Chapter Five main management of memory .ppt

Silberschatz, Galvin and Gagne ©2013
Operating System Concepts – 9th
Edition
Chapter 5: Main Memory
Managment

8.2 Silberschatz, Galvin and Gagne ©2013
Edition
Chapter 5: Memory Management
 Background
 Swapping
 Contiguous Memory Allocation
 Segmentation
 Paging

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Background
 Program must be brought (from disk) into memory and
placed within a process for it to be run
 Main memory and registers are only storage CPU can
access directly
 Memory unit only sees a stream of addresses + read
requests, or address + data and write requests
 Register access in one CPU clock (or less)
 Main memory can take many cycles, causing a stall
 Cache sits between main memory and CPU registers
 Protection of memory required to ensure correct operation

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Base and Limit Registers
 A pair of base and limit registers define the logical address space
 CPU must check every memory access generated in user mode to
be sure it is between base and limit for that user

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Hardware Address Protection

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Address Binding
 Programs on disk, ready to be brought into memory to execute form an input
queue
 Without support, must be loaded into address 0000
 Inconvenient to have first user process physical address always at 0000
 How can it not be?
 Further, addresses represented in different ways at different stages of a
program’s life
 Source code addresses usually symbolic
 Compiled code addresses bind to relocatable addresses
 i.e. “14 bytes from beginning of this module”
 Linker or loader will bind relocatable addresses to absolute addresses
 i.e. 74014
 Each binding maps one address space to another

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Binding of Instructions and Data to Memory
 Address binding of instructions and data to memory addresses
can happen at three different stages
 Compile time: If memory location known a priori, absolute
code can be generated; must recompile code if starting
location changes
 Load time: Must generate relocatable code if memory
location is not known at compile time
 Execution time: Binding delayed until run time if the process
can be moved during its execution from one memory segment
to another
 Need hardware support for address maps (e.g., base and
limit registers)

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Multistep Processing of a User Program

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Logical vs. Physical Address Space
 The concept of a logical address space that is bound to a
separate physical address space is central to proper memory
management
 Logical address – generated by the CPU; also referred to
as virtual address
 Physical address – address seen by the memory unit
 Logical and physical addresses are the same in compile-time
and load-time address-binding schemes; logical (virtual) and
physical addresses differ in execution-time address-binding
scheme
 Logical address space is the set of all logical addresses
generated by a program
 Physical address space is the set of all physical addresses
generated by a program

Edition
Memory-Management Unit (MMU)
 Hardware device that at run time maps virtual to physical
address
 Many methods possible, covered in the rest of this chapter
 To start, consider simple scheme where the value in the
relocation register is added to every address generated by a
user process at the time it is sent to memory
 Base register now called relocation register
 MS-DOS on Intel 80x86 used 4 relocation registers
 The user program deals with logical addresses; it never sees the
real physical addresses
 Execution-time binding occurs when reference is made to
location in memory
 Logical address bound to physical addresses

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Dynamic relocation using a relocation register
 Routine is not loaded until it is
called
 Better memory-space utilization;
unused routine is never loaded
 All routines kept on disk in
relocatable load format
 Useful when large amounts of
code are needed to handle
infrequently occurring cases
 No special support from the
operating system is required
 Implemented through program
design
 OS can help by providing libraries
to implement dynamic loading

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Dynamic Linking
 Static linking – system libraries and program code combined by
the loader into the binary program image
 Dynamic linking –linking postponed until execution time
 Small piece of code, stub, used to locate the appropriate
memory-resident library routine
 Stub replaces itself with the address of the routine, and executes
the routine
 Operating system checks if routine is in processes’ memory
address
 If not in address space, add to address space
 Dynamic linking is particularly useful for libraries
 System also known as shared libraries
 Consider applicability to patching system libraries
 Versioning may be needed

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Swapping
 A process can be swapped temporarily out of memory to a
backing store, and then brought back into memory for continued
execution
 Total physical memory space of processes can exceed
physical memory
 Backing store – fast disk large enough to accommodate copies
of all memory images for all users; must provide direct access to
these memory images
 Roll out, roll in – swapping variant used for priority-based
scheduling algorithms; lower-priority process is swapped out so
higher-priority process can be loaded and executed
 Major part of swap time is transfer time; total transfer time is
directly proportional to the amount of memory swapped
 System maintains a ready queue of ready-to-run processes
which have memory images on disk

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Swapping (Cont.)
 Does the swapped out process need to swap back in to same
physical addresses?
 Depends on address binding method
 Plus consider pending I/O to / from process memory space
 Modified versions of swapping are found on many systems (i.e.,
UNIX, Linux, and Windows)
 Swapping normally disabled
 Started if more than threshold amount of memory allocated
 Disabled again once memory demand reduced below
threshold

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Schematic View of Swapping

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Context Switch Time including Swapping
 If next processes to be put on CPU is not in memory, need to
swap out a process and swap in target process
 Context switch time can then be very high
 100MB process swapping to hard disk with transfer rate of
50MB/sec
 Swap out time of 2000 ms
 Plus swap in of same sized process
 Total context switch swapping component time of 4000ms
(4 seconds)
 Can reduce if reduce size of memory swapped – by knowing
how much memory really being used
 System calls to inform OS of memory use via
request_memory() and release_memory()

