Lecture1 Memory Mgmt

8/12/2019 Lecture1 Memory Mgmt

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CHAROTAR INSTITUTE OF TECHNOLOGY

DEPARTMENT OF INFORMATION TECHNOLOGY

IT502OPERATING SYSTEM

DEPARTMENT OF INFORMATION TECHNOLOGY

CHAROTAR INSTITUTE OF TECHNOLOGY, CHANGA


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IT502-OPERATING SYSTEMS INFORMATION TECHNOLOGY

Classified e-Material Copyrights Charotar Institute of Technology, Changa 2

Topics for discussion

Functions Of Operating System

What is Memory Manager?

The notion Of Address Space

Approaches Of Memory Management

Memory management without Swapping Or Paging

Multiprogramming and memory usageModeling Multiprogramming

Multiprogramming with Fixed Partitions

Relocation and Protection

Swapping


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Major Functions Of Operating System

Processor management

That is assignment of processor to different tasksbeing performed by the computer system.

Memory management

allocation of main memory and other storage areas tothe system programmes as well as user programmesand data.

File management,

that is, the storage of file of various storage devices toanother. It also allows all files to be easily changedand modified through the use of text editors or someother files manipulation routines.


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Input/output management,

that is, co-ordination and assignment of the differentoutput and input device while one or more programmesare being executed.

Interpretation of commands and instructions

Establishment and enforcement of a priority system.That is, it determines and maintains the order in whichjobs are to be executed in the computer system.

In this session we are suppose to study the detailsabout Memory mangement.


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So the first question that arises in our minds is:

What do you mean by memory?

Memory refers to storage needed by the kernel, theother components of the operating system and theuser programs.

Ideally memory should beLarge, Fast and Non volatile

Then next question arises is:

What are the different types of memory devicesinherent in computer?

Cache, Main memory(RAM),ROM and hard Disks


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How do different types of memory differ?

In practice, a memory hierarchy is used

1. Small amount of fast, expensive memory cache

2. Some medium-speed, medium price main memory

3. Gigabytes of slow, cheap disk storage

Now there is need of a controllling structure

to decide what memory to use and when?

This decision is taken by memory manager

Definition: Thepart of operating system that manages

memory is called the memory manager. It handles the memory hierarchy


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Classified e-Material Copyrights Charotar Institute of Technology, Changa 73/30/2014Page 7

The notion of address space

An address space is set of addresses that a process

can use to address memory.

Before executing any process cpu has to allocateaddress space to it

For example, consider changa campus as memory and

out department as a process CITC building has been allocated some space. Our

address space is then Building 1.


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Memory Management Approaches

Monoprogramming vs. Multiprogramming

One program at a time vs. many

Fixed program set vs. swapping

When programs are switched out, save state to disk

Complete image vs. partial image (Virtual memory)

Some program images wont fit in available RAM

Modern systems are all multi-programmed, withswapping and virtual memory

But first we will learn monoprogramming without

swapping or paging


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The simplest possible scheme is to have just one

process in memory at a time, and allow that processto use all memory

The user loads entire memory with a program from adisk or a tape and it takes over whole machine

The actual technique used on microcomputers is asfollows

The memory is divided between operating system anda single user process

The location may of memory may be at different

places as shown in figure in next slide

Monoprogramming without swapping or paging


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Fig. 3-1. Three simple ways of organizing memory-

Simplest Memory Management

Monoprogramming without Swapping or Paging


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Uses And Advantages

Uses:-

It was used for

1. (a) was used for mainframes and minicomputers

2. (b) is used on some palmtop computers andembedded systems

3. (c) was used by early PCs running MS-DOS whereBIOS device drivers were in ROM

Advantages:-

1. The sole advantage of monoprogramming is itssimplicity.

2. There is very little software required in the operatingsystem for memory management in this case. Theprocessis given all of the available memory which itcan then use as it desires.


