LecturePPT_Unit_3b_AY2021-22_TechngVrsn.ppt

Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th Edition
U3_Virtual Memory

8.2 Silberschatz, Galvin and Gagne ©2013
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
 To examine the relationship between shared
memory and memory-mapped files
 To explore how kernel memory is managed

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

Background (Cont.)
 Virtual 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

Background (Cont.)
 Virtual memory:
 Physical memory organized in page frames
 MMU must map logical to physical
 Virtual memory can be implemented via:
 Demand paging
 Demand segmentation

Demand Paging
 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
 Page is needed
 invalid reference  abort
 not-in-memory  bring to memory and reference it
 Already-in-memory  reference it
 Lazy swapper – never swaps a page into memory unless page
will be needed
 Swapper that deals with pages is a pager

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

Demand paging: 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
1. Operating system looks at page table to decide:
 Invalid reference  abort
 Not in memory: means page fault
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

Steps in Handling a Page Fault

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

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 )

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

What Happens if There is no Free Frame?
 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
 Use modify (dirty) bit to reduce overhead of page
transfers – only modified pages are written to disk

Why page replacement?

Page Replacement Algorithms
 Page-replacement algorithm
 Want lowest number of page-faults
 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

Graph of Page Faults Versus The Number of Frames

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)
 Number of page faults: 15
 To track ages of pages, just use a FIFO queue

 Consider string: 1,2,3,4,1,2,5,1,2,3,4,5
 Generally, adding more frames reduces page faults
If adding more frames causes more page faults, then it is
known as
Belady’s Anomaly

FIFO Illustrating Belady’s Anomaly

Optimal Algorithm
 Replace page that will not be used for longest period of time

Optimal Algorithm
 Replace page that will not be used for longest period of time
 Optimal page replacement algorithm gives minimum page faults
 Limitation: Can’t read the future
 Used for measuring how well any algorithm performs when
compared with optimal algorithm.
 Number of page faults for the above reference string: 9

Least Recently Used (LRU) Algorithm
 Use past knowledge rather than future
 Replace page that has not been used in the most amount of
time

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

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
 Stack implementation
 Keep a stack of page numbers in a double link form:
 But each update more expensive
 No search for replacement
 LRU and OPT are cases of stack algorithms that don’t have
Belady’s Anomaly

Use Of A Stack to Record Most Recent Page References

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
 Second-chance algorithm
 Generally FIFO, plus hardware-provided reference bit
 Known as Clock page replacement algorithm
 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

Second-Chance (clock) Page-Replacement Algorithm

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

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

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

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
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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 Page Replacement
 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

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:
 Low CPU utilization
 Operating system thinking 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.)

How to handle thrashing:
Using Page-Fault Frequency
 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

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