, but discard rather
than paging out when freeing frame
● Used in Solaris and current BSD
● Still need to write to swap space
4 Pages not associated with a file (like stack and heap) –
anonymous memory
4 Pages modified in memory but not yet written back to the file
system
● Mobile systems
● Typically don’t support swapping
● Instead, demand page from file system and reclaim read-only
pages (such as code)
Operating System Concepts – 9th 9. 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
● In general, free pages are allocated from a pool of zero-fill-on-
demand pages
● Pool should always have free frames for fast demand page
execution
4 Don’t want to have to free a frame as well as other
processing on page fault
● Why zero-out a page before allocating it?
● vfork() variation on fork() system call has parent suspend and
child using copy-on-write address space of parent
● Designed to have child call exec()
● Very efficient

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Before Process 1 Modifies Page C

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 After Process 1 Modifies Page C

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 What Happens if There is no Free Frame?

● Used up by process pages

● Also in demand from the kernel, I/O buffers, etc
● How much to allocate to each?
● Page replacement – find some page in memory, but not
really in use, page it out
● Algorithm – terminate? swap out? replace the page?
● 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 – 9th 9. Silberschatz, Galvin and Gagne

 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

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Need For Page Replacement

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 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
- Write victim frame to disk if dirty

3. Bring the desired page into the (newly) free frame; update
the page and frame tables

4. Continue the process by restarting the instruction that

caused the trap

Note now potentially 2 page transfers for page fault – increasing

EAT

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Page Replacement

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Page and Frame Replacement Algorithms

● Frame-allocation algorithm determines

● How many frames to give each process
● Which frames to replace
● Page-replacement algorithm
● Want lowest page-fault rate on both first access and re-
access
● Evaluate algorithm by running it on a particular string of
memory references (reference string) and computing the
number of page faults on that string
● String is just page numbers, not full addresses
● Repeated access to the same page does not cause a
page fault
● Results depend on number of frames available
● In all our examples, the reference string of referenced
page numbers is
7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Graph of Page Faults Versus The Number of Frames

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 First-In-First-Out (FIFO) Algorithm
● Reference string:
7,0,1,2,0,3,0,4,2,3,0,3,0,3,2,1,2,0,1,7,0,1
● 3 frames (3 pages can be in memory at a time per
process)

15 page
faults

● Can vary by reference string: consider

1,2,3,4,1,2,5,1,2,3,4,5
● Adding more frames can cause more page faults!
4 Belady’s Anomaly
● How to track ages of pages?
● Just use a FIFO queue

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 FIFO Illustrating Belady’s Anomaly

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Optimal Algorithm
● Replace page that will not be used for longest period of time
● 9 is optimal for the example
● How do you know this?
● Can’t read the future
● Used for measuring how well your algorithm performs

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Least Recently Used (LRU) Algorithm
● Use past knowledge rather than future
● Replace page that has not been used in the most amount of
time
● Associate time of last use with each page

● 12 faults – better than FIFO but worse than OPT

● Generally good algorithm and frequently used
● But how to implement?

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 LRU Algorithm (Cont.)
● 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 find smallest value
4 Search through table needed
● Stack implementation
● Keep a stack of page numbers in a double link form:
● Page referenced:
4 move it to the top
4 requires 6 pointers to be changed
● But each update more expensive
● No search for replacement
● LRU and OPT are cases of stack algorithms that don’t
have Belady’s Anomaly

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Use Of A Stack to Record Most Recent Page References

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 LRU Approximation Algorithms
● LRU needs special hardware and still slow
● Reference bit
● With each page associate a bit, initially = 0
● When page is referenced bit set to 1
● Replace any with reference bit = 0 (if one exists)
4 We do not know the order, however
● Second-chance algorithm
● Generally FIFO, plus hardware-provided reference bit
● Clock replacement
● If page to be replaced has
4 Reference bit = 0 -> replace it
4 reference bit = 1 then:
– set reference bit 0, leave page in memory
– replace next page, subject to same rules

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Second-Chance (clock) Page-Replacement Algorithm

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Enhanced Second-Chance Algorithm

● Improve algorithm by using reference bit and modify bit

(if available) in concert
● Take ordered pair (reference, modify)
1. (0, 0) neither recently used not modified – best page to
replace
2. (0, 1) not recently used but modified – not quite as good,
must write out before replacement
3. (1, 0) recently used but clean – probably will be used
again soon
4. (1, 1) recently used and modified – probably will be used
again soon and need to write out before replacement
● When page replacement called for, use the clock scheme
but use the four classes replace page in lowest non-
empty class
● Might need to search circular queue several times

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Counting Algorithms
● Keep a counter of the number of references that have been
made to each page
● Not common

● Lease Frequently Used (LFU) Algorithm: replaces

page with smallest count

● 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 – 9th 9. Silberschatz, Galvin and Gagne

