2. Virtual memory
Consider a typical, large application:
There are many components that are mutually
exclusive. Example: A unique function selected
dependent on user choice.
Error routines and exception handlers are very
rarely used.
Most programs exhibit a slowly changing
locality of reference. There are two types of
locality: spatial and temporal.
3. Characteristics of Paging
and Segmentation
Memory references are dynamically
translated into physical addresses at run time
a process may be swapped in and out of main
memory such that it occupies different regions
A process may be broken up into pieces
(pages or segments) that do not need to be
located contiguously in main memory
Hence: all pieces of a process do not need to
be loaded in main memory during execution
computation may proceed for some time if the
next instruction to be fetch (or the next data to be
accessed) is in a piece located in main memory
4. Process Execution
The OS brings into main memory only a
few pieces of the program (including its
starting point)
Each page/segment table entry has a
present bit that is set only if the
corresponding piece is in main memory
The resident set is the portion of the
process that is in main memory
An interrupt (memory fault) is generated
when the memory reference is on a piece
not present in main memory
5. Process Execution (cont.)
OS places the process in a Blocking state
OS issues a disk I/O Read request to bring
into main memory the piece referenced to
another process is dispatched to run while
the disk I/O takes place
an interrupt is issued when the disk I/O
completes
this causes the OS to place the affected process
in the Ready state
6. Advantages of Partial
Loading
More processes can be maintained in
main memory
only load in some of the pieces of each
process
With more processes in main memory, it
is more likely that a process will be in the
Ready state at any given time
A process can now execute even if it is
larger than the main memory size
it is even possible to use more bits for
logical addresses than the bits needed
for addressing the physical memory
7. Virtual Memory: large as you
wish!
Ex: 16 bits are needed to address a physical
memory of 64KB
lets use a page size of 1KB so that 10 bits are
needed for offsets within a page
For the page number part of a logical address
we may use a number of bits larger than 6, say
22 (a modest value!!)
The memory referenced by a logical
address is called virtual memory
is maintained on secondary memory (ex: disk)
pieces are bring into main memory only when
needed
8. Virtual Memory (cont.)
For better performance, the file system is
often bypassed and virtual memory is stored
in a special area of the disk called the swap
space
larger blocks are used and file lookups and
indirect allocation methods are not used
By contrast, physical memory is the
memory referenced by a physical address
is located on DRAM
The translation from logical address to
physical address is done by indexing the
appropriate page/segment table with the
help of memory management hardware
9. Possibility of trashing
To accommodate as many processes as
possible, only a few pieces of each process is
maintained in main memory
But main memory may be full: when the OS
brings one piece in, it must swap one piece out
The OS must not swap out a piece of a process
just before that piece is needed
If it does this too often this leads to trashing:
The processor spends most of its time swapping
pieces rather than executing user instructions
10. Locality
Temporal locality: Addresses that are referenced
at some time Ts will be accessed in the near
future (Ts + delta_time) with high probability.
Example : Execution in a loop.
Spatial locality: Items whose addresses are near
one another tend to be referenced close together
in time. Example: Accessing array elements.
How can we exploit this characteristics of
programs? Keep only the current locality in the
main memory. Need not keep the entire program
in the main memory.
11. Locality and Virtual Memory
Principle of locality of references: memory
references within a process tend to cluster
Hence: only a few pieces of a process will
be needed over a short period of time
Possible to make intelligent guesses about
which pieces will be needed in the future
This suggests that virtual memory may
work efficiently (ie: trashing should not
occur too often)
13. Demand paging
Main memory (physical address space) as well as
user address space (virtual address space) are
logically partitioned into equal chunks known as
pages. Main memory pages (sometimes known as
frames) and virtual memory pages are of the same
size.
Virtual address (VA) is viewed as a pair (virtual page
number, offset within the page). Example: Consider a
virtual space of 16K , with 2K page size and an
address 3045. What the virtual page number and
offset corresponding to this VA?
14. Virtual Page Number and
Offset
3045 / 2048 = 1
3045 % 2048 = 3045 - 2048 = 997
VP# = 1
Offset within page = 997
Page Size is always a power of 2? Why?
15. Page Size Criteria
Consider the binary value of address 3045 :
1011 1110 0101
for 16K address space the address will be 14
bits. Rewrite:
00 1011 1110 0101
A 2K address space will have offset range 0 -
2047 (11 bits)
Offset within page
Page#
001 011 1110 0101
16. Demand paging (contd.)
There is only one physical address space but as many
virtual address spaces as the number of processes in
the system. At any time physical memory may
contain pages from many process address space.
Pages are brought into the main memory when
needed and “rolled out” depending on a page
replacement policy.
