CPTR 352 - Virtual Memory
What are the characteristics of paging?
- All memory references within a process are logical address that are dynamically translated into physical addresses at run time. - A process may be broken up into several pages and those pages need not be contiguously located in main memory - One process can be swapped out to make room for another process
What happens in Prepaging?
- Attempt to anticipate upcoming page requests - More pages in memory - Payoff if prediction is accurate
What are the implications of localality?
- Do we really need to set aside space for the entire process if some of it is never used? - More processes may be maintained in main memory - A process may be larger than main memory * The programmer is no longer limited by user's hardware * User's experience deteriorates gradually with size * * Swapping to disk still works * * Versus abrupt, hard limit
Variable Allocation, Global Scope
- Easiest to implement * One global policy * All frames available to all processes - Risk of Wealth inequity
FIFO Replacement Policy
- Evict longest resident - Easy, but early page might still be popular
Least Recently Used (LRU) Repl. Policy
- Evict page that has been ignored the longest - Maintain a sorted list of pages * Resort on every memory reference Much more expensive - Best of implementable policies * Effective due to locality
Optimal Replacement Policy
- Evict page that will not be referenced for the longest time in the future - Theoretical policy used for comparison
Fixed-Allocation
- Gives a process a fixed number of frames in main memory within which to execute. - fair but risk of waste
Fixed Allocation
- Gives process a fixed number of frames - When a page fault occurs, one of the pages of that process must be replaced
Aging Page Replacement Policy
- Hardware sets flag in page table when used (Referenced bit (r-bit)) - Keep an aging register for each page - Periodically shift aging register right and shift in page's referenced bit - Look for register with smallest value * Most shifts right without a set r-bit coming in
Page-Fault-Frequency
- If many page faults in t time units, increase size - If few page faults in t time units, decrease size
Fixed Allocation, Local Scope
- Initial decision applies to lifetime of process - Small values gives high page fault rate * More time swapping * Less CPU time for process
Variable Allocation, Local Scope
- Initial frame count allocation - Replacement selected from resident set of process with page fault - Periodic re-evaluation of allocation to increase or decrease * Attempt to predict future demand
What are the properties of small pages?
- Less internal fragmentation - More likely to leave room for other processes - More pages per process - Larger page table (swapped out to disk?) - Less efficient disk I/O
Spatial Locality
- Memory references tend to cluster in space - Nearby values are more likely to be needed soon
Temporal Locality
- Memory references tend to cluster in time - Recently-reference values are more likely to be needed soon
Variable-allocation
- Number of frames can vary over process' lifetime - Risk of few processes hogging the system - Allocating scarce resources
Optimal Working Set
- Pages that will be needed in next t time units - Impossible to know
What is a page fault?
- Reference to a page that is not in memory - Hardware trap to switch from user mode to kernel mode and have OS load the page and restart
What happens in Preclearing?
- Scan frames for dirty pages and write out - Keeps disk busy - Pages more likely to be clean - Urgent I/O may be delayed for less urgent I/O
Second-Change Page Repl. Policy (Clock / Clock Hand)
- Uses r-bit to divide all memory into: * Recently/ not recently referenced - On page fault: * If pointer is at page p with r = 0, then select p and advance the pointer to the next page - Otherwise, keep looking (but first clear r and advance pointer)
What happens in demand clearing?
- Wait till a page is evicted to write - I/O cost is doubled
What happens in Demand Paging?
- Wait till a page is requested - Fewer unused pages in memory - Longer wait time
Memory has 3 frames containing pages 0, 1, and 2, where 0 is the oldest page. RS = {0 3 0 1} 1) When page 3 is referenced, FIFO will replace page _____.
0
Pages 0 - 2 reside in frames 0 - 2. All r-bits are 0 and the pointer is at page 0: Page | r Frame 0 0 | 0 Frame 1 1 | 0 Frame 2 2 | 0 Assume the reference string RS = {3, 2, 1, 0}. 1) The first page fault will replace page _____.
0
Pages 0 - 2 reside in frames 0 - 2. All r-bits are 0 and the pointer is at page 0: Page | r Frame 0 0 | 0 Frame 1 1 | 0 Frame 2 2 | 0 Assume the reference string RS = {3, 2, 1, 0}. 2) The second page fault will replace page _____.
1
RS = {0 1 2 1 2 3 4 5 2 2 2 2 2 1 0} 1) With d = 4, the minimum working set size is _____.
1
RS = {0 1 2 1 2 3 4 5 2 2 2 2 2 1 0} 3) With d = 5, the minimum working set size is _____.
1
Third-chance algorithm With 3 frames, RS = {0 1 2 1 3} where the first reference modifies the page 0, and the pointer at page 0, the first page fault will replace page _____.