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Disadvantages of Monoprogramming

Monoprogramming is wasteful of CPU and memory

resources.

While the program is performing I/O, the CPU is idle

Likewise, if the program doesnt use all of theavailable memory, the excess memory in the system

is wasted. Programs cannot be larger than the size of the

available physical memory.


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Multiprogramming

In multiprogramming, as you know, multiple

processes are conceptually executed at the

same time.

This permits the operating system to switch the CPUto a different process when the current process

becomes blocked. The difficulty, of course, is that memory is

now required to hold the images of each

process that is a candidate for execution.


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Multiprogramming Questions

How effective is multiprogramming? That is,

how much more quickly can we execute processesthan if monoprogramming was used?

How much higher is CPU utilization?

How much higher is memory utilization?

Is the additional memory needed to supportmultiple processes worth the cost?


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Answering the Questions

One obvious way to answer these questions

is to run the same set of processes using

monoprogramming, and then again usingmultiprogramming.

Unfortunately, this is a very expensive and time

consuming procedure, and only answers thequestions relative to the particular set of

processes used in the analysis.

A more general, mathematical analysis is

desired.


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A Simple Multiprogramming Model

Assumption 1: there are n processes in

memory.

Assumption 2: the probability of each process beingblocked is p (a number between 0 and 1).

Assumption 3: all processes are independent.

Assumption 4: there is one CPU, and weignore time spent doing context switches.


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Performing the Analysis

We know the CPU will be idle only if all n

processes are blocked.

Using elementary probability, we know that

probability of all processes being blocked is Pn

and probability of at least one process not being

blocked is 1 pn (CPU utilization) The chart on the next slide illustrates CPU

utilization for three different processing

blocking probabilities: 20%, 50% and 80%.


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Modeling Multiprogramming

CPU utilization as a function of number of processes in memory

Degree of multiprogramming

Suppose that a process spends a fraction pof its time waiting for I/O to completewith nprocesses in memory at a time, the probability that all n processes are waitingfor I/O is p^n. CPU utilization is 1 - p^n (with the assumption that the processes areindependent)

Modeling Multiprogramming


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Memory Management in Multiprogramming

With monoprogramming, memory management

was simple: give all available memory to the job

and were done.

With multiprogramming we must decide how to

arrange the memory images of the jobs in the

primary memory of the system. With n jobsthere are n! orderings of the jobs in memory.

We must also consider what happens when one

of the jobs finishes can we fit a new job in the

memory space just vacated?


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Fixed Partitioning of Memory

One approach to the problem is to

statically divide the available memory into a numberof regions or partitions, the size of each is fixed.

This approach is used in the IBM OS/MFT(Multiprogramming with a Fixed number of Tasks)operating system (now largely obsolete).


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Multiprogramming with Fixed Partitions

Fixed memory partitions

separate input queues for each partition

single input queue (strategies: next convenient process,biggest process, dont ignore more than k times)


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Multiprogramming with Fixed Tasks Characteristics

Each job (with a maximum memory requirement

specified) is run in the smallest partition that meetsits needs.

Underutilization of memory can still occur, since job

sizes dont necessarily exactly match partition sizes.

Suppose we have two 40K partitions and a 39K and a41K job. Only the 39K job can be run, wasting 41K.

Concern must be given to memory protection, since a

program in one partition might access memory in

another partition by accident or on purpose


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MFT: Advantages & Disadvantages

Advantages:

1. Easy to implement.

2. Provides multiprogramming (faster turnaround,

better use of memory and CPU).

Disadvantages:

1. Scheduling can be complicated.2. Memory resources are still wasted.

3. Job size is limited to physical memory size.

4. Extra hardware and/or software are needed for

relocation and protection.


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Relocation and Protection

Cannot be sure where program will be loaded in memory address locations of variables, code routines cannot be

absolute(address translation: during loading/calculation duringexecution)

must keep a program out of other processes partitions


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Program Relocation

To achieve relocation, OS/MFT modifies the addresses

in instructions when the program is loaded to reflectthe location at which the program is loaded.

