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Computer Systems
2009-2010
Week 14: Memory Management
Amanda Oddie
2
Session Aims Understand how main memory
(RAM) can be shared between programs running in time slices
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Main Memory Programs can only execute efficiently if
they are present in main memory during execution
Only main memory is fast enough to feed the CPU with enough instructions and data to keep it usefully occupied.
Thus main memory is a precious resource which must be managed carefully on a multiprogramming machine
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Main Memory In addition to holding instructions
RAM is also a workspace, a transient storage repository.
Space must also be made available for: Operating System Code Operating System Data Video Storage Space etc
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Imagine loading ... Microsoft Windows Microsoft Internet Explorer An email Application Microsoft Word And listening to music using media
player whilst you do all that multi-tasking!It doesn’t stop there. You need memory for your hardware. For example you require main memory to display images on your monitor
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Not enough memory Each time a PROCESS starts it
REQUESTS an amount of RAM The amount of total RAM requested
is usually more than the amount of Physical RAM available in Multiprogramming Operating Systems such as Windows and Linux
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Hard disk, Main memory and CPU
CPU
0
?
Hard disk
Fetch and execute
Memory
Load program into main memory
Start executing the program
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Single Process System Early method Operating system
loaded first Just one user
program loaded into locations above the operating system
Inefficient use of memory – small programs leave much unused memory space
UNUSED
User Process
Operating System
Memory
0
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Fixed Memory Partition Memory divided
into fixed partitions
Several programs could run simultaneously
Space still wasted inside each partition
Process C
Process B
Process A
Operating System100k
300k
600k
Memory address
Partition 1 200k
Partition 2 300k
Partition 3 400k
1000k
Memory
0
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Sharing by swapping Program and data
copied into memory at beginning of each time slice
copied back to disc at end of time slice
All main memory available to program
But, high overhead of swapping time
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Solution Paging
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Sharing by paging Divide program and data into pages
normally resident on the hard disk Each page is typically 4kB in size Only swap in the pages needed
during time slice Memory divided into pageframes Each pageframe can hold one page
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Loading pages
Pages can be loaded in any order Use a page table for each program to keep
track of where in memory its pages are located
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Translating addresses Given an instruction like JMP 314 314 means 314 locations from beginning
of program This does not refer to a physical address
in real memory but to a virtual address measured from the program start
Assume each page 100 locations long JMP 314 means
jump to page 3 offset 14 Where is the location in real memory ?
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Virtual Memory The Operating System has a
Memory Management Unit MMU that translates virtual addresses to physical addresses in RAM.
This is known as Address Translation
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Carrying out the translation
JMP 314
Page number
Real memory address: 2514
Page table
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Page fault Interrupt generated when required
page is not in main memory It causes:
Required page to be fetched from disc into a spare pageframe
If no pageframe is spare then an existing page must be freed up by copying its current contents back to the disk
Page table updated Program continues
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Thrashing Page faults are too frequent Most time spent swapping pages Caused by:
Bad program design Too many programs attempting to run
concurrently
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Paging strategies Fetch strategy
When to fetch a page Replacement strategy
When to swap page out
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Fetch strategies Demand fetch Anticipatory fetch
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Replacement strategies Optimal page replacement Longest resident Least recently used Least frequently used
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Page File The area of the hard disk that stores
the RAM image is called a page file. It holds pages of RAM on the hard disk The Operating System moves data back
and forth between the page file and RAM. Previous versions of Windows called
this a swap file and had a .SWP extension.
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Total Size of the Page File The size of the page file is based on
how much RAM is installed in the computer.
By default, Windows XP creates a page file which is 1.5 times the amount of installed RAM
The page file is placed on the hard drive in the same folder as the Windows XP operating system
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Question What is never paged out?
The Kernel remains resident in RAM,This is never paged out.
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Invalid Page Faults? The CPU trying to access an address
out of range The CPU trying to access an address
which is already occupied by a different process
When the page that the CPU is trying to access does not exist in either RAM or VM
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Invalid Page Faults Sometimes, through program or
hardware error, the page is not there.
The system then has an ‘Invalid Page Fault’ error.
This will be a fatal error if detected in a program
The Application is normally shut down by the Operating System
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Improving Performance The page file is a reserved section of the
hard drive where data may be written and retrieved as needed.
Since the paging file and operating system files are by default located on the same drive, parallel access to both locations is impossible.
One or the other has to wait, slowing down overall system performance.
Moving the page file to another drive will improve performance.
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Question How else could you improve
performance?
Installing more RAM
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Memory Management in Windows Processes
Processes Memory Usage Virtual Memory Usage
Performance Commit Charge Physical Memory Kernel Memory
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Windows Task Manager
Process NameVirtual Memory Usage
Each Process has a Amount of Virtual Memory Limit of 4GB
Physical Memory Usage
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Commit Charge
Physical Memory
Kernel Memory
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Question What is Fragmentation?
The scattering of parts of a file throughout a disk,e.g. when the operating system breaks up the file and fits it into the spaces left vacant by previously deleted files.
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Non-Contiguous Page Files Paging files are normally created when a
drive is relatively empty finding a large contiguous block of space is not
a problem. A page file is dynamic by default
it can be expanded and contracted depending on the amount of extra virtual memory that's needed.
If the initial block of drive space that was allocated at setup becomes surrounded by additional files that have been saved to the drive ... ...a split page file can occur when the operating
system expands it past the initial size.
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Question How do you defragment files?
Windows comes with a utility called Disk Defragmenter which will reorganise the files stored on disk
35
Question What creates the Page File?
a) The CPUb) The RAMc) The Operating Systemd) It’s created by magic
36
Question What creates the Page File?
a) The CPUb) The RAMc) The Operating Systemd) It’s created by magic
Answer C: With help from the CPU
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So What do we Know? Main memory (RAM) is a precious resource Every process and device requires RAM Virtual Memory through paging gives the
impression that there is more RAM than physical RAM
The Kernel is never Paged Out Paging is Managed by the Memory
Management Unit (MMU) Page Faults are normal interrupts Invalid Page Faults are bad How to view Memory Allocation and
Resources in Windows