Lecture Operating systems: A concept-based approach (2/e): Chapter 5 - Dhananjay M. Dhamdhere
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Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 1
Copyright © 2008
Managing the memory hierarchy
•
The memory hierarchy is comprised of several memory
units with different speeds and cost
– Efficient operation of a process and the system depends on
effective use of the memory hierarchy
* Efficient operation of a process depends on hit ratios in faster
memories of the hierarchy, i.e., the cache and memory
* Efficient operation of the system requires many processes to be
present in memory
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 2
Copyright © 2008
Managing the memory hierarchy
•
How different levels in the hierarchy are managed
– L1 cache and L2 cache
* Allocation and use is managed by hardware to ensure high hit ratios
– Memory
* Use is managed by run time libraries of programming languages
* Allocation is managed by the kernel. It must
Accommodate many processes in memory
Ensure high hit ratios
– Disk
* Allocation and use is managed by the kernel
Quick loading and storing of process address spaces is
important
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 3
Copyright © 2008
Managing the memory hierarchy
•
Efficient use of memory
– Involves sharing of memory among processes and reuse of
memory previously allocated to other processes
– Requires speedy allocation and de-allocation within the memory
allocated to a process
– We discuss these aspects in Chapter 5 and this set of slides
•
The memory hierarchy consisting of memory and a disk
is called virtual memory
– We discuss it separately in Chapter 6
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 4
Copyright © 2008
Memory binding
•
Each entity has a set of attributes; e.g., a variable has
type, size and dimensionality
– Binding is the action of specifying values of attributes of an
entity, e.g.
* Declaration of type of a variable is the binding of its type attribute
* Memory allocation is the binding its memory address attribute
– Two types of binding are used in practice:
* Early binding (static binding ─ binding before operation begins)
Restrictive, but leads to efficient operation of a process
* Late binding (dynamic binding ─ binding during operation)
Flexible, but may lead to less efficient operation of a process
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 5
Copyright © 2008
Features of static and dynamic memory allocation
•
Static memory allocation
– Allocation is performed before a process starts operating
– Size of memory required should be known a priori; otherwise,
* Wastage may occur if size is overestimated
* A process may have to be terminated if it requires more memory
than allocated to it
– No allocation or de-allocation actions required during operation
•
Dynamic memory allocation
– Allocation is performed during operation of a process
– Allocation equals size; hence no wastage of memory
– Allocation / de-allocation overhead is incurred during operation
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 6
Copyright © 2008
Memory allocation preliminaries
•
Stack
– LIFO allocation
– A ‘contiguous’ data structure—data occupy adjoining locations
– Used for data allocated ‘automatically’ on entering a function, such as
parameters of a function call and local variables of a function
•
Heap
– Non-contiguous data structure—data may not occupy adjoining
locations
* Pointer-based access to allocated data
– Used for program controlled data (PCD data) that is explicitly allocated
and de-allocated in a program, e.g. malloc / calloc
– ‘holes’, i.e., unused areas, may develop in memory during operation
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 7
Copyright © 2008
A hole develops in a heap due to de-allocation
(a) Three variables are allocated in the heap
(b) Deallocation of the memory of floatptr2 creates a hole in memory
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 8
Copyright © 2008
Memory allocation for a process
•
Memory requirements of the components of a process
– Sizes of code and static data of the program to be executed are
known a priori
* Hence code and static data can be allocated statically
– Sizes of stack and program controlled data (PCD) data vary
during operation
* Hence stack and PCD data require dynamic allocation
– The memory allocation model for a process incorporates both
these requirements
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 9
Copyright © 2008
Memory allocation model for a process
• The code and static data are given a fixed allocation
• Stack and PCD data can grow in opposite directions
• Their sizes can vary independently; problem arises only if they overlap
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 10
Copyright © 2008
Memory allocation for a process
•
A process is created when a program is to be executed.
Two issues concerning program loading and execution
are:
– If the start address of the program is different from the start
address of the allocated memory area
* Addresses in instructions should be changed so that they would
access the intended operands
* This requirement is called relocation of a program
– Programs should not interfere with each other’s execution
* This requirement is called memory protection
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 11
Copyright © 2008
Relocation register
•
A computer provides a relocation register in the CPU to
assist in relocation of a program
– When the current instruction wishes to access memory, the CPU
* takes the memory address from the instruction, say address bbb
* adds contents of the relocation register, say aaa, to it
* accesses the byte with the resulting address bbb + aaa
Q: How to execute a program with the start address 50,000 in the
memory area starting on address 70,000?
