Chapter 9: Virtual Memory
Chapter 9: Virtual Memory
Background
Demand Paging
Page Replacement
Allocation of Frames
Thrashing
Memory-Mapped Files
Allocating Kernel Memory
Other Considerations
Operating-System Examples
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Objectives
To describe the benefits of a virtual memory system
To explain the concepts of demand paging, pagereplacement algorithms, and allocation of page frames
To discuss the principle of the working-set model
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Background
The instructions being executed must be in physical
Memory.
The first approach to meeting this requirement is to place
the entire logical address space in physical memory.
Dynamic loading can help to ease this restriction, but it
generally requires special precautions and extra work by
the programmer.
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Background
The requirement that instructions must be in physical
memory to be executed seems both necessary and
reasonable
but it is also unfortunate, since it limits the size of a program
to the size of physical memory.
in many cases, the entire program is not needed to load into
memory
program often have code to handle unusual error
conditions.
Arrays, lists, and tables are often allocated more memory
than they actually need.
P316
Certain options and features of a program may be used
rarely.
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Background
The ability to execute a program that is only partially in
memory would confer many benefits:
A program would no longer be constrained by the amount
of physical memory that is available. Users would be able
to write programs for an extremely large virtual address
space, simplifying the programming task.
More programs could be run at the same time, with the
corresponding increase in CPU utilization and throughput
but with no increase in response time or turnaround time.
Less I/O would he needed to load or swap each user
program into memory; so each user program would run
faster.
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Virtual Memory That is Larger Than Physical Memory
⇒
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Background
Virtual memory involves the separation of logical memory
as perceived by users from physical memory.
This separation allows an extremely large virtual
memory to be provided for programmers when only a
smaller physical memory is available
the programmer no longer needs to worry about the
amount of physical memory available
The virtual address space of a process refers to the
logical (or virtual) view of how a process is stored in
memory. Typically, this view is that a process begins
at a certain logical address-say, address 0-and exists
in contiguous memory
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Virtual--address Space
Virtual
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Demand Paging
pages are only loaded when they are demanded during
program execution; pages that are never accessed are
thus never loaded into physical memory.
Lazy swapper – never swaps a page into memory unless
page will be needed ---partial exchange
Rather than swapping the entire processes into
memory---entire exchange
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Transfer of a Paged Memory to Contiguous Disk Space
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Valid--Invalid Bit
Valid
to distinguish between the pages that are in memory and
the pages that are on the disk.
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
During address translation, if valid–invalid bit in page
table entry is i ⇒ page fault
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Page Table When Some Pages Are Not in Main Memory
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Page Fault
If there is a page that was not brought into memory, first
reference to that page will trap to operating system: pagefault trap.
procedure for handling page fault:
We check an internal table (usually kept with the
process control block) for this process to determine
whether the reference was a valid or an invalid
memory access.
If the reference was invalid, we terminate the process.
if it was valid, but we have not yet brought in that
page, we now page it in.
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Page Fault
We find a free frame (by taking one from the freeframe list, for example).
We schedule a disk operation to read the desired
page into the newly allocated frame.
When the disk read is complete, we modify the
internal table kept with the process and the page table
to indicate that the page is now in memory.
We restart the instruction that was interrupted by the
trap. The process can now access the page as though
it had always been in memory.
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Steps in Handling a Page Fault
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Pure demand paging
in the extreme case, we can start executing a process
with no pages in memory.
When the operating system sets the instruction pointer
to the first instruction of the process, which is on a
non-memory-resident page, the process immediately
faults for the page.
After this page is brought into memory; the process
continues to execute, faulting as necessary until every
page that it needs is in memory. At that point, it can
execute with no more faults.
Never bring a page into memory until it is required.
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Hardware to support demand paging
Page table
This table has the ability to mark an entry invalid
through a valid-invalid bit or special value of protection
bits.
页表增加状态位，标识该页是否在内存，在内存的页，
frame项指出物理地址。不在内存的页，应用另外字段
标识出该页在外存的地址。
Secondary memory
This memory holds those pages that are not present
in main memory.
The secondary memory is usually a high-speed disk.
It is known as the swap device, and the section of disk
used for this purpose is known as swap space.
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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)
EAT = (1 – p) x memory access
+ p (page fault overhead
+ swap page out (optional)
+ swap page in
+ restart overhead
)
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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.
it is important to keep the page-fault rate low in a demandpaging system.
Otherwise, the effective access time increases, slowing
process execution dramatically.