1
With 4 frames and RS = {1 2 3 4 2 }, the queue for LRU replacement will at the end contain the sequence of pages _____ . The left-most page is the Q head (least recent) and the right-most page is the Q end (most recent).
1 3 4 2
The aging registers have 4 bits. The parameter d is 100. A page p is accessed at times 50, 150, and 160. 1) At time 100 the register will contain _____.
1000
The aging registers have 4 bits. The parameter d is 100. A page p is accessed at times 50, 150, and 160. 2) At time 170 the register of page p will contain _____.
1000
Third-chance algorithm When a page with the bits 00 is modified, the bits are set to ______.
11
The aging registers have 4 bits. The parameter d is 100. A page p is accessed at times 50, 150, and 160. 3) A time 200 the register of page p will contain _____.
1100 (The page was accessed during the first 100 and the second 100 references. Thus the 2 leading bits are set to 1.)
Memory has 3 frames containing pages 0, 1, and 2, where 0 is the oldest page. RS = {0 3 0 1} 3) The best page to remove when page 3 is referenced would be ______.
2
RS = {0 1 2 1 2 3 4 5 2 2 2 2 2 1 0} 5) With d = 6, the minimum working set size is _____.
2
The VM address uses: s1: 3 bits s2: 3 bits p1: 3 bits p2: 3 bits w: 8 bits Q: The total number of words that make up the VM is _____.
2^20 (3+3+3+3+8)
The VM addresses use: s1: 3 bits s2: 4 bits p: 5 bits 2) The number of pages per segment is _____.
2^5 (p)
The VM addresses use: s: 5 bits p1: 3 bits p2: 4 bits 1) The number of segments is _____.
2^5 (s)
What is the potential size of VM for a 64-bit virtual address?
2^64
The VM addresses use: s: 5 bits p1: 3 bits p2: 4 bits 2) The number of pages per segment is _____.
2^7 (p1 + p2)
The VM addresses use: s1: 3 bits s2: 4 bits p: 5 bits 1) The number of segments is _____.
2^7 (s1 + s2)
Memory has 3 frames containing pages 0, 1, and 2, where 0 is the oldest page. RS = {0 3 0 1} 2) The total number of page faults for the RS is _____.
3
Pages 0 - 2 reside in frames 0 - 2. All r-bits are 0 and the pointer is at page 0: Page | r Frame 0 0 | 0 Frame 1 1 | 0 Frame 2 2 | 0 Assume the reference string RS = {3, 2, 1, 0}. 3) With LRU, the second page fault would replace page _____.
3
RS = {0 1 2 1 2 3 4 5 2 2 2 2 2 1 0} 2) With d = 4, the maximum working set size is _____.
4
memory with 2 page frames contains pages 5 and 3. Time: 1 2 3 4 5 6 7 RS: 5 5 5 1 3 5 3 1) The first page fault will occur at time (optimal) _____
4
RS = {0 1 2 1 2 3 4 5 2 2 2 2 2 1 0} 4) With d = 5, the maximum working set size is _____.
5
RS = {0 1 2 1 2 3 4 5 2 2 2 2 2 1 0} 6) With d = 6, the maximum working set size is _____.
5
memory with 2 page frames contains pages 5 and 3. Time: 1 2 3 4 5 6 7 RS: 5 5 5 1 3 5 3 2) Page _____ will be replaced by the first page fault.
5
Page
A fixed-size contiguous block of a logical address space identified by a page number
Frame
A fixed-size contiguous block of physical memory identified by a frame number
With 3 page frames and RS = {0 1 2 0 3 0}, FIFO will cause _____ LRU.
A larger number of page faults than
A process executes the code br m ... m: ld R,x where the branch instruction is on a resident page and the instruction "ld" loads the content of the variable x into a register R. Q: Executing the code _____.
Can trigger 0, 1, or 2 page faults (If both the ld instruction and x are on the same page as br then no page fault is triggered. If ld or x is on a different page then 1 page fault is triggered. If ld and x are both on different pages from br then 2 page faults are triggered.)
What does a page table do?
Converts page to frame
The aging registers have 4 bits. The parameter d is 100. Page p1 has not been accessed for more than 400 references and page p2 has not been accessed for more than 600 references. Q: When a page fault occurs, the aging algorithm will replace _____.
Either page chosen at random
T/F: The size of the working set grows linearly with d.
F
T/F: VM cannot be implemented without the modified bit.
F
Order the replacement policies LRU, CLOCK, OPT, FIFO, and in order of least effective to most effective (based on page faults per 1000 references vs number of frames allocated)
FIFO, CLOCK, LRU, and OPT
How can page policies be divided?
Into fixed or variable resident set size
Why must moving pages between memory and disk must be minimized?
