Memory Management

Chapter 7

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 Memory Management

 Is the task carried out by the OS and

hardware to accommodate multiple
processes in main memory
 If only a few processes can be kept in main
memory, then much of the time all
processes will be waiting for I/O and the
CPU will be idle
 Hence, memory needs to be allocated
efficiently in order to pack as many
processes into memory as possible
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 Memory Management

 In most schemes, the kernel occupies

some fixed portion of main memory and
the rest is shared by multiple processes

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 Memory Management Requirements

 Relocation
 programmer cannot know where the program
will be placed in memory when it is executed
 a process may be (often) relocated in main
memory due to swapping
 swapping enables the OS to have a larger pool
of ready-to-execute processes
 memory references in code (for both
instructions and data) must be translated to
actual physical memory address

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 Memory Management Requirements

 Protection
 processes should not be able to reference
memory locations in another process without
permission
 impossible to check addresses at compile time
in programs since the program could be
relocated
 address references must be checked at run
time by hardware

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 Memory Management Requirements

 Sharing
 mustallow several processes to access a
common portion of main memory without
compromising protection
 cooperating processes may need to share
access to the same data structure
 better to allow each process to access the same
copy of the program rather than have their own
separate copy

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 Memory Management Requirements

 Logical Organization
 userswrite programs in modules with different
characteristics
 instruction
modules are execute-only
 data modules are either read-only or read/write

 some modules are private others are public

 To effectively deal with user programs, the OS

and hardware should support a basic form of
module to provide the required protection and
sharing

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 Memory Management Requirements

 Physical Organization
 secondary memory is the long term store for
programs and data while main memory holds
program and data currently in use
 moving information between these two levels of
memory is a major concern of memory
management (OS)
 itis highly inefficient to leave this responsibility
to the application programmer

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 Simple Memory Management
 In this chapter we study the simpler case where
there is no virtual memory
 An executing process must be loaded entirely in
main memory (if overlays are not used)
 Although the following simple memory
management techniques are not used in modern
OS, they lay the ground for a proper discussion of
virtual memory (next chapter)
 fixed partitioning
 dynamic partitioning
 simple paging
 simple segmentation

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 Fixed Partitioning

 Partition main memory

into a set of non
overlapping regions
called partitions

 Partitions can be of
equal or unequal sizes

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 Fixed Partitioning
 any process whose size is less than or equal to a
partition size can be loaded into the partition
 if all partitions are occupied, the operating system
can swap a process out of a partition
 a program may be too large to fit in a partition.
The programmer must then design the program
with overlays
 when the module needed is not present the user
program must load that module into the program’s
partition, overlaying whatever program or data are
there

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 Fixed Partitioning
 Main memory use is inefficient. Any
program, no matter how small, occupies an
entire partition. This is called internal
fragmentation.
 Unequal-size partitions lessens these
problems but they still remain...
 Equal-size partitions was used in early
IBM’s OS/MFT (Multiprogramming with a
Fixed number of Tasks)

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 Placement Algorithm with Partitions
 Equal-size partitions
 If
there is an available partition, a process can
be loaded into that partition
 because all partitions are of equal size, it does
not matter which partition is used
 Ifall partitions are occupied by blocked
processes, choose one process to swap out to
make room for the new process

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 Placement Algorithm with Partitions
 Unequal-size
partitions: use of
multiple queues
 assign each process to
the smallest partition
within which it will fit
 A queue for each
partition size
 tries to minimize
internal fragmentation
 Problem: some queues
will be empty if no
processes within a size
range is present
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 Placement Algorithm with Partitions
 Unequal-size
partitions: use of a
single queue
 When its time to load a
process into main
memory the smallest
available partition that
will hold the process is
selected
 increases the level of
multiprogramming at the
expense of internal
fragmentation

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 Dynamic Partitioning
 Partitions are of variable length and number
 Each process is allocated exactly as much
memory as it requires
 Eventually holes are formed in main
memory. This is called external
fragmentation
 Must use compaction to shift processes so
they are contiguous and all free memory is
in one block
 Used in IBM’s OS/MVT (Multiprogramming
with a Variable number of Tasks)
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 Dynamic Partitioning: an example