A: Load 20,000 in the relocation register (see next slide)
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 12
Copyright © 2008
Program relocation using the relocation register
(a) Program has been coded to have the start address 50,000
(b) Execution of the program in memory area with start address 70,000:
• Relocation register contains 20,000
• Operand that had address 65,784 in program exists in memory 85,784
∙ 20,000 is added to the address in the Add instruction during execution
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 13
Copyright © 2008
Memory protection using bound registers
• CPU has two registers: Lower bound register (LBR), and Upper bound
register (UBR) containing start and end addresses of memory for a program
• An interrupt is raised if the program accesses an address outside the
range defined by contents of LBR and UBR
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 14
Copyright © 2008
Kernel actions for memory protection
•
The kernel performs the following actions:
– While allocating memory to a process
* Write the start and end addresses of the memory area allocated to
the program in the PCB of the process
– While dispatching a process
* Access the PCB and load the start and end addresses of the
memory area in the LBR and UBR registers
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 15
Copyright © 2008
Reuse of memory
•
The kernel keeps track of free memory areas in a heap
and reuses them while making fresh memory allocations
– It maintains a free list to store information about free memory areas
– A memory request is satisfied by using a part of a free memory area
* The remaining memory area is put back into the free list
– Three techniques for reuse of free memory
* First fit technique:
Use the first free area in the list that can satisfy the request
* Best fit technique:
Use the free area that leaves smallest remaining free area
* Next fit technique:
Use the next free area in the list that satisfies the request
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 16
Copyright © 2008
Free area management in a heap
(a) Singly linked free list
(b) Doubly linked free list
∙ Permits fast insertions and deletions
∙ Consider allocation of memory area y in best fit allocation
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 17
Copyright © 2008
Heap management
(a) Free list
(b)–(d) Allocations using first, best and next fit allocation
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 18
Copyright © 2008
Memory fragmentation
•
Fragmentation is the existence of unusable free areas in
memory
– For example, fragmentation may occur because free memory
areas are too small to be used
– Fragmentation has several consequences
* Memory is wasted
* The kernel may run out of memory to be allocated
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 19
Copyright © 2008
Memory fragmentation
•
Two forms of fragmentation
– External fragmentation
* A memory area is too small to be allocated to any process
– Internal fragmentation
* Some part of an allocated memory area is not used; however, the
kernel cannot allocate it to another process
It arises when the kernel allocates a larger memory area than
requested by a process
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 20
Copyright © 2008
How to counter external fragmentation?
•
Do not allow free memory areas to become too small
– Best fit leads to successively smaller memory areas!
– Perform merging by combining adjoining free areas into a single
larger memory area
* Boundary tags are used to assist in merging
– Perform compaction of allocated memory so that a single free
memory area exists
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 21
Copyright © 2008
Boundary tags
• A tag is a descriptor of a memory area
• Tags are stored at both boundaries of a memory area
– Hence tags of adjoining memory areas are in adjoining memory locations
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 22
Copyright © 2008
Merging using boundary tags
(a) Singly linked free list
(b)–(d) Freeing of areas X, Y or Z
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 23
Copyright © 2008
Memory compaction
* Free areas are hatched in the figure
• All occupied free areas are moved towards one end of memory
• All free areas move to the other end of memory
• Compaction involves relocation of programs that are moved
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 24
Copyright © 2008
Buddy system allocator
•
The allocator works as follows:
– Status of memory blocks is recorded in a status map
– When memory is to be allocated
* If an appropriate free block exists, allocate it.
* Otherwise, split a larger free block into two (or more) free areas
These blocks are buddies of one another
One of the buddies is used for satisfying a memory request
(either directly or through successive splitting)
The other buddy block remains free
– When a memory block is to be freed
* If its buddy block is already free, the two are merged to form a
single free block. The resulting free block is similarly merged
– Binary buddy: A block is split into two blocks of same size
Chapter 5:
Memory Management
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 25
Copyright © 2008