It's time consuming. When the page to be removed has not been modified during execution then an exact copy still exists on the disk and the page does not have to be copied back to the disk.
How can the replacement scope be divided?
Local or global
What is less likely with OO programs?
Locality
How do you determine the logical address?
Logical address = page * pageSize + offset
Page Table
Map from page number to frame number
Are all pages necessarily in memory?
No
Will further allocations of pages always significantly decreased the page fault rate?
No, beyond a certain size, further allocations of pages will not significantly decrease the page fault rate
What is the unintended bonus of paging and what happens?
Not all pages need to be in memory at once. - Load first page of process and start executing - Attempt to reference a non-resident page will be trapped by the hardware and reported to the OS - OS can load the page, update the page table, and retry the instruction
What are the page replacement algorithms?
Optimal working set, working set, page-fault-frequency
What makes up a logical address space?
Pages
Working Set
Pages that were needed in past t time units
How do you determine the physical address?
Physical address = frame * frameSize + offset
______ number of pages loaded increases page faults
Small
What was the original use of swapping?
Swap in or out an entire process
T/F: VM cannot be implemented without demand paging.
T
T/F: VM cannot be implemented without page replacement.
T
What is resident set management?
The OS must decide how many pages to bring into main memory
Demand Paging
The principle of loading a page into memory only when the page is needed, rather than at the start of the execution.
How is overhead reduced when dealing with page tables?
To reduce the overhead, a table can be divided into individual pages and brought into memory as needed using demand paging and page replacement.
How to determine which frame to use?
Use a replacement policy algorithms - Optimal, FIFO, Least Recently Used, Aging Page Bit, Second-Chance (Clock), Third-Chance (not-recently-used)
Local Replacement Scope
When a page fault occurs, choose only among the resident pages of the process that generated the page fault
Global Replacement Scope
When a page fault occurs, consider all pages in main memory
modified-bit (m-bit)
a binary flag in each page table entry that indicates whether the corresponding page has been modified during execution. The modified bit is set to 1 automatically by any instruction that stores data into the page and is used by the operating system to minimize the movement of data to disk.
Present bit
a binary flag in each page table entry that indicates whether the corresponding page is currently resident in memory. If a page is resident, then the entry points to the frame that holds the page.
referenced bit (r-bit)
a bit associated with a page and is set automatically by the hardware whenever the page is referenced by any instruction.
VM
a collection of one or more logical address spaces, each of which may exceed the size of physical memory. A logical address is then referred to as a virtual address. A paged VM creates a single large contiguous address space per process. A paged VM with segmentation creates multiple large address spaces per process, each of which is paged.
Page fault
an interrupt that occurs when a program attempts to reference a non-resident page. The interrupt triggers the operating system to find the page on disk, copy the page into a frame, and set the present bit to 1.
aging register
associated with a page and is shifted periodically to the right by 1 bit. Unless the most significant bit is set to 1, the page is aging in the sense that the associated register value is steadily decreasing.
The page fault rate _____ as d increases.
decreases
aging page replacement algorithm
does not maintain pages sorted in the exact LRU order, but groups together pages referenced during a period of d consecutive references. Each period is represented by 1 bit in a periodically shifting aging register.
Pages are loaded into ____________
frames
When p = 1 then the corresponding page resides _____.
in both physical memory and on disk
When p = 0 then the corresponding page resides _____.
on disk only
p = 0 means the page is currently not resident in memory. p = 1 means the page is currently resident in memory. m = 0 means the page has not been modified. m = 1 means the page has been modified. 3) A page fault is triggered when _____.
p = 0 and m = 0
p = 0 means the page is currently not resident in memory. p = 1 means the page is currently resident in memory. m = 0 means the page has not been modified. m = 1 means the page has been modified. 2) Immediately after a page replacement, the bits in the page table are _____.
p = 1 and m = 0
p = 0 means the page is currently not resident in memory. p = 1 means the page is currently resident in memory. m = 0 means the page has not been modified. m = 1 means the page has been modified. 1) A page must be written back to disk when _____.
p = 1 and m = 1
Third-chance algorithm A page with the r/m-bits set to 01 has been _____.
referenced and modified
The _______ the amount of memory allocated to each process, the _______ processes can reside in memory
smaller, more
Page replacement
the act of overwriting a page in memory with a different page loaded from the disk when needed.
reference string
the sequence of page numbers referenced by an executing program during a given time interval. Reference strings are used to compare different page replacement algorithms by counting the number of page faults generated.
The optimal working set of a process
the set of pages that will be needed in the immediate future and thus should be resident. The size of the working set varies with the program's behavior. When execution is highly localized in long tight loops and static data structures, the set contains a small number of repeated page numbers. When execution involves many branches or highly dynamic data structures, the set contains many different page numbers.