 A hole of 64K is left after loading 3 processes: not

enough room for another process
 Eventually each process is blocked. The OS
swaps out process 2 to bring in process 4
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 Dynamic Partitioning: an example

 another hole of 96K is created

 Eventually each process is blocked. The OS
swaps out process 1 to bring in again process 2
and another hole of 96K is created...
18  Compaction would produce a single hole of 256K
Placement Algorithm
 Used to decide
which free block to
allocate to a process
 Goal: to reduce
usage of compaction
(time consuming)
 Possible algorithms:
 Best-fit: choose
smallest hole
 First-fit: choose first
hole from beginning
 Next-fit: choose first
hole from last
placement
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 Placement Algorithm: comments
 Next-fit often leads to allocation of the
largest block at the end of memory
 First-fit favors allocation near the
beginning: tends to create less
fragmentation then Next-fit
 Best-fit searches for smallest block: the
fragment left behind is small as possible
 main memory quickly forms holes too small to
hold any process: compaction generally needs
to be done more often

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 Replacement Algorithm

 When all processes in main memory are

blocked, the OS must choose which
process to replace
 A process must be swapped out (to a Blocked-
Suspend state) and be replaced by a new
process or a process from the Ready-Suspend
queue
 We will discuss later such algorithms for
memory management schemes using virtual
memory

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 Buddy System
 A reasonable compromize to overcome
disadvantages of both fixed and variable
partitionning schemes
 A modified form is used in Unix SVR4 for
kernal memory allocation
 Memory blocks are available in size of
2^{K} where L <= K <= U and where
 2^{L}= smallest size of block allocatable
 2^{U} = largest size of block allocatable
(generally, the entire memory available)

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 Buddy System
 We start with the entire block of size 2^{U}
 When a request of size S is made:
 If 2^{U-1} < S <= 2^{U} then allocate the entire
block of size 2^{U}
 Else, split this block into two buddies, each of size
2^{U-1}
 If 2^{U-2} < S <= 2^{U-1} then allocate one of the
2 buddies
 Otherwise one of the 2 buddies is split again
 This process is repeated until the smallest block
greater or equal to S is generated
 Two buddies are coalesced whenever both of
them become unallocated
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 Buddy System

 The OS maintains several lists of holes

 the i-list is the list of holes of size 2^{i}
 whenever a pair of buddies in the i-list occur,
they are removed from that list and coalesced
into a single hole in the (i+1)-list
 Presented with a request for an allocation
of size k such that 2^{i-1} < k <= 2^{i}:
 the i-list is first examined
 if the i-list is empty, the (i+1)-list is then
examined...
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 Example of Buddy System

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 Buddy Systems: remarks
 On average, internal fragmentation is 25%
 each memory block is at least 50% occupied
 Programs are not moved in memory
 simplifies memory management
 Mostly efficient when the size M of memory
used by the Buddy System is a power of 2
M = 2^{U} “bytes” where U is an integer
 then the size of each block is a power of 2
 the smallest block is of size 1
 Ex: if M = 10, then the smallest block would be
of size 5
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 Relocation

 Because of swapping and compaction, a

process may occupy different main
memory locations during its lifetime
 Hence physical memory references by a
process cannot be fixed
 This problem is solved by distinguishing
between logical address and physical
address

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 Address Types
 A physical address (absolute address) is a
physical location in main memory
 A logical address is a reference to a
memory location independent of the
physical structure/organization of memory
 Compilers produce code in which all
memory references are logical addresses
 A relative address is an example of logical
address in which the address is expressed
as a location relative to some known point
in the program (ex: the beginning)
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 Address Translation
 Relative address is the most frequent type
of logical address used in pgm modules
(ie: executable files)
 Such modules are loaded in main memory
with all memory references in relative form
 Physical addresses are calculated “on the
fly” as the instructions are executed
 For adequate performance, the translation
from relative to physical address must by
done by hardware