 Page-Buffering Algorithms
● Keep a pool of free frames, always
● Then frame available when needed, not found at fault
time
● Read page into free frame and select victim to evict
and add to free pool
● When convenient, evict victim
● Possibly, keep list of modified pages
● When backing store otherwise idle, write pages there
and set to non-dirty
● Possibly, keep free frame contents intact and note what is
in them
● If referenced again before reused, no need to load
contents again from disk
● Generally useful to reduce penalty if wrong victim
frame selected

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Applications and Page Replacement

● All of these algorithms have OS guessing about future

page access
● Some applications have better knowledge – i.e.
databases
● Memory intensive applications can cause double
buffering
● OS keeps copy of page in memory as I/O buffer
● Application keeps page in memory for its own work
● Operating system can given direct access to the disk,
getting out of the way of the applications
● Raw disk mode
● Bypasses buffering, locking, etc

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Allocation of Frames
● Each process needs minimum number of frames
● 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
● Maximum of course is total frames in the system
● Two major allocation schemes
● fixed allocation
● priority allocation
● Many variations

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Fixed Allocation
● Equal allocation – For example, if there are 100 frames
(after allocating frames for the OS) and 5 processes, give
each process 20 frames
● Keep some as free frame buffer pool

● Proportional allocation – Allocate according to the size of

process
● Dynamic as degree of multiprogramming, process
sizes change

Operating System Concepts – 9th 9. 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 – 9th 9. 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
● But then process execution time can vary greatly
● But greater throughput so more common

● Local replacement – each process selects from only

its own set of allocated frames
● More consistent per-process performance
● But possibly underutilized memory

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Non-Uniform Memory Access
● So far all memory accessed equally
● Many systems are NUMA – speed of access to memory
varies
● Consider system boards containing CPUs and
memory, interconnected over a system bus
● Optimal performance comes from allocating memory
“close to” the CPU on which the thread is scheduled
● And modifying the scheduler to schedule the thread
on the same system board when possible
● Solved by Solaris by creating lgroups
4 Structure to track CPU / Memory low latency
groups
4 Used my schedule and pager
4 When possible schedule all threads of a process
and allocate all memory for that process within the
lgroup

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Thrashing
● If a process does not have “enough” pages, the page-fault
rate is very high
● Page fault to get page
● Replace existing frame
● But quickly need replaced frame back
● This leads to:
4 Low CPU utilization
4 Operating system thinking that it needs to increase
the degree of multiprogramming
4 Another process added to the system

● Thrashing ≡ a process is busy swapping pages in and out

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Thrashing (Cont.)

Operating System Concepts – 9th 9. 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?

Σ size of locality > total memory size
● Limit effects by using local or priority page
replacement

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Locality In A Memory-Reference Pattern

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Working-Set Model
● Δ ≡ working-set window ≡ a fixed number of page references
Example: 10,000 instructions
● WSSi (working set of Process Pi) =
total number of pages referenced in the most recent Δ (varies in
time)
● if Δ too small will not encompass entire locality
● if Δ too large will encompass several localities
● if Δ = ∞ ⇒ will encompass entire program
● D = Σ WSSi ≡ total demand frames
● Approximation of locality
● if D > m ⇒ Thrashing
● Policy if D > m, then suspend or swap out one of the processes

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Keeping Track of the Working Set
● Approximate with interval timer + a reference bit
● Example: Δ = 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 – 9th 9. Silberschatz, Galvin and Gagne

 Page-Fault Frequency
● More direct approach than WSS
● Establish “acceptable” page-fault frequency
(PFF) rate and use local replacement policy
● If actual rate too low, process loses frame
● If actual rate too high, process gains frame

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Working Sets and Page Fault Rates
● Direct relationship between working set of a process
and its page-fault rate
● Working set changes over time
● Peaks and valleys over time

Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne

 Summary
• Virtual memory abstracts physical memory into an extremely large uniform array
of storage.
• The benefits of virtual memory include the following: (1) a program can be larger
than physical memory, (2) a program does not need to be entirely in memory, (3)
processes can share memory, and (4) processes can be created more efficiently.
• Demand paging is a technique whereby pages are loaded only when they are
demanded during program execution. Pages that are never demanded are thus
never loaded into memory.
• A page fault occurs when a page that is currently not in memory is accessed. The
page must be brought from the backing store into an available page frame in
memory.
• Copy-on-write allows a child process to share the same address space as its
parent. If either the child or the parent process writes (modifies) a page, a copy of
the page is made.
• When available memory runs low, a page-replacement algorithm selects an
existing page in memory to replace with a new page. Page replacement algorithms
include FIFO, optimal, and LRU. Pure LRU algorithms are impractical to implement,
and most systems instead use LRU-approximation algorithms.
• Global page-replacement algorithms select a page from any process in the system
for replacement, while local page-replacement algorithms select a page from the
faulting process.
• Thrashing occurs when a system spends more time paging than executing.
Operating System Concepts – 9th 9. Silberschatz, Galvin and Gagne
 End of Chapter 9

Operating System Concepts – 9th Silberschatz, Galvin and Gagne