Operating system- memory management this ppt elaborated the concepts related to OS memory management various schemes

Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th
Edition
Chapter 8: Main Memory

8.2 Silberschatz, Galvin and Gagne ©2009
Edition
Chapter 8: Memory Management
 Background
 Swapping
 Contiguous Memory Allocation
 Paging
 Structure of the Page Table
 Segmentation
 Example: The Intel Pentium

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Objectives
 To provide a detailed description of various ways of organizing memory hardware
 To discuss various memory-management techniques, including paging and segmentation
 To provide a detailed description of the Intel Pentium, which supports both pure segmentation and
segmentation with 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
 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

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Hardware Address Protection with Base and Limit Registers

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Address Binding
 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

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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

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Dynamic Loading
 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
 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
 Plus disk latency of 8 ms
 Swap out time of 2008 ms
 Plus swap in of same sized process
 Total context switch swapping component time of 4016ms (> 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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Contiguous Allocation
 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
 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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Contiguous Allocation (Cont.)
 Multiple-partition allocation
 Degree of multiprogramming limited by number of partitions
 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)
OS
process 5
process 8
process 2
OS
process 5
process 2
OS
process 5
process 2
OS
process 5
process 9
process 2
process 9
process 10

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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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Paging
 Physical address space of a process can be noncontiguous; process is allocated physical memory
whenever the latter is available
 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

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Free Frames
Before allocation After allocation

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Implementation of Page Table
 Page table is kept in main memory
 Page-table base register (PTBR) points to the page table
 Page-table length register (PTLR) indicates size of the page table
 In this scheme every data/instruction access requires two memory accesses
 One for the page table and one for the data / instruction
 The two memory access problem can be solved by the use of a special fast-lookup hardware cache called
associative memory or translation look-aside buffers (TLBs)
 Some TLBs store address-space identifiers (ASIDs) in each TLB entry – uniquely identifies each process
to provide address-space protection for that process
 Otherwise need to flush at every context switch
 TLBs typically small (64 to 1,024 entries)
 On a TLB miss, value is loaded into the TLB for faster access next time
 Replacement policies must be considered
 Some entries can be wired down for permanent fast access

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Associative Memory
 Associative memory – parallel search
 Address translation (p, d)
 If p is in associative register, get frame # out
 Otherwise get frame # from page table in memory
Page # Frame #

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Paging Hardware With TLB

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Effective Access Time
 Associative Lookup =  time unit
 Can be < 10% of memory access time
 Hit ratio = 
 Hit ratio – percentage of times that a page number is found in the associative registers; ratio related to
number of associative registers
 Consider  = 80%,  = 20ns for TLB search, 100ns for memory access
 Effective Access Time (EAT)
EAT = (1 + )  + (2 + )(1 – )
= 2 +  – 
 Consider  = 80%,  = 20ns for TLB search, 100ns for memory access
 EAT = 0.80 x 120 + 0.20 x 220 = 140ns
 Consider slower memory but better hit ratio ->  = 98%,  = 20ns for TLB search, 140ns for memory
access
 EAT = 0.98 x 160 + 0.02 x 300 = 162.8ns

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Memory Protection
 Memory protection implemented by associating protection bit with each frame to indicate if read-only or
read-write access is allowed
 Can also add more bits to indicate page execute-only, and so on
 Valid-invalid bit attached to each entry in the page table:
 “valid” indicates that the associated page is in the process’ logical address space, and is thus a legal
page
 “invalid” indicates that the page is not in the process’ logical address space
 Any violations result in a trap to the kernel

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Valid (v) or Invalid (i)
Bit In A Page Table

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Shared Pages
 Shared code
 One copy of read-only (reentrant) code shared among processes (i.e., text editors, compilers, window
systems)
 Similar to multiple threads sharing the same process space
 Also useful for interprocess communication if sharing of read-write pages is allowed
 Private code and data
 Each process keeps a separate copy of the code and data
 The pages for the private code and data can appear anywhere in the logical address space

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Shared Pages Example

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Structure of the Page Table
 Memory structures for paging can get huge using straight-forward methods
 Consider a 32-bit logical address space as on modern computers
 Page size of 4 KB (212
)
 Page table would have 1 million entries (232
/ 212)
 If each entry is 4 bytes -> 4 MB of physical address space / memory for page table alone
 That amount of memory used to cost a lot
 Don’t want to allocate that contiguously in main memory
 Hierarchical Paging
 Hashed Page Tables
 Inverted Page Tables

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Hierarchical Page Tables
 Break up the logical address space into multiple page tables
 A simple technique is a two-level page table
 We then page the page table

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Two-Level Page-Table Scheme

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Two-Level Paging Example
 A logical address (on 32-bit machine with 1K page size) is divided into:
 a page number consisting of 22 bits
 a page offset consisting of 10 bits
 Since the page table is paged, the page number is further divided into:
 a 12-bit page number
 a 10-bit page offset
 Thus, a logical address is as follows:
 where p1 is an index into the outer page table, and p2 is the displacement within the page of the inner page
table
 Known as forward-mapped page table
page number page offset
p1
p2 d
12 10 10

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Address-Translation Scheme

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64-bit Logical Address Space
 Even two-level paging scheme not sufficient
 If page size is 4 KB (212
)
 Then page table has 252
entries
 If two level scheme, inner page tables could be 210
4-byte entries
 Address would look like
 Outer page table has 242
entries or 244
bytes
 One solution is to add a 2nd
outer page table
 But in the following example the 2nd
outer page table is still 234
bytes in size
 And possibly 4 memory access to get to one physical memory location
outer page page offset
p1
p2 d
42 10 12
inner page

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Three-level Paging Scheme

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Hashed Page Tables
 Common in address spaces > 32 bits
 The virtual page number is hashed into a page table
 This page table contains a chain of elements hashing to the same location
 Each element contains (1) the virtual page number (2) the value of the mapped page frame (3) a pointer to
the next element
 Virtual page numbers are compared in this chain searching for a match
 If a match is found, the corresponding physical frame is extracted

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Hashed Page Table

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Inverted Page Table
 Rather than each process having a page table and keeping track of all possible logical pages, track all
physical pages
 One entry for each real page of memory
 Entry consists of the virtual address of the page stored in that real memory location, with information
about the process that owns that page
 Decreases memory needed to store each page table, but increases time needed to search the table
when a page reference occurs
 Use hash table to limit the search to one — or at most a few — page-table entries
 TLB can accelerate access
 But how to implement shared memory?
 One mapping of a virtual address to the shared physical address

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Inverted Page Table Architecture

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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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Example of Segmentation

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Example: The Intel Pentium
 Supports both segmentation and segmentation with paging
 Each segment can be 4 GB
 Up to 16 K segments per process
 Divided into two partitions
 First partition of up to 8 K segments are private to process (kept in local descriptor table LDT)
 Second partition of up to 8K segments shared among all processes (kept in global descriptor
table GDT)
 CPU generates logical address
 Given to segmentation unit
 Which produces linear addresses
 Linear address given to paging unit
 Which generates physical address in main memory
 Paging units form equivalent of MMU
 Pages sizes can be 4 KB or 4 MB

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Logical to Physical Address
Translation in Pentium

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Intel Pentium Segmentation

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Pentium Paging Architecture

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Linear Address in Linux
 Linux uses only 6 segments (kernel code, kernel data, user code, user data,
task-state segment (TSS), default LDT segment)
 Linux only uses two of four possible modes – kernel and user
 Uses a three-level paging strategy that works well for 32-bit and 64-bit systems
 Linear address broken into four parts:
 But the Pentium only supports 2-level paging?!

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Three-level Paging in Linux

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