Consider figure A, if a program has entered partition 2and user wants to access 100thinstruction in theprogram then, instruction needs to be modified as

200K +100

This has some obvious disadvantages:

1)the loader must mark those instructions needingmodification, and

2)the modification of the program is a slowoperation


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Another Relocation Technique

Another approach to relocation causes a base

register (one of the processors general

registers) to be set to the address at which the

program is loaded. The instructions in the

program then use this base register much like an

index register when they need to referencememory.

Disadvantages:

Additional base registers are required The approachconsumes one of the CPUs registers.

A program cant be easily relocated once its started.


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Perhaps the most general approach is to have a

special CPU register used as the base register,

and automatically add its content to every memoryaddress generated by a program.

This register is protected from modification by

the user and is, in fact, entirely transparent tothe users program.

The users program now appears to be

addressing a region of memory starting at location 0.

Still Another Technique


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Protection

In the IBM 360 (and others), each 2K block of

memory has a special 4-bit code called the lock.

Only kernel-mode programs can alter these

locks. All locks for each programs memory are set

to a unique value.

A four-bit key is present in the Program StatusWord (PSW, essentially a register containing the

next instruction address, interrupt enable

information, and so forth). The PSW is included in

the context of a process.


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Another Memory Protection Scheme

Another approach is to add a special limits

register (which can only be set by supervisor modeprograms).

Every memory address is automatically comparedwith the limits register.

If the limits register is exceeded, a fault is generated.

Both these approaches require special dedicatedhardware, and that implies additional expense inproducing the CPU.

IT502 OPERATING SYSTEMS INFORMATION TECHNOLOGY


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Swapping

Swapping is used to increase the probability that a

process will be ready to run when the CPU wouldotherwise be idle.

That is ,swap in process from hard disk to mainmemory

Most of the time there is not enough memory to hold

all the active processes at the same time So in that case we need to take that process back to

hard disk. That is swap out process from mainmemory to hard disk

Thus, Swapping is the process of moving a process

between primary memory and secondary storage A process can be swappedtemporarily out of memory

to a backing store, and then brought back intomemory for continued execution. There is specialplace allocated for this is hard disk called swap area



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Schematic View of Swapping



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Variable Partition Sizes

Fig. 3-4. Memory allocation changes as processes come into memory

leave memory

Shaded regions are unused memory



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Compaction

Collectively the holes in memory might be large

enough to allow the execution of another program. But our programs assume they will be executed in a

contiguous region of memory and the holes are notcontiguous

When Swapping creates multiple holes in thememory, it is possible to coalesce all the holes intoone larger hole that might be useful. This technique ismemory compaction

To perform compaction we must move the occupied

memory regions together to eliminate the holes. Compaction can consume a great deal of CPU time

during which processes cannot execute.



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Advantages & Disadvantages

Advantages:

1. Conceptually there is less wasted memory.

2. Scheduling is possibly simpler.

Disadvantages:1. Memory resources are still wasted (holes).

2. Job sizes are still tied to the physical memory size.

3. Extra hardware and/or software are still needed for

relocation and protection.



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How Much Memory To Allocate?

Many programs have predictable memory

requirements: code, static data, and stack (to holdprocedure invocation records and local variables)

However many others have less predictable memoryrequirements due to the need for dynamic storageallocation and recursion.

As a result, processes may request execution in apartition larger than their static memoryrequirements.

To solve this problem a traditional memory layout is

used which is as shown in the next slide



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To accommodate the potential for growth ofsome parts of a programs memory allocation,the code, static data and heap (dynamic data)

in the lowest part of a memory region.

The stack is then placed at the high end of theregion, growing downward as items are pushed,and contracting upward as items are removed.

This allows expansion of the heap and the stackinto the region between them.



