Chapter 9: Virtual Memory
Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013
Chapter 9: 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
Operating System Concepts – 9th Edition 9.2 Silberschatz, Galvin and Gagne ©2013
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 Edition 9.3 Silberschatz, Galvin and Gagne ©2013
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
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 Edition 9.4 Silberschatz, Galvin and Gagne ©2013
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 Edition 9.5 Silberschatz, Galvin and Gagne ©2013
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
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Virtual Memory That is Larger Than Physical Memory
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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
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 Edition 9.8 Silberschatz, Galvin and Gagne ©2013
Shared Library Using Virtual Memory
Operating System Concepts – 9th Edition 9.9 Silberschatz, Galvin and Gagne ©2013
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 Edition 9.10 Silberschatz, Galvin and Gagne ©2013
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
Without changing program behavior
Without programmer needing to change code
Operating System Concepts – 9th Edition 9.11 Silberschatz, Galvin and Gagne ©2013
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 Edition 9.12 Silberschatz, Galvin and Gagne ©2013
Page Table When Some Pages Are Not in Main Memory
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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
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Steps in Handling a Page Fault
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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
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Need For Page Replacement
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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 Edition 9.18 Silberschatz, Galvin and Gagne ©2013
Page Replacement
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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 Edition 9.20 Silberschatz, Galvin and Gagne ©2013
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!
Belady’s Anomaly
How to track ages of pages?
Just use a FIFO queue
Operating System Concepts – 9th Edition 9.21 Silberschatz, Galvin and Gagne ©2013
FIFO Illustrating Belady’s Anomaly
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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
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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 Edition 9.24 Silberschatz, Galvin and Gagne ©2013
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
Search through table needed
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
But each update more expensive
No search for replacement
LRU and OPT are cases of stack algorithms that don’t have
Belady’s Anomaly
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Use Of A Stack to Record Most Recent Page References
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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)
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
Reference bit = 0 -> replace it
reference bit = 1 then:
– set reference bit 0, leave page in memory
– replace next page, subject to same rules
Operating System Concepts – 9th Edition 9.27 Silberschatz, Galvin and Gagne ©2013
Second-Chance (clock) Page-Replacement Algorithm
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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 Edition 9.29 Silberschatz, Galvin and Gagne ©2013
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 Edition 9.30 Silberschatz, Galvin and Gagne ©2013
End of Chapter 9
Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013
