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Operating Systems
Memory Management
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Purpose of Memory Management
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Provide the memory space to enable several
processes to be executed at the same time.
Provide a satisfactory level of performance for the
system users.
Protect each process from one another.
Enable sharing of memory space between
processes.
Make the addressing of memory space transparent
to programmers.
Caching
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Main memory is the primary area for holding the
instructions and data that processes are using.
The cache memory is smaller (< 64 kB) and faster
than main memory.
Cache memory is used to store data and instructions
that are currently being used, or are predicted to be
used shortly.
The processor will look for information in the cache
before looking in main memory.
Caching
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Modern computer systems can make use of
a cache in three places :–
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The Level One (L1) Cache is inside the CPU
chip.
The Level Two (L2) Cache is external to the
CPU chip but is between the L1 Cache and the
main memory.
A Hard Drive Cache is between the Hard Drive
and the CPU.
Memory Management Algorithms
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Single-Process System
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Fixed Partition Memory
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Process to be executed is loaded into the free space area of
the memory.
Used by MS-DOS.
Divide memory into a number of fixed areas of different size
Each area holds an active process.
Each area may contain unused space resulting in internal
fragmentation.
A process may not be able to run because it cannot find a
large enough partition.
Used by IBM 360s.
Memory Management Algorithms
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Variable Partition Memory
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A process is allocated the exact amount of memory it
requires.
Processes are loaded into consecutive areas until memory
is full.
As a process terminates, the space it occupies is available
for the use of a new process.
A new process may not need the entire free area, leaving
holes, resulting in external fragmentation.
A process may not be able to run because there are not
enough consecutive free areas.
Memory Management Algorithms
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Simple Paging
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Each process is divided into a number of fixed chunks,
called pages.
Memory is divided into a set of page frames of the same
size.
The size of a page is typically 4 kB.
A process does not have to be loaded into consecutive
page frames.
Simple Paging is the most common algorithm in use.
Virtual Memory
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Using simple paging a process can be loaded in
parts.
Only the portions of a process that are being
referenced at an instant need to be present in
memory.
Therefore a process can have available more
memory than is physically present.
The memory that a process has available to it is
referred to as its virtual memory space.
Virtual Memory
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The size of the virtual memory space is determined
by the processor’s bit-width.
A 32-bit processor, like the Pentium IV, has 232 bytes
or 4 GB of virtual memory. A 64-bit processor can
have 264 bytes or 16 exabytes of virtual memory.
Most desktop computers have 512 MB (229 bytes) of
physical memory (RAM) or less.
The operating system uses an area on disk called
the paging file or swap file to allocate additional
memory to processes.
Managing the Swap File in Windows
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Microsoft recommends that the
paging file size be at least 1.5 times
the size of real memory.
On Windows 95/98 there was a
choice between letting the OS
automatically control the paging file
size or to specify a custom size.
On Windows 2000, a custom
paging file size is created by setting
an initial and maximum size.
On Windows XP, either a custom,
system managed or no paging file
can be created.
Memory Addressing
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A virtual memory address is in the form (p,o)
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For a 32-bit address space with a 4 kB page
size:
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p is the number of the page containing the
memory location
o is the offset of the location from the start of the
page
o will occupy 12 bits (4 kB = 212)
p will occupy 20 bits.
The Memory Management Unit (MMU)
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When a page is loaded into real memory, the virtual
page number is translated into a physical page
number.
This translation is done by a module of the CPU
called the Memory Management Unit (MMU).
The MMU maintains page tables that map how the
virtual page translates to a physical page.
The page tables also keep track of whether a virtual
page is associated with a physical page and when it
was last accessed.
Address Translation
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The page number portion of the address is further
sub-divided into a page directory index and a page
table index.
The MMU first locates a table known as the page
directory.
Each process has its own private page directory, and
the address of that directory is stored in its PCB.
The MMU uses the page directory index to locate an
entry in the page directory table.
The MMU retrieves from the page directory the
location of the page table.
Address Translation
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The MMU uses the page
index to locate an entry in
the page table.