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Keeping Track of Memory Usage

There are three common ways to keep track of

memory usage:1. bit maps

2. linked lists

The basic goals of each system are to:1. identify all used/unused memory regions,

2. support the allocation of a contiguous unused regionof n blocks of memory, and

3. handle the deallocation (freeing) of a region ofmemory.



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Memory Management with Bit Maps or chained lists

Fig. 3-6a. Part of memory with 5 processes, 3 holes tick marks show allocation units

shaded regions are free

Fig. 3-6b. Corresponding bit map

Fig. 3-6c. Same information as a list



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Memory Management with Bit map

We can use an array of bits is used to keep track

of used/unused blocks of memory.

We might choose to set a bit to 0 to represent anunused block, with a 1 bit meaning the block is used.

The size of the block has several effects.

Larger block sizes yield smaller bitmaps, but memorywill likely be wasted in the last block for each region.

Smaller block sizes result in larger bitmaps, but less

waste will occur in a regions last block.



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Bit Maps: Advantages & Disadvantages

Advantages:

1. easy to implement,

2. easy to deallocate a region, and the size is dependentonly on the size of primary memory and of a block.

Disadvantage:

1. it is difficult to allocate a contiguous region of size n,since the system must search the bit map for nconsecutive 0 bits.



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Classified e-Material Copyrights Charotar Institute of Technology, Changa 41 41

Memory Management with Linked Lists

Fig. 3-7. Four neighbor combinations for the terminatingprocess X



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Linked Lists

Using linked lists, the basic idea is to maintain a list of

free regions of contiguous memory and another list ofoccupied regions.

The free list can be ordered in various ways:

1. ordered by memory address,

2. or ordered by size. We explicitly assume region sizes are random.

Allocation of a region can be done using any of manydifferent algorithms (e.g. first fit, best fit, next fit,worst fit, quick fit).



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First Fit Allocation Technique

Scan the free list from its head, looking for the first

unused region that satisfies (is larger than or equal

to the size of) the allocation request.

Then split that region, if necessary, to produce oneregion of the requested size. other unused) region

back on the list. Advantages:

1. Easy to implement

2. Fast

Disadvantage:

1. Produces lots of very small holes



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Next Fit Allocation Technique

This approach is similar to the first fit algorithm, but

each new search for an acceptable unused memoryregion begins (in the list) where the last one ended.

Disadvantage

1. In simulations, it produces slightly worse results than

first fit.



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Best Fit Allocation Technique

Scan the entire list to locate the smallest unused

region that will accommodate the request formemory.

Disadvantages:

1. it is potentially very slow; or

2. requires the list to be ordered on size; whichcomplicates the deallocation algorithm.

Expected advantage:

It should reduce the number of holes.



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Worst Fit Allocation Technique

Pick the largest available hole to satisfy each

allocation, splitting it as necessary into the allocatedregion and a new hole.

Disadvantage:

1. Simulation results for this technique are negative.



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Quick Fit Allocation Technique

Maintain multiple lists, each list having holes of the

same common sizes. To satisfy an allocation request, look first in the list

with the appropriate size, then in lists of largerunused regions.

Disadvantage:

1. Deallocation is costly, since all lists must be scannedto find neighboring unused regions.

Advantage:

1. Allocation is fast.



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Fragmentation

External Fragmentationtotal memory space exists to

satisfy a request, but it is not contiguous. Internal Fragmentationallocated memory may be

slightly larger than requested memory; this size differenceis memory internal to a partition, but not being used.

Reduce external fragmentation by compaction

Shuffle memory contents to place all free memorytogether in one large block.

Compaction is possible onlyif relocation is dynamic,and is done at execution time.



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Related Exam Based Questions

Monoprogramming without Swapping or Paging [4]

Explain Swapping in detail [6] Define

1. Memory Manager

2. Compaction

3. Degree of MultiProgrmming Explain Relocation problem in detail [2]

Lecture1 Memory Mgmt

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