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Over--allocation of Memory
Over
If a process of ten pages actually uses only half of
them, then demand paging saves the I/O necessary
to load the five pages that are never used.
We could also increase our degree of multiprogramming by running twice as many processes.
Thus, if we had forty frames, we could run eight
processes, rather than the four that could run if
each required ten frames (five of which were never
used).
each of these processes may suddenly try to use all
ten of its pages, resulting in a need for sixty frames
when only forty are available.
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What happens if there is no free frame?
Terminate the user process--- this option is not the
best choice.
The operating system could instead swap out a
process, freeing all its frames and reducing the level
of multiprogramming. ---this option is a good one in
certain circumstances
Page replacement---the most common solution
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Need For Page Replacement
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Page Replacement
If no frame is free, we find one that is not currently being
used and free it.
We can free a frame by writing its contents to swap space
and changing the page table (and all other tables) to
indicate that the page is no longer in memory
We can now use the freed frame to hold the page for
which the process faulted.
P328
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Page-fault service routine including 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
write the victim frame to the disk; change the page and
frame tables accordingly
Bring the desired page into the (newly) free frame; update
the page and frame tables
Restart the process
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Page Replacement
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Overhead of page fault service time
if no frames are free, two page transfers (one out and one
in) are required. This situation effectively doubles the pagefault service time
reduce this overhead by using a modify bit (or dirty bit)
each page or frame has a modify bit associated with it in
the hardware
The modify bit for a page is set by the hardware
whenever any word or byte in the page is written into,
indicating that the page has been modified.
When we select a page for replacement, we examine its
modify bit.
If the bit is set, we know that the page has been
modified since it was read in from the disk. In this
case, we must write that page to the disk.
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Overhead of page fault service time
If the modify bit is not set, if the copy of the page on
the disk has not been overwritten (by some other
page, for example), then we need not write the
memory page to the disk
This technique also applies to read-only pages (for
example, pages of binary code). Such pages cannot be
modified; thus, they may be discarded when desired.
This scheme can significantly reduce the time required
to service a page fault, since it reduces I/O time by onehalf if the page has not been modified.
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Implement demand paging
frame-allocation algorithm
If we have multiple processes in memory, we must
decide how many frames to allocate to each process
page-replacement algorithm
when page replacement required, we must select the
frames that are to be replaced.
Designing appropriate algorithms to solve these problems
is an important task, because disk I/O is so expensive.
Even slight improvements in demand-paging methods
yield large gains in system performance
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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
At 100 bytes per page
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Graph of Page Faults Versus The Number of Frames
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First--In
First
In--First
First--Out (FIFO) Algorithm
associates with each page the time when that page was
brought into memory.
When a page must be replaced, the oldest page is
chosen.
Notice that it is not strictly necessary to record the time
when a page is brought in. We can create a FIFO queue
to hold all pages in memory. We replace the page at the
head of the queue. When a page is brought into
memory, we insert it at the tail of the queue.
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FIFO Page Replacement
Three frames are initially empty
There are 15 faults altogether
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First--In
First
In--First
First--Out (FIFO) Algorithm
its performance is not always good.
the page replaced may be an initialization module
that was used a long time ago and is no longer
needed. ---好的方面
it could contain a heavily used variable that was
initialized early and is in constant use.---差的方面
Belady‘s anomaly: the page-fault rate may increase
as the number of allocated frames increases---极端
情况
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Belady‘s anomaly
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)
1
2
9 page faults
3
1
2
4 frames
10 page faults
3
4
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FIFO Illustrating Belady’s Anomaly
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Optimal (OPT) Algorithm
Replace page that will not be used for longest period of
time
With only nine page faults!
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Optimal (OPT) Algorithm
this page-replacement algorithm guarantees the lowest
possible page fault rate for a fixed number of frames.
difficult to implement, because it requires future
knowledge of the reference string. (We encountered a
similar situation with the SJF CPU-scheduling algorithm)
Only used for measuring how well your algorithm
performs
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Least Recently Used (LRU) Algorithm
The FIFO algorithm uses the time when a page was
brought into memory
Whereas the OPT algorithm uses the time when a page
is to be used.
If we use the recent past as an approximation of the
near future, then we can replace the page that has not
been used for the longest period of time ---the leastrecently-used (LRU) algorithm.
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LRU Page Replacement
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Implement LRU
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
P335
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implement LRU
Stack implementation – keep a stack of page numbers in
a double link form
Whenever a page is referenced, it is removed from the
stack and put on the top.
the most recently used page is always at the top of the
stack and the least recently used page is always at the
bottom
No search for replacement
the tail pointer points to the bottom of the stack,
which is the LRU page.