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 Simple example of hardware
translation of addresses
 When a process is assigned to the running state, a
base register (in CPU) gets loaded with the
starting physical address of the process
 A bound register gets loaded with the process’s
ending physical address
 When a relative addresses is encountered, it is
added with the content of the base register to
obtain the physical address which is compared
with the content of the bound register
 This provides hardware protection: each process
can only access memory within its process image

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Example Hardware for Address Translation

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 Simple Paging

 Main memory is partition into equal fixed-

sized chunks (of relatively small size)
 Trick: each process is also divided into
chunks of the same size called pages
 The process pages can thus be assigned
to the available chunks in main memory
called frames (or page frames)
 Consequence: a process does not need to
occupy a contiguous portion of memory

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 Example of process loading

 Now suppose that process B is swapped out

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 Example of process loading (cont.)
 When process A and C
are blocked, the pager
loads a new process D
consisting of 5 pages
 Process D does not
occupied a contiguous
portion of memory
 There is no external
fragmentation
 Internal fragmentation
consist only of the last
page of each process

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 Page Tables

 The OS now needs to maintain (in main memory) a

page table for each process
 Each entry of a page table consist of the frame
number where the corresponding page is
physically located
 The page table is indexed by the page number to
obtain the frame number
 A free frame list, available for pages, is maintained
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 Logical address used in paging

 Within each program, each logical address

must consist of a page number and an
offset within the page
 A CPU register always holds the starting
physical address of the page table of the
currently running process
 Presented with the logical address (page
number, offset) the processor accesses the
page table to obtain the physical address
(frame number, offset)
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Logical address in paging
 The logical address becomes
a relative address when the
page size is a power of 2
 Ex: if 16 bits addresses are
used and page size = 1K, we
need 10 bits for offset and
have 6 bits available for page
number
 Then the 16 bit address
obtained with the 10 least
significant bit as offset and 6
most significant bit as page
number is a location relative to
the beginning of the process
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 Logical address in paging

 By using a page size of a power of 2, the

pages are invisible to the programmer,
compiler/assembler, and the linker
 Address translation at run-time is then
easy to implement in hardware
 logicaladdress (n,m) gets translated to
physical address (k,m) by indexing the page
table and appending the same offset m to the
frame number k

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 Logical-to-Physical Address
Translation in Paging

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 Simple Segmentation
 Each program is subdivided into blocks of
non-equal size called segments
 When a process gets loaded into main
memory, its different segments can be
located anywhere
 Each segment is fully packed with
instructs/data: no internal fragmentation
 There is external fragmentation; it is
reduced when using small segments

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 Simple Segmentation
 In contrast with paging, segmentation is
visible to the programmer
 provided as a convenience to organize logically
programs (ex: data in one segment, code in
another segment)
 must be aware of segment size limit

 The OS maintains a segment table for each

process. Each entry contains:
 the starting physical addresses of that
segment.
 the length of that segment (for protection)

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 Logical address used in segmentation
 When a process enters the Running state, a CPU
register gets loaded with the starting address of
the process’s segment table.
 Presented with a logical address (segment
number, offset) = (n,m), the CPU indexes (with n)
the segment table to obtain the starting physical
address k and the length l of that segment
 The physical address is obtained by adding m to k
(in contrast with paging)
 the hardware also compares the offset m with the length
l of that segment to determine if the address is valid

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 Logical-to-Physical Address
Translation in segmentation

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 Simple segmentation and paging
comparison
 Segmentation requires more complicated
hardware for address translation
 Segmentation suffers from external fragmentation
 Paging only yield a small internal fragmentation
 Segmentation is visible to the programmer
whereas paging is transparent
 Segmentation can be viewed as commodity
offered to the programmer to organize logically a
program into segments and using different kinds
of protection (ex: execute-only for code but read-
write for data)
 for this we need to use protection bits in segment
table entries
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