04 ch02 understandos memory1

8/20/2019 04 Ch02 UnderstandOS Memory1

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Chapter 2Memory Management:

Early Systems

Understanding Operating Systems,Fourth Edition


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Memory Management:Memory Management: Early SystemsEarly Systems

“Memory is second only to processes in

importance (and in intensity) with which its

managed by the OS” Tanenbaum.


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Memory Management

Ideally programmers wantmemory that is

large

fast

non volatile

Memory management done by

OS memory manager

Special purpose hardware

Memory hierarchy

small amount of fast,expensive memory – cache

some medium-speed, mediumprice main memory

Gigabytes of slow, cheap diskstorage

Translate these speeds tohuman scale


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Memory Management

• Memory Management means

– Being aware of what parts of the memory are in useand which parts are not

– Allocating memory to processes when they request it

and de-allocating memory when a process releasesit

– Moving data from memory to disc, when the physical

capacity becomes full, and vice versa.

– Stopping programs interfering with memory that

‘doesn’t belong to them’


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• Types of memory allocation schemes: – Single-user systems

– Fixed partitions

– Dynamic partitions – Relocatable dynamic partitions


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SingleSingle--UserUser

Contiguous SchemeContiguous Scheme• Program is loaded in its entirety into memory and allocated as

much contiguous space in memory as it needs

– Jobs processed sequentially in single-user systems – Requires minimal work by the Memory Manager

• Register to store the base address

• Accumulator to keep track of the program size

Oneie BIOS

Embedded

System

DOS


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Memory Protection inMemory Protection in SingleSingle--User ContiguousUser Contiguous

SchemeScheme• To properly implement this scheme

a boundary register is needed – Special purpose H/W must be

designed into CPUs

– the contents of the register canonly be modified by specialprivileged instruction

• When a process referencesmemory the system checks if its

outside its boundary – Inside: then service the request

– Outside: Error message andterminate program

• Of course the user program must

access the OS from time to time toperform I/O for example – Does this through system calls

– The system detects the call andswitches to ‘kernel mode’ or‘executive mode’

boundary register

CPU

0x123

0x123


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SingleSingle--User Contiguous SchemeUser Contiguous Scheme(continued)(continued)

• Disadvantages of Single-User ContiguousScheme:

– Doesn’t support multiprogramming

• Mono-programming is unacceptable for modern user

• Modern users will not suffer the delay of swapping

programs in/out to the hard disk

– Not cost effective

• Multiprogramming makes more effective use of theCPU .... When a single process is doing I/O the CPU

is inactive


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The secret isThe secret is

multiprogramming can improve theutilization of the CPU

But now the problem is :

how do we to organize the available memory so allthe programs can reside there ?


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Fixed PartitionsFixed Partitions• Main memory is partitioned; one partition/job

– Partitions can be different sizes

• The partition sizes remain static unless and until computersystem is shut down, reconfigured, and restarted

– Must remember where each space is in memory

• Also requires protection of the job’s memory space

– Requires matching job size with partition size

– Allows multiprogramming


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Fixed PartitionsFixed Partitions (continued)(continued)

Table 2.1: A simplified fixed partition memory table with the

free partition shaded

To allocate memory spaces to jobs, the operating system’s

Memory Manager must keep some sort of table. A very simple example is shown below:


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• Job1 is allocated first space

large enough to accommodate it

• Job 2 is allocated next

available space large enough

to accommodate it

•Job 3 ?•No partition big enough to

accommodate it even though

70K of free space is

available in Partition 1 where

Job 1 occupies only 30K of

the 100K available but

unfortunately Job 1 owns all

of this partition.


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• Programmers – In the earliest multi-programming systems the programmer

translated the job using an absolute assemble or compiler

• This meant that the process had a pre-defined preciselocation defined for it in memory

• The process could only run in that space• If that partition was occupied (while others were free)then this resulted in inefficiencies.