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Stack implementation
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Evaluation of LRU Implementation
Hardware assistance beyond the standard TLB registers.
The updating of the clock fields or stack must be done for
every memory reference.
If we were to use an interrupt for every reference to allow
software to update such data structures, it would slow
every memory reference by a factor of at least ten, hence
slowing every user process by a factor of ten.
Few systems could tolerate that level of overhead for
memory management.
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LRU Approximation Algorithms
The reference bit for a page is set by the hardware
whenever that page is referenced (either a read or a write
to any byte in the page).
Reference bits are associated with each entry in the page
table.
Initially, all bits are cleared (to 0) by the operating system.
As a user process executes, the bit associated with each
page referenced is set (to 1) by the hardware
after some time, we can determine which pages have been
used and which have not been used by examining the
reference bits, although we do not know the order of use.
This information is the basis for approximate LRU
replacement.
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Additional--ReferenceAdditional
Reference-Bits Algorithm
We can keep an 8-bit byte for each page in a table in
memory.
At regular intervals, a timer interrupt transfers control to the
operating system. The operating system shifts the
reference bit for each page into the high-order bit of its 8bit byte, shifting the other bits right by 1 bit and discarding
the low-order bit.
These 8-bit shift registers contain the history of page use
for the last eight time periods.
If we interpret these 8-bit bytes as unsigned integers, the
page with the lowest number is the LRU page, and it can
be replaced.
Same value---FIFO
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Second--chance replacement
Second
The basic algorithm of second-chance replacement is a FIFO
replacement of algorithm.
When a page has been selected, however, we inspect its
reference bit. If the value is 0, we proceed to replace this
page; but if the reference bit is set to 1, we give the page a
second chance and move on to select the next FIFO page.
When a page gets a second chance, its reference bit is
cleared, and its arrived time is reset to the current time.
Thus, a page that is given a second chance will not be
replaced until all other pages have been replaced (or given
second chances).
In addition, if a page is used often enough to keep its
reference bit set, it will never- be replaced.
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Implement the secondsecond-chance algorithm
a circular queue------Clock algorithm
A pointer (that is, a hand on the clock) indicates which page is
to be replaced next.
When a frame is needed, the pointer advances until it finds a
page with a 0 reference bit.
As it advances, it clears the reference bits.
Once a victim page is found, the page is replaces, and the
new page is inserted in the circular queue in that position.
In the worst case, when all bits are set, the pointer cycles
through the whole queue, giving each page a second chance.
it clears all the reference bits before selecting the next page
for replacement.
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Second--Chance (clock) PageSecond
Page-Replacement Algorithm
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Enhanced SecondSecond-Chance Algorithm
considering the reference bit and the modify bit as an
ordered pair.
four possible classes
(0,0) neither recently used nor modified---best page to
replace
(0, 1) not recently used but modified---not quite as good,
because the page will need to be written out before
replacement
(1,0) recently used but clean---probably will be used
again soon
(1,1) recently used and modified---probably will be used
again soon, and the page will be need to be written out
to disk before it can be replaced
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Enhanced SecondSecond-Chance Algorithm
When page replacement is called for, we use the same
scheme as in the clock algorithm
examine the class to which that page belongs. We replace
the first page encountered in the lowest nonempty class.
Notice that we may have to scan the circular queue several
times before we find a page to be replaced.
we give preference to those pages that have been
modified
to reduce the number of I/Os required.
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Counting--Based Page Replacement
Counting
Keep a counter of the number of references that have been
made to each page
LFU (least frequently used) Algorithm: replaces page
with smallest count. The reason for this selection is that an
actively used page should have a large reference count.
MFU (most frequently used) Algorithm: based on the
argument that the page with the smallest count was
probably just brought in and has yet to be used
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Page--Buffering Algorithms
Page
Not a standalone page replacement algorithm, it
cooperates with a page replacement algorithm
systems commonly keep a pool of free frames.
When a page fault occurs, a victim frame is chosen as
before.
However; the desired page is read into a free frame from
the pool before the victim is written out.
This procedure allows the process to restart as soon as
possible, without waiting for the victim page to be written
out.
When the victim is later written out, its frame is added to
the free-frame pool.
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Minimum Number of Frames
Each process needs minimum number of pages
保证进程正常运行所需的最小页框数，少于此值，进程将无法
运行。
reasons for allocating at least a minimum number of frames
Involving performance. Obviously, as the number of
frames allocated to each process decreases, the pagefault rate increases, slowing process execution.
when a page fault occurs before an executing instruction is
complete, the instruction must be restarted. Consequently,
we must have enough frames to hold all the different
pages that any single instruction can reference.