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Context Switch Time and Swapping (Cont.)
 Other constraints as well on swapping
 Pending I/O – can’t swap out as I/O would occur to wrong
process
 Or always transfer I/O to kernel space, then to I/O device
 Known as double buffering, adds overhead
 Standard swapping not used in modern operating systems
 But modified version common
 Swap only when free memory extremely low

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Contiguous Allocation
 Main memory must support both OS and user processes
 Limited resource, must allocate efficiently
 Contiguous allocation is one early method
 Main memory usually into two partitions:
 Resident operating system, usually held in low memory with
interrupt vector
 User processes then held in high memory
 Each process contained in single contiguous section of
memory

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Contiguous Allocation (Cont.)
 Relocation registers used to protect user processes from each
other, and from changing operating-system code and data
 Base register contains value of smallest physical address
 Limit register contains range of logical addresses – each
logical address must be less than the limit register
 MMU maps logical address dynamically
 Can then allow actions such as kernel code being transient
and kernel changing size

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Hardware Support for Relocation and Limit Registers

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Multiple-partition allocation
 Multiple-partition allocation
 Degree of multiprogramming limited by number of partitions
 Variable-partition sizes for efficiency (sized to a given process’ needs)
 Hole – block of available memory; holes of various size are scattered
throughout memory
 When a process arrives, it is allocated memory from a hole large enough to
accommodate it
 Process exiting frees its partition, adjacent free partitions combined
 Operating system maintains information about:
a) allocated partitions b) free partitions (hole)

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Dynamic Storage-Allocation Problem
 First-fit: Allocate the first hole that is big enough
 Best-fit: Allocate the smallest hole that is big enough; must
search entire list, unless ordered by size
 Produces the smallest leftover hole
 Worst-fit: Allocate the largest hole; must also search entire list
 Produces the largest leftover hole
How to satisfy a request of size n from a list of free holes?
First-fit and best-fit better than worst-fit in terms of speed and storage
utilization

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Fragmentation
 External Fragmentation – total memory space exists to
satisfy a request, but it is not contiguous
 Internal Fragmentation – allocated memory may be slightly
larger than requested memory; this size difference is memory
internal to a partition, but not being used
 First fit analysis reveals that given N blocks allocated, 0.5 N
blocks lost to fragmentation
 1/3 may be unusable -> 50-percent rule

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Fragmentation (Cont.)
 Reduce external fragmentation by compaction
 Shuffle memory contents to place all free memory together
in one large block
 Compaction is possible only if relocation is dynamic, and is
done at execution time
 I/O problem
 Latch job in memory while it is involved in I/O
 Do I/O only into OS buffers
 Now consider that backing store has same fragmentation
problems

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Segmentation
 Memory-management scheme that supports user view of memory
 A program is a collection of segments
 A segment is a logical unit such as:
main program
procedure
function
method
object
local variables, global variables
common block
stack
symbol table
arrays

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User’s View of a Program

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Logical View of Segmentation
1
3
2
4
1
4
2
3
user space physical memory space

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Segmentation Architecture
 Logical address consists of a two tuple:
<segment-number, offset>,
 Segment table – maps two-dimensional physical addresses; each
table entry has:
 base – contains the starting physical address where the
segments reside in memory
 limit – specifies the length of the segment
 Segment-table base register (STBR) points to the segment table
’s location in memory
 Segment-table length register (STLR) indicates number of
segments used by a program;
segment number s is legal if s < STLR

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Segmentation Architecture (Cont.)
 Protection
 With each entry in segment table associate:
 validation bit = 0  illegal segment
 read/write/execute privileges
 Protection bits associated with segments; code sharing
occurs at segment level
 Since segments vary in length, memory allocation is a
dynamic storage-allocation problem
 A segmentation example is shown in the following diagram

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Segmentation Hardware

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Paging
 Physical address space of a process can be noncontiguous;
process is allocated physical memory whenever the latter is
available
 Avoids external fragmentation
 Avoids problem of varying sized memory chunks
 Divide physical memory into fixed-sized blocks called frames
 Size is power of 2, between 512 bytes and 16 Mbytes
 Divide logical memory into blocks of same size called pages
 Keep track of all free frames
 To run a program of size N pages, need to find N free frames and
load program
 Set up a page table to translate logical to physical addresses
 Backing store likewise split into pages
 Still have Internal fragmentation

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Address Translation Scheme
 Address generated by CPU is divided into:
 Page number (p) – used as an index into a page table which
contains base address of each page in physical memory
 Page offset (d) – combined with base address to define the
physical memory address that is sent to the memory unit
 For given logical address space 2m
and page size 2n
page number page offset
p d
m -n n

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Paging Hardware

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Paging Model of Logical and Physical Memory

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Paging Example
n=2 and m=4 32-byte memory and 4-byte pages

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Paging (Cont.)
 Calculating internal fragmentation
 Page size = 2,048 bytes
 Process size = 72,766 bytes
 35 pages + 1,086 bytes
 Internal fragmentation of 2,048 - 1,086 = 962 bytes
 Worst case fragmentation = 1 frame – 1 byte
 On average fragmentation = 1 / 2 frame size
 So small frame sizes desirable?
 But each page table entry takes memory to track
 Page sizes growing over time
 Solaris supports two page sizes – 8 KB and 4 MB
 Process view and physical memory now very different
 By implementation process can only access its own memory

Chapter Five main management of memory .ppt

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Chapter Five main management of memory .ppt