The MMU retrieves from the
page table the address of
the page frame in physical
memory.
The MMU uses the page
byte offset as an index into
the physical page and
isolates the data that the
process wants to reference.
Address Translation
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Why a 3-step process?
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Only a process' page directory must be fully defined.
Page tables are defined only as necessary.
If the majority of a process' 4 GB address space is
unallocated, a significant saving in memory results because
page tables are not allocated to define the unused space.
Otherwise 4 MB would be required to allocate each process’
page tables in a 32-bit address space.
Translation Look-Aside Buffer (TLB)
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The three-step translation process would cause a system's
performance to be unbearably poor if the process occurred on
every memory access.
Instead the processor has a translation look-aside buffer (TLB)
which stores the most recent virtual page to physical page
translation.
When a process makes a memory reference, the MMU takes
the virtual page number and simultaneously compares it with
the virtual page number of every translation pair stored in the
TLB.
If there is a match, the MMU can bypass the page directory and
page table lookups because it has already obtained the page
frame number from the TLB.
Mechanics of Virtual Memory
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If a process needs an instruction or data from page
p1 which is not already in memory, a page fault is
generated, the OS then does the following:
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Find out where the contents of page p1 are stored on disk.
Use a page replacement algorithm to choose another page
p2 mapped to some frame f of physical memory.
Copy the contents of frame f out to disk.
Update the page tables to indicate that p2 is no longer
associated with a physical page.
Copy p1's data from disk to frame f.
Update the page tables so that p1 is mapped to frame f.
Locality of Reference
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Processes exhibit a characteristic known as
locality of reference.
Over intervals of time address references
made by a process tend to cluster around
narrow ranges contained in a few pages.
If memory references jumped around virtual
space at random, there would be a disk read
and write for each new reference and virtual
memory would be as slow as a disk.
Page Replacement Policy
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Pages must be removed whenever physical memory
is full of in-use pages and a process requires a page
not in physical memory.
Two characterizations for replacement policies are
global and local.
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In a global replacement policy, the MMU considers all pages
of physical memory as replacement candidates.
In a local replacement policy, the MMU considers as
replacement candidates only pages belonging to the
process that is accessing the page to be brought in.
Page Replacement Policy
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Page Replacement Policies are further characterized
by the algorithm used to choose a page to remove.
First-in First-Out (FIFO) algorithm
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Select for removal the page which has been in memory for
the longest time.
Implement as a linked-list queue, the page at the head of
the queue is the oldest.
A heavily used page will be periodically removed, thereby
resulting in a page fault the next time it is needed.
Page Replacement Policy
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Clock or Second Chance algorithm
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Variation of FIFO that uses a circular queue.
An entry in the queue has a “used” bit.
When a page is first loaded set its used bit to zero.
When the page frame is subsequently referenced set its used bit
to one.
When a page replacement is required go round the list until a
used bit of zero is found, change all intermediate used bits from
one to zero.
Essentially, what second-chance does is, as its name
suggests, giving every page a "second-chance" - an
old page which has been referenced is probably in
use, and should not be swapped out over a new page
which has not been referenced
Page Replacement Policy
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Least Recently Used (LRU) algorithm
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Select for replacement the page whose time since
last reference is greatest.
Requires that a time stamp recording is made for
a page frame at the time of each reference.
The overhead of maintaining a time stamp and
finding the oldest value is very high.
Page Replacement Policy
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Not Recently Used (NRU) algorithm
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Each page frame has associated with it a “page referenced”
bit.
At intervals, the OS resets all if these bits to zero.
Subsequent reference to a page will set its page referenced
bit to one.
When a page fault occurs, a page with the bit set to zero is
selected for replacement.
If a page fault occurs right after the bits are reset, unable to
determine if a page was recently used.
Thrashing
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Thrashing occurs when the total memory
requirement of all the processes is
significantly greater than physical memory.
The CPU spends more time swapping pages
than doing productive work.
This is usually an indication that the
computer requires more physical memory
(RAM).