– To overcome this problem programmers created (more

complex) relocating compilers, assemblers, and loaders.• These programs produced a relocatable program that

could run in any available (big enough) partition


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• Disadvantages: – Requires entire program to be stored contiguously

– Jobs are allocated space on the basis of firstavailable partition of required size

– Works well only if all of the jobs are of the same sizeor if the sizes are known ahead of time

– Arbitrary partition sizes lead to undesired results

• Too small a partition size results in large jobs havingto wait such that their ‘turnaround time’ is extended

• Too large a partition size results in memory waste or‘internal fragmentation’


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Dynamic PartitionsDynamic Partitions

• Dynamic Partitions: Jobs are given only as muchmemory as they request when they are loaded

– Available memory is kept in contiguous blocks

– Memory waste is comparatively small

• Disadvantages:

– At first its great and it fully utilizes memory when thefirst jobs are loaded

•

Subsequent allocation leads to memory waste or‘external fragmentation’


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Dynamic PartitionsDynamic Partitions (continued)(continued)

Figure 2.2: Main memory use during dynamic partition allocation


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Dynamic PartitionsDynamic Partitions (continued)(continued)

Figure 2.2 (continued): Main memory use during dynamic partition allocation

5 K

10 K

20K

35K of memory available but

Job 8 still has to wait!


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Where to put the jobsWhere to put the jobs

• With Dynamic memory allocation we give jobs justenough room

• Easy for first job

• What happens when we have lots of jobs allarriving one after the other and all with different

requirements

• We need some sort of allocation algorithm


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BestBest--FitFit andand FirstFirst--FitFit Allocation Allocation

• Free partitions are allocated on the following basis: – First-fit memory allocation: First partition fitting the

requirements

• Leads to fast allocation of memory space

– Best-fit memory allocation: Smallest partition

fitting the requirements

• Results in least wasted space• Internal fragmentation reduced but not eliminated


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FirstFirst--Fit AllocationFit Allocation

Figure 2.3: An example of a first-fit free scheme


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BestBest--FitFit

Figure 2.4: An example of a best-fit free scheme

?


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A Note: The Memory Manager A Note: The Memory Manager

In the following slides we assume that the Memory Manager keeps twolists,

– one for free memory (LHS) and

– one for busy memory blocks (RHS)

– Assume the two lists are of a fixed size and the elements in the list cancontain values or be empty.

– For simplicity the lists are often merged in the diagrams that follow


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Free Memory List

FirstFirst

--Fit AlgorithmFit Algorithm

A new Jobrequires

a 200 size

block

After Job is placed inmemory the list of

addresses where free

space is available

changes

.........The variousaddresses

where we

can getfree space.

Please note that 4075 + 105 = 4180

( so the space (1045) up to 5225

must be filled with jobs)

Where

can we

put theJob?


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BestBest--Fit AlgorithmFit Algorithm

• Algorithm for Best-Fit: – Goal: find the smallest memory block into which the

job will fit

– NB: Entire table must be searched before allocation


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BestBest--Fit AlgorithmFit Algorithm A Jobrequires200 spaces

Where will it beplaced?


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More Allocation SchemesMore Allocation Schemes

• Hypothetical allocation schemes: – Next-fit: Starts searching from last allocated block, for the nextavailable block when a new job arrives

– Worst-fit: Allocates the largest free available block to the new

job

• Opposite of best-fit

• Good way to explore the theory of memory allocation; might

not be the best choice for an actual system

.


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Another way to do it? Another way to do it?

• Alternative search strategy – Search the entire input queue looking for the largest job that fitsinto the partition

– Do not waste a large partition on a small job

• but smaller jobs are discriminated against• ☺ Have at least one small partition or ensure that small jobs

only get skipped a certain number of times


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When Jobs finishWhen Jobs finish

• How do we reclaim space when jobs finish?


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DeallocationDeallocation

• Deallocation: Freeing an allocated memory space

– For fixed-partition system: ☺

• Straightforward process. When job completes,

Memory Manager resets the status of the job’s

memory block to “free”

• Any code—for example, binary values with 0indicating free and 1 indicating busy—may be used


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DeallocationDeallocation (continued)(continued)

• For dynamic-partition system:

– Algorithm tries to combine free areas of memory

whenever possible. This makes it more complex.

– The system must prepare for three possible cases:

• Case 1: When the block to be deallocated is adjacent

to another free block

• Case 2: When the block to be deallocated is between

two free blocks• Case 3: When the block to be deallocated is isolated

from other free blocks


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Case 1: Joining Two Free BlocksCase 1: Joining Two Free Blocks

This has to be de-allocated. It ‘buts-up’ to the

next highest free memory location (althoughnot to the free space below it... there is

probably another process in the green area).

Join


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– The free memory list must bechanged to reflect starting

address of the new free block• In the example, 7600

– this was the addressof the first instructionof the job that just

released this block – Memory block size for the new

free space must be changedto show its new size—that is,the combined total of the two

free partitions• In the example, (200 + 5)

Case 1: Joining Two Free BlocksCase 1: Joining Two Free Blocks


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Case 2: Joining Three Free BlocksCase 2: Joining Three Free Blocks

This has to be de-allocated

Now notice that the memory we will free is butted-up to free memory above and below it. We’ll end up

with three free spaces. These can be joined together.