Consider a 8K main (physical) memory and three
virtual address spaces of 2K, 3K and 4K each. Page
size of 1K. The status of the memory mapping at
some time is as shown.
18. Issues in demand paging
How to keep track of which logical page goes
where in the main memory? More specifically,
what are the data structures needed?
Page table, one per logical address space.
How to translate logical address into physical
address and when?
Address translation algorithm applied every time a
memory reference is needed.
How to avoid repeated translations?
After all most programs exhibit good locality. “cache
recent translations”
19. Issues in demand paging
(contd.)
What if main memory is full and your process
demands a new page? What is the policy for
page replacement? LRU, MRU, FIFO, random?
Do we need to roll out every page that goes
into main memory? No, only the ones that
are modified. How to keep track of this info
and such other memory management
information? In the page table as special bits.
20. Support Needed for
Virtual Memory
Memory management hardware must
support paging and/or segmentation
OS must be able to manage the movement
of pages and/or segments between
secondary memory and main memory
We will first discuss the hardware aspects;
then the algorithms used by the OS
21. Paging
Each page table entry contains a present bit to indicate
whether the page is in main memory or not.
If it is in main memory, the entry contains the frame
number of the corresponding page in main memory
If it is not in main memory, the entry may contain the
address of that page on disk or the page number may
be used to index another table (often in the PCB) to
obtain the address of that page on disk
Typically, each process has its own page table
22. Paging
A modified bit indicates if the page has
been altered since it was last loaded into
main memory
If no change has been made, the page does
not have to be written to the disk when it
needs to be swapped out
Other control bits may be present if
protection is managed at the page level
a read-only/read-write bit
protection level bit: kernel page or user
page (more bits are used when the
processor supports more than 2 protection
levels)
23. Page Table Structure
Page tables are variable in length
(depends on process size)
then must be in main memory instead of
registers
A single register holds the starting
physical address of the page table of
the currently running process
25. Sharing Pages
If we share the same code among different
users, it is sufficient to keep only one copy in
main memory
Shared code must be reentrant (ie: non self-
modifying) so that 2 or more processes can
execute the same code
If we use paging, each sharing process will have
a page table who’s entry points to the same
frames: only one copy is in main memory
But each user needs to have its own private data
pages
27. Translation Lookaside Buffer
Because the page table is in main memory,
each virtual memory reference causes at
least two physical memory accesses
one to fetch the page table entry
one to fetch the data
To overcome this problem a special cache is
set up for page table entries
called the TLB - Translation Lookaside Buffer
Contains page table entries that have been most
recently used
Works similar to main memory cache
28. Translation Lookaside Buffer
Given a logical address, the processor examines
the TLB
If page table entry is present (a hit), the frame
number is retrieved and the real (physical)
address is formed
If page table entry is not found in the TLB (a
miss), the page number is used to index the
process page table
if present bit is set then the corresponding frame is
accessed
if not, a page fault is issued to bring in the referenced
page in main memory
The TLB is updated to include the new page entry
30. TLB: further comments
TLB use associative mapping hardware to
simultaneously interrogates all TLB entries
to find a match on page number
The TLB must be flushed each time a new
process enters the Running state
The CPU uses two levels of cache on each
virtual memory reference
first the TLB: to convert the logical address to
the physical address
once the physical address is formed, the CPU
then looks in the cache for the referenced word
31. Page Tables and Virtual
Memory
Most computer systems support a very large
virtual address space
32 to 64 bits are used for logical addresses
If (only) 32 bits are used with 4KB pages, a page
table may have 2^{20} entries
The entire page table may take up too much main
memory. Hence, page tables are often also stored
in virtual memory and subjected to paging
When a process is running, part of its page table
must be in main memory (including the page table
entry of the currently executing page)
32. Inverted Page Table
Another solution (PowerPC, IBM Risk 6000) to the
problem of maintaining large page tables is to use
an Inverted Page Table (IPT)
We generally have only one IPT for the whole
system
There is only one IPT entry per physical frame
(rather than one per virtual page)
this reduces a lot the amount of memory
needed for page tables
The 1st entry of the IPT is for frame #1 ... the nth
entry of the IPT is for frame #n and each of these
entries contains the virtual page number
Thus this table is inverted
33. Inverted Page Table
The process ID with the
virtual page number could
be used to search the IPT
to obtain the frame #
For better performance,
hashing is used to obtain a
hash table entry which
points to a IPT entry
A page fault occurs if
no match is found
chaining is used to
manage hashing
overflow d = offset within page
34. The Page Size Issue
Page size is defined by hardware; always a
power of 2 for more efficient logical to
physical address translation. But exactly
which size to use is a difficult question:
Large page size is good since for a small page
size, more pages are required per process
More pages per process means larger page tables.
