2. Agenda
Introduction to windows os
Virtual memory
Virtual address translation
Page files / page faults
Working set
Physical memory
Conclusion
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3. Introduction to windows os
Microsoft Windows is a series of graphical interface operating systems
developed, marketed, and sold by Microsoft.
Microsoft Windows came to dominate the world's personal computer
market.
Microsoft introduced an operating environment named /Windows/ on
November 20, 1985 as an add-on to MS-DOS in response to the growing
interest in graphical user interfaces.
The most recent version of Windows is Windows 7.
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4. Windows uses demand paging with Clustering.
Clustering handles page faults by bringing in not only the
faulting page but also the multiple pages surrounding the
faulting page.
Windows uses clock algorithm.
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5. What is Virtual Memory?
CPU can address up to 3GB of memory, using its full 32 bits.
This is normally far more than the RAM of the machine.
The hardware provides for programs to operate in terms of as
much as they wish of this full 4GB space as Virtual Memory, those
parts of the program and data which are currently active being
loaded into Physical Random Access Memory (RAM).
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6. Why virtual memory?
The first is to allow the use of programs that are too big to
physically fit in memory.
The other reason is to allow for multitasking – multiple
programs running at once.
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7. Virtual Memory in Windows
In Windows the processor manages the mapping in terms of pages of
4 Kilobytes each a size that has implications for managing virtual
memory by the system.
Only some parts of the program and data that are currently in active
use need to be held in physical RAM.
Other parts are then held in a swap file(in as it’s called in Windows
95/98/ME: Win386.swp )or page file(in Windows NT versions
including Windows 2000 and XP).
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8. Address space of Windows
32-bit Address Space
32-bits = 2^32 = 4 GB
3 GB for address space
1 GB for kernel mode
64-bit Address space
64-bits = 2^64 = 17,179,869,184 GB
x64 today supports 48 bits virtual = 262,144 GB = 256 TB
IA-64 today support 50 bits virtual = 1,048,576 GB = 1024 TB
64-bit Windows supports 44 bits = 16,384 GB = 16 TB
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9. Virtual Address Space (V.A.S.)
00000000
Process space contains:
User Unique per
Accessible process
The application you are running 7FFFFFFF
(.EXE + .DLLs) 80000000
A user-mode stack for each thread Kernel-mode System-
accessible wide
All static storage defined by the
FFFFFFFF
application
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10. Virtual Address Space (V.A.S.)
00000000
System space contains:
Executive, Kernel User Unique per
Accessible process
Statically-allocated system-
wide data cells 7FFFFFFF
Page tables 80000000
Kernel-mode device drivers Kernel-mode System-
accessible wide
File system cache
FFFFFFFF
A kernel-mode stack
for every thread
in every
process
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11. What is loaded in RAM?
Items of RAM can be divided into two parts :
-Non paged area
• Parts of system which are very important . This cannot
be paged out.
-page pool
• Program code, Data pages that had actual data written
to them.
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12. Memory Organization
Two-level hierarchical memory map
Page directory table
Page directory entries (PDEs) point to page table
One page directory table per process
Location in page directory register
Page table
Page table entries (PTEs) point to page frames
Page frame
Contains page of data
TLB (translation look aside buffer) accelerates address translation
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13. Virtual Address Translation
Hardware converts each valid virtual address to a physical
address
virtual address
Virtual page number Byte within page
Page
Directory
Address translation
(hardware) If page Page
not valid fault
Page
Tables
Physical page number Byte within page
Translation physical address
Lookaside
Buffer
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15. Page file
The page file is a hidden file called pagefile.sys.
It is regenerated at each boot .
there is no need to include it in a backup
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16. The paging file and the RAM
The VMM creates a file on the hard disk that holds the extra memory that
is needed by the O.S.
This file is called a paging file (also known as a swap file), and plays an
important role in virtual memory.
The paging file combined with the RAM accounts for all of the memory.
Whenever the O.S. needs a ‘block’ of memory that’s not in the real (RAM)
memory, the VMM takes a block from the real memory that hasn’t been
used recently, writes it to the paging file.
Then it reads the block of memory that the O.S. needs from the paging
file.
