Chapter 5: Memory Management Dhamdhere: Operating Systems— A Concept-Based Approach Slide No: 1 Copyright ©2005 Memory Management Chapter 5

Chapter 5: Memory Management

Dhamdhere: Operating Systems—A Concept-Based Approach

Slide No: 1Copyright ©2005

Memory Management

Chapter 5




Memory binding

• Each entity has a set of attributes, e.g. a variable has type, size, dimensionality.

• Binding is the action of specifying values of attributes of an entity.

• Two types of binding are used in practice:– Early binding: Restrictive, but leads to efficient execution– Late binding: Flexible, but may lead to less efficient execution




Memory binding

• Memory allocation is a binding of the `memory address’ attribute of a data entity

• Static and dynamic binding are examples of early and late binding, respectively




Features of static and dynamic memory allocation




Memory allocation preliminaries

• Stack – LIFO allocation

– A `contiguous’ data structure

– Used for data allocated `automatically’ on entering a block

• Heap – Non-contiguous data structure

– 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




A heap before and after freeing an allocation area




Memory allocation model for a process




Linking, loading and execution of programs




Memory binding in programs

• The origin of a program is the address of its first instruction or data byte

• A compiler is given an origin specification

• It binds instructions and data of a program in accordance with its origin specification




Assembly program P and its generated code




Linking and relocation of programs

• Linking– A program may wish to use library functions and other programs

– These library functions and other programs should become a part of the program

– Linking is the function that performs this action

• Relocation– Many program may have the same origin specification, so the address

binding of some of them has to be changed

– A program may have to be executed from a memory area that is different from the area for which it is compiled




Linking and relocation of programs

• Linking and relocation could be performed– Statically

– Dynamically

• What are the advantages of dynamic linking or relocation?

• How is dynamic linking or relocation performed?




Program relocation using the relocation register: (a) program, (b) its view during execution




Memory protection using bound registers




Memory protection using memory protection leys




Kernel actions for memory protection




Free area management in a heap:(a) singly linked free list, (b) doubly linked free list




Heap management: (a) free list, (b)-(d) allocation using first, best and next fit




Memory fragmentation

• Fragmentation is the development of un-usably small areas in memory

• It has several consequences– Memory is wasted

– The OS may run out of space to be allocated

• Two forms of memory fragmentation– Internal fragmentation

– External fragmentation




Forms of memory fragmentation




How to counter external fragmentation?

• Do not allow free memory areas to become too small– Best fit leads to successively smaller memory areas!

• Combine adjoining free areas into a single larger memory area

• Perform compaction




Boundary tags




Merging using boundary tags (a) Free list, (b) -(d) freeing of areas X, Y or Z




Memory compaction




Buddy system

• When memory is to be allocated– Allocate the entire free area to satisfy a request, or

– Split a free area into two free areas of equal size* These areas are called buddies

* One of the buddies is considered for satisfying a memory request (either completely, or through successive splitting)

• When memory is to be freed– It is merged with its buddy, if the buddy is also free

• Status of blocks is saved in a status map




A buddy system




Buddy system operation when a block is released




Powers of two allocators

• Allocation is in terms of blocks whose sizes are different powers of 2

• Blocks are not split or merged

• Free lists are maintained for different block sizes

• The header element contains information about block status and size. It is stored in the block itself

• Header element occupies memory, hence memory efficiency is low for requests that are powers of two in size




Header element in the powers-of-two allocator




Approaches to memory allocation

• Contiguous memory allocation– Allocates a single contiguous memory area to a request– Suffers from fragmentation– Requires provision to counter fragmentation

• Noncontiguous memory allocation– Allocates a set of disjoint memory areas to a request– Overcomes certain forms of fragmentation (internal ? External?)– Requires address translation during execution of programs




Contiguous memory allocation

• Fixed partitioned memory allocation– Partitions are fixed once and for all

– A process is allocated a memory partition that is larger in size than its maximum requirement

• Variable partitioned memory allocation– A process is allocated only as much memory as it needs

– Memory is reused through first-fit, best-fit, etc.




Internal fragmentation in fixed partitioned allocation




Memory compaction in variable partitioned allocation




Noncontiguous memory allocation

• Several disjoint memory areas may be allocated in response to a request. However, hardware aids in the execution of programs.

– User or a process is not aware of noncontiguity– Hence two views of a process exist

* Logical view: View by the user or process itself. * Physical view: Actual situation regarding allocation

– The organization of components in the logical view is called logical organization, and addresses used in it are called logical addresses

– Addresses used in the physical view are called physical addresses

• Avoids external fragmentation as no memory area is too small to be allocated





• How is noncontiguous memory allocation performed?– A process is considered to consist of a set of components– Each component is allocated a contiguous area of memory that can

accommodate it– Each logical address in an instruction is considered to consist of a pair

(component #, byte #)

• During operation of the process, a hardware unit called the memory management unit (MMU) converts a (component #, byte #) pair into an absolute memory address








Logical and physical views of a process in a noncontiguous memory allocation




Schematic of address translation in non-contiguous memory allocation




Approaches to Noncontiguous memory allocation

• Paging– A process consists of components of a fixed size, called pages

– The page size is a power of 2 and is fixed in the architecture

– Memory is divided into parts called page frames, whose size matches the size of a page

– The kernel allocates memory to all pages of a process

– The kernel builds a page table that stores information about addresses of page frames allocated to processes

– The MMU uses information in the page table to perform address translation




Paging

• A logical address is split into a pair (page #, word #)

• This splitting is performed using bit-splitting since the page size is a power of 2




Processes in paging




Approaches to noncontiguous memory allocation

• Segmentation– Each component in a process is a logical unit called a segment,

e.g., a function, a module or an object

– Components have different sizes

– The kernel allocates memory to all components and builds a segment table

– Each logical address in a segment is a pair of the form (segment #, byte #)

– The MMU uses the segment table to perform address translation




A process Q in segmentation




Comparison of continuous and noncontinuous memory allocation




Kernel memory allocation

• The kernel creates and destroys data structures used to store information about user processes and resources at a very high rate, e.g. PCBs, ECBs.

• Hence efficiency of kernel memory allocation directly influences its overhead

• Kernel uses special techniques to perform memory allocation for its data structures

• These techniques exploit the fact that the sizes of many of these data structures are known in advance, e.g. PCBs, ECBs.




Slab allocator

• A slab is a fixed sized area of memory

• A slab contains data structures of the same kind

• A slab is preformatted to contain standard sized slots for these data structures

• A free list indicates which slots are free

• If all slabs containing data structures of a specific kind are full, new slabs are allocated by the kernel

• Allocation and de-allocation of memory is very fast




Format of a slab




Program forms employed in operating systems




(a) a program, and (b) its equivalent overlay structured program in execution




Sharing of a program (a) static sharing, (b) dynamic sharing




Reentrant program (a) structure of program C, (b)-(c) invocation by A and B.

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Chapter 5: Memory Management Dhamdhere: Operating Systems— A Concept-Based Approach Slide No: 1 Copyright ©2005 Memory Management Chapter 5