Hence, a large portion of page tables in virtual memory
Small page size is good to minimize internal
fragmentation
Large page size is good since disks are designed
to efficiently transfer large blocks of data
Larger page sizes means less pages in main
memory; this increases the TLB hit ratio
35. The Page Size Issue
With a very small page
size, each page matches
the code that is actually
used: faults are low
Increased page size
causes each page to
contain more code that
is not used. Page faults
rise.
Page faults decrease if
we can approach point P
were the size of a page
is equal to the size of
the entire process
36. The Page Size Issue
Page fault rate is also
determined by the
number of frames
allocated per process
Page faults drops to a
reasonable value when
W frames are allocated
Drops to 0 when the
number (N) of frames
is such that a process
is entirely in memory
37. The Page Size Issue
Page sizes from 1KB to 4KB are most
commonly used
But the issue is non trivial. Hence
some processors are now supporting
multiple page sizes. Ex:
Pentium supports 2 sizes: 4KB or 4MB
R4000 supports 7 sizes: 4KB to 16MB
38. Operating System Software
Memory management software depends
on whether the hardware supports paging
or segmentation or both
Pure segmentation systems are rare.
Segments are usually paged -- memory
management issues are then those of
paging
We shall thus concentrate on issues
associated with paging
To achieve good performance we need a
low page fault rate
39. Fetch Policy
Determines when a page should be brought
into main memory. Two common policies:
Demand paging only brings pages into main
memory when a reference is made to a
location on the page (ie: paging on demand
only)
many page faults when process first started but
should decrease as more pages are brought in
Prepaging brings in more pages than needed
locality of references suggest that it is more
efficient to bring in pages that reside
contiguously on the disk
efficiency not definitely established: the extra
pages brought in are “often” not referenced
40. Placement policy
Determines where in real memory a
process piece resides
For pure segmentation systems:
first-fit, next fit... are possible choices (a real
issue)
For paging (and paged segmentation):
the hardware decides where to place the page:
the chosen frame location is irrelevant since all
memory frames are equivalent (not an issue)
41. Replacement Policy
Deals with the selection of a page in
main memory to be replaced when a
new page is brought in
This occurs whenever main memory is
full (no free frame available)
Occurs often since the OS tries to
bring into main memory as many
processes as it can to increase the
multiprogramming level
42. Replacement Policy
Not all pages in main memory can be
selected for replacement
Some frames are locked (cannot be paged
out):
much of the kernel is held on locked frames as
well as key control structures and I/O buffers
The OS might decide that the set of pages
considered for replacement should be:
limited to those of the process that has suffered
the page fault
the set of all pages in unlocked frames
43. Replacement Policy
The decision for the set of pages to be
considered for replacement is related to
the resident set management strategy:
how many page frames are to be allocated to
each process? We will discuss this later
No matter what is the set of pages
considered for replacement, the
replacement policy deals with algorithms
that will choose the page within that set
44. Basic algorithms for the replacement
policy
The Optimal policy selects for
replacement the page for which the time
to the next reference is the longest
produces the fewest number of page faults
impossible to implement (need to know the
future) but serves as a standard to compare
with the other algorithms we shall study:
Least recently used (LRU)
First-in, first-out (FIFO)
Clock
45. The LRU Policy
Replaces the page that has not been referenced
for the longest time
By the principle of locality, this should be the page
least likely to be referenced in the near future
performs nearly as well as the optimal policy
Example: A process of 5 pages with an OS that
fixes the resident set size to 3
46. Note on counting page
faults
When the main memory is empty, each new
page we bring in is a result of a page fault
For the purpose of comparing the different
algorithms, we are not counting these initial
page faults
because the number of these is the same for all
algorithms
But, in contrast to what is shown in the
figures, these initial references are really
producing page faults
47. Implementation of the LRU
Policy
Each page could be tagged (in the page table
entry) with the time at each memory
reference.
The LRU page is the one with the smallest time
value (needs to be searched at each page fault)
This would require expensive hardware and a
great deal of overhead.
Consequently very few computer systems
provide sufficient hardware support for true
LRU replacement policy
Other algorithms are used instead
48. The FIFO Policy
Treats page frames allocated to a
process as a circular buffer
When the buffer is full, the oldest page is
replaced. Hence: first-in, first-out
This is not necessarily the same as the LRU
page
A frequently used page is often the oldest, so
it will be repeatedly paged out by FIFO
Simple to implement
requires only a pointer that circles through
the page frames of the process