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17. The paging file and the RAM
The VMM then takes the block of memory from the paging file, and
moves it into the real memory – in place of the old block.
This process is called swapping (also known as paging),
The blocks of memory that are swapped are called pages.
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18. Page fault
When the program needs the page which is not in main
memory the page fault interrupt will be invoked.
If the this is available on disk then it will be swapped.
If it is not available due to some hardware problems the
system will have ‘invalid page fault error’.
It may manifest itself as a ‘blue screen’ failure with a STOP
code.
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19. Page Faults
A page fault occurs when there is a reference to a page that isn’t
mapped to a physical page
The system goes to the appropriate block in the associated file to find
the contents of the page:
Physical page is allocated
Block is read into physical page
Page table entry is filled in
Exception is dismissed
Processor re-executes the instruction that caused the page fault
The page has now been “faulted into” the process “working set”
Pages are only brought into memory as a result of page faults
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20. Disadvantages of virtual memory
Virtual memory can slow down performance.
If the size of virtual memory is quite large in comparison to the real
memory, then more swapping to and from the hard disk will occur as a
result.
Accessing the hard disk is far slower than using system memory.
Using too many programs at once in a system with an insufficient
amount of RAM results in constant disk swapping – also
called thrashing.
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21. Working Set List
newer pages older pages
Working Set
All the physical pages “owned” by a process
E.g. the pages the process can reference without incurring a page fault
A process always starts with an empty working set
It then incurs page faults when referencing a page that isn’t in its
working set
Many page faults may be resolved from memory
22. Working Set
Each process has a default working set minimum and maximum
Can change with SetProcessWorkingSet
Working set minimum controls maximum number of locked
pages (VirtualLock)
Minimum is also reserved from RAM as a guarantee to the
process
Working set maximum is ignored
If there’s ample memory, process working set represents all the
memory it has referenced (but not freed)
23. Working Set Replacement
To standby
or modified
page list
Working Set
When memory manager decides the process is large enough, it
give up pages to make room for new pages
Local page replacement policy
Means that a single process cannot take over all of physical
memory unless other processes aren’t using it
Page replacement algorithm is least recently accessed
(pages are aged when available memory is low)
24. Working Set Breakdown
Consists of 2 types of pages:
Shareable (of which some may be shared)
Private
Four performance counters available:
Working Set Shareable
Working Set Shared (subset of shareable that are currently shared)
Working Set Private
Working Set Size (total of WS Shareable+Private)
Note: adding this up for each process overcounts shared pages
25. Managing Physical Memory
System keeps unassigned physical pages on one of several lists
Free page list
Modified page list
Standby page lists (8 as of Vista & later)
Zero page list
Bad page list - pages that failed memory test at system startup
Lists are implemented by entries in the “PFN database”
Maintained as FIFO lists or queues
26. Standby and Modified Page Lists
Modified pages go to modified (dirty) list
Avoids writing pages back to disk too soon
Unmodified pages go to standby (clean) lists
They form a system-wide cache of “pages likely to be needed again”
Pages can be faulted back into a process from the standby and
modified page list
These are counted as page faults, but not page reads
27. Modified Page Writer
When modified list reaches certain size, modified page writer system
thread is awoken to write pages out
Also triggered when memory is overcommitted (too few free pages)
Does not flush entire modified page list
Pages move from the modified list to the standby list
E.g. can still be soft faulted into a working set
28. Free and Zero Page Lists
Free Page List
Used for page reads
Private modified pages go here on process exit
Pages contain junk in them (e.g. not zeroed)
On most busy systems, this is empty
Zero Page List
Used to satisfy demand zero page faults
References to private pages that have not been created yet
When free page list has 8 or more pages, a priority zero thread is awoken
to zero them
On most busy systems, this is empty too
29. Super Fetch
Superfetch proactively repopulates RAM with the most useful data
Sets priority of pages to optimal value, based on the page history and
other analysis that it performs
Takes into account frequency of page usage, usage of page in context
of other pages in memory
Scenarios SuperFetch improves include
Resume from hibernate and suspend
Fast user switching
Performance after infrequent or low priority tasks execute
Application launch