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Case 2: Joining Three Free BlocksCase 2: Joining Three Free Blocks..

• Deallocated memory space is

between two free memory blocks – Change list to reflect the starting

address of the new free block

• In the example, 7560— whichwas the smallest beginningaddress

– Sizes of the three free partitions mustbe combined

• In the example, (20 + 20 + 205)

– Because location 7600 in our list hasbeen combined the pointer in that list

location must be changed to null toindicate that it no longer points to freememory space, hence ‘null’.


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Case 3:Case 3: DeallocatingDeallocating an Isolated Blockan Isolated Block

• Space to be deallocated is

isolated from other free

areas

1. System learns that the

memory block to be

released is ‘tight’ between

two other busy areas

2. Null entry in the busy list

3. Must search the free

memory list for a null entry

4. And point that list element

to the new free area

1

2

3

4


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– Dynamic partitions

– Relocatable dynamic partitions


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Problems so far Problems so far

• The fixed and dynamic memory managementsystems described so far share some unacceptable

fragmentation characteristics that must be resolved

• We’ll likely end up with lots of slivers of un-used

memory


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RelocatableRelocatable Dynamic PartitionsDynamic Partitions

• Relocatable Dynamic Partitions: – Memory Manager relocates programs to gathertogether all of the empty blocks

– Compact the empty blocks to make one block of

memory large enough to accommodate some or allof the jobs waiting to get in

• Sometimes called garbage collection

– Its not easy


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(continued)(continued)

• Compaction: Reclaiming fragmented sections ofthe memory space

– Every program in memory must be relocated so they

are contiguous

– So every address so must be relocated

– Every reference to an address within the program

must be updated – Data must not be harmed


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Figure 2.5: An assembly language program that performs

a simple incremental operation


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The assembly language program (RHS) after it has been

processed by the assembler appear on LHS. Notice the

memory locations are flagged by apostrophe ‘


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• Compaction issues:

– What goes on behind the scenes when relocation

and compaction take place?

– What keeps track of how far each job has moved

from its original storage area?

– What lists have to be updated?


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• What lists have to be updated?

– Free list must show the partition for the new block of

free memory

– Busy list must show the new locations for all of the

jobs already in process that were relocated

– Each job will have a new address except for those

that were already at the lowest memory locations


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• Special-purpose registers are used for relocation:

– Bounds register

• Stores highest location accessible by each program

– Relocation register

• Contains the value that must be added to each

address referenced in the program so it will be able to

access the correct memory addresses after relocation

• If the program isn’t relocated, the value stored in theprogram’s relocation register is zero


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Figure 2.7: Three snapshots of memory before and after

compaction


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Figure 2.8: Contents of relocation register and close-up of

Job 4 memory area (a) before relocation and

(b) after relocation and compaction


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• Compacting and relocating optimizes the use of

memory and thus improves throughput

• Options for when and how often it should be done:

– When a certain percentage of memory is busy

– When there are jobs waiting to get in

– After a prescribed amount of time has elapsed

Goal: Optimize processing time and memory use while

keeping overhead as low as possible


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SummarySummary

• Four memory management techniques were usedin early systems: single-user systems, fixedpartitions, dynamic partitions, and relocatabledynamic partitions

• Memory waste in dynamic partitions iscomparatively small as compared to fixed partitions

• First-fit is faster in making allocation but leads tomemory waste

• Best-fit makes the best use of memory space butslower in making allocation


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Summary (continued)Summary (continued)

• Compacting and relocating optimizes the use of

memory and thus improves throughput• All techniques require that the entire program

must:

– Be loaded into memory

– Be stored contiguously

– Remain in memory until the job is completed

• Each technique puts severe restrictions on the size

of the jobs: can only be as large as the largestpartitions in memory


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Objectives of this chapter wereObjectives of this chapter were

You will be able to describe:

• The basic functionality of the three memoryallocation schemes presented in this chapter: fixedpartitions, dynamic partitions, relocatable dynamic

partitions• Best-fit memory allocation as well as first-fit

memory allocation schemes

• How a memory list keeps track of available memory

• The importance of deallocation of memory in adynamic partition system


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Objectives (continued)Objectives (continued)

You should be able to describe:

• The importance of the bounds register in memory

allocation schemes

• The role of compaction and how it improves

memory allocation efficiency

04 ch02 understandos memory1

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