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Allocation 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
the minimum number of frames per process is defined by
the architecture
the maximum number is defined by the amount of
available physical memory.
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Frame Allocation Method
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
Use a proportional allocation scheme using
priorities rather than size
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Global vs. Local Replacement
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
One problem with a global replacement algorithm is
that a process cannot control its own page-fault rate.
Local replacement might hinder a process, however,
by not making available to it other, less used pages
of memory
Global replacement generally results in greater
system throughput and is therefore the more
common method.
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Thrashing
现象：If the process does not have the number of
frames it needs to support pages in active use, it
will quickly page-fault. At this point, it must replace
some page. However, since all its pages are in
active use, it must replace a page that will be
needed again right away. Consequently, it quickly
faults again, and again, and again, replacing
pages that it must bring back in immediately.
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Thrashing
定义：This high paging activity is called thrashing.
本质： A process is thrashing if it is spending more time
paging than executing.
原因: (a) If CPU utilization is too low, we increase the
degree of multiprogramming by introducing a new
process to the system. (b) A global page-replacement
algorithm is used
The new process tries to get started by taking frames
from running processes, causing more page faults and
a longer queue for paging service. As a result, CPU
utilization drops even further; and the CPU scheduler
tries to increase the degree of multiprogramming even
more. Thrashing has occurred
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Thrashing (Cont.)
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Limit the effects of thrashing
by using a local replacement algorithm
if one process starts thrashing, it cannot steal frames
from another process and cause the latter to thrash as
well.
However, the problem is not entirely solved. If
processes are thrashing, they will be in the queue for
the paging device most of the time. The average
service time for a page fault will increase because of
the longer average queue for the paging device.
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Prevent Thrashing
must provide a process with as many frames as it needs.
The working-set strategy starts by looking at how many
frames a process is actually using. This approach defines
the locality model of process execution.
The locality model states that, as a process executes, it
moves from locality to locality. A locality is a set of pages
that are actively used together. A program is generally
composed of several different localities, which may
overlap.
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Locality In A MemoryMemory-Reference Pattern
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Working--Set Model
Working
based on the assumption of locality.
∆ ≡ working-set window ≡ a fixed number of page
references
The set of pages in the most recent ∆ page
references is the working set
If a page is in active use, it will be in the working set.
If it is no longer being used, it will drop from the
working set ∆ time units after its last reference
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Working--set model
Working
∆ = 10 memory references
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Size of ∆
if ∆ too small will not encompass entire locality
if ∆ too large will encompass several localities
if ∆ = ∞ ⇒ will encompass entire program
The operating system monitors the working set of each
process and allocates to that working set enough frames
to provide it with its working-set size.
This working-set strategy prevents thrashing while
keeping the degree of multiprogramming as high as
possible. Thus, it optimizes CPU utilization.
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Page--Fault Frequency Scheme
Page
Establish “acceptable” page-fault rate
If actual rate too low, process loses frame
If actual rate too high, process gains frame
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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
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Other Issues – Page Size
Page size selection must take into consideration:
fragmentation
table size
I/O overhead
locality
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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
The virtual memory manager maintains a list of free page
frames. Associated with this list is a threshold value that
is used to indicate whether sufficient free memory is
available.
If a page fault occurs for a process that is below its
working-set maximum, the virtual memory manager
allocates a page from this list of free page frames.
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Windows XP
If a process is at its working-set maximum and it incurs a
page fault, it must select a page for replacement using a
local page-replacement policy.
When the amount of free memory falls below the
threshold, the virtual memory manager uses a tactic
known as automatic working-set trimming to restore the
value above the threshold.
Automatic working-set trimming works by evaluating the
number of pages allocated to processes. lf a process has
been allocated snore pages than its working-set minimum,
the virtual memory management removes pages until the
process reaches its working-set minimum.
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Windows XP
A process that is at its working-set minimum may be
allocated pages from the free-page frame list once
sufficient free memory is available.
The algorithm used to determine which page to remove
from a working set depends on the type of processor.
On single-processor 80x86 systems, Windows XP uses
a variation of the clock algorithm.
On Alpha and multiprocessor x86 systems, Windows
XP uses a variation on the FIFO algorithm.
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Homework: 2、
2、4、5、7、8、10、
10、12、
12、13
End of Chapter 9