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Chapter 6: Memory Management
Purpose of memory manager is to administer the use the use of the primary memory.
Primary memory and CPU are the fundamental resources used by every process.
Basics Requirements that drive memory designs
REQUIREMENT ON PRIMARY MEMORY
•The primary memory access time must be as small as possible. This need influences both software and hardware design
•The primary memory must be as large as possible. Using virtual memory, software and hardware can make the memory appear to be larger than it actually is
•The primary memory must be cost-effective. The cost cannot be more than a small percentage of the total cost of the computer.
The purpose of the memory manager is to allocate primary memory space to
processes to map the process address space into the
allocated portion of the primary memory to minimize access times using a cost-
effective amount of primary memory
3 Functions of Memory Manager
In an environment that supports dynamic memory allocation, the memory manager must keep a record of the usage of each allocatable block of memory.
Memory allocation is the process assigning blocks of memory on request.
There are 3 memory allocation: i. First Fit, Worst Fit, Best Fit ii. Buddy System Iii. Suballocators
Memory allocation
There are three algorithms for searching the list of free (free list) blocks for a specific amount of memory.◦ First Fit ◦ Best Fit ◦ Worst Fit
When recycling free blocks, there is a choice as to where to add the blocks to the free list. Free list kept:
Memory allocation(address)Increasing size(best fit)Decreasing size (worst fit)Increasing time
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3.2 Variable Partitioning
First Fit : Allocate the first free block that is large enough for the new process.
This is a fast algorithm. Another strategy is first fit, which simply
scans the free list until a large enough hole is found. Despite the name, first-fit is generally better than best-fit because it leads to less fragmentation.
Variation of first fit known ad next fit, continues each search for a suitable block
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first fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
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first fit
Initial memory mapping
Initial memory mapping
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
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first fit
P4 of 3KB arrives
P4 of 3KB arrives
OS
P1 12 KB
P4 3 KB
<FREE> 7 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
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first fit
P4 of 3KB loaded here
by FIRST FIT
P4 of 3KB loaded here
by FIRST FIT
OS
P1 12 KB
P4 3 KB
<FREE> 7 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
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first fit
P5 of 15KB arrives
P5 of 15KB arrives
OS
P1 12 KB
P4 3 KB
<FREE> 7 KB
P2 20 KB
P5 15 KB
<FREE> 1 KB
P3 6 KB
<FREE> 4 KB
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first fit
P5 of 15 KB loaded here
by FIRST FIT
P5 of 15 KB loaded here
by FIRST FIT
Best Fit : Allocate the smallest block among those that are large enough for the new process.
In this method, the OS has to search the entire list, or it can keep it sorted and stop when it hits an entry which has a size larger than the size of new process.
This algorithm produces the smallest left over block. Problem: Requires more time for searching all the list or
sorting it. It leads to the creation of lots of little holes that are
not big enough to satisfy any requests. This situation is called fragmentation, and is a problem for all memory-management strategies, although it is particularly bad for best-fit.
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Best fit
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
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best fit
Initial memory mapping
Initial memory mapping
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
14
best fit
P4 of 3KB arrives
P4 of 3KB arrives
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OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
P4 3 KB
<FREE> 1 KB
best fit
P4 of 3KB loaded here
by BEST FIT
P4 of 3KB loaded here
by BEST FIT
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OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
P4 3 KB
<FREE> 1 KB
best fit
P5 of 15KB arrives
P5 of 15KB arrives
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OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P5 15 KB
<FREE> 1 KB
P3 6 KB
P4 3 KB
<FREE> 1 KB
best fit
P5 of 15 KB loaded here
by BEST FIT
P5 of 15 KB loaded here
by BEST FIT
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Worst Fit : Allocate the largest block among those that are large enough for the new process.
The memory manager places process in the largest block of unallocated memory available.
The ideas is that this placement will create the largest hole after the allocations, thus increasing the possibility that, compared to best fit, another process can use the hole created as a result of external fragmentation.
worst fit
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OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
worst fit
Initial memory mapping
Initial memory mapping
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OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
<FREE> 16 KB
P3 6 KB
<FREE> 4 KB
worst fit
P4 of 3KB arrives
P4 of 3KB arrives
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OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
worst fit
P4 of 3KB Loaded here
by WORST FIT
P4 of 3KB Loaded here
by WORST FIT
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OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
worst fit
No place to load P5 of 15K
No place to load P5 of 15K
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
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worst fit
No place to load P5 of 15K
No place to load P5 of 15K
Compaction is needed !!Compaction is needed !!
Compaction is a method to overcome the external fragmentation problem.
All free blocks are brought together as one large block of free space.
Compaction requires dynamic relocation. Certainly, compaction has a cost and selection of
an optimal compaction strategy is difficult. One method for compaction is swapping out
those processes that are to be moved within the memory, and swapping them into different memory locations
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compaction
OS
P1 12 KB
<FREE> 10 KB
P2 20 KB
P4 3 KB
<FREE> 13 KB
P3 6 KB
<FREE> 4 KB
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compaction
Memory mapping before
compaction
Memory mapping before
compaction
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
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compaction
Swap out P2
Secondarystorage
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
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compaction
Swap in P2
Secondarystorage
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
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compaction
Swap out P4
Secondarystorage
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
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compaction
Swap in P4 with a different starting address
Secondarystorage
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
30
compaction
Swap out P3
Secondary
storage
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
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compaction
Swap in P3
Secondary
storage
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
<FREE> 27 KB
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compaction
Memory mapping after compaction
Memory mapping after compaction
Now P5 of 15KB can be loaded here
Now P5 of 15KB can be loaded here
OS
P1 12 KB
P2 20 KB
P4 3 KB
P3 6 KB
P5 12 KB
<FREE> 12 KB
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compaction
P5 of 15KB is loaded
P5 of 15KB is loaded
Static relocation: A process may be loaded into memory, each time possibly having a different starting address
Necessary for variable partitioning Dynamic relocation: In addition to static
relocation, the starting address of the process may change while it is already loaded in memory
Necessary for compaction
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relocation
The allocator will only allocates blocks of certain sizes and has many free lists, one for each permitted size.
The permitted sizes are usually either powers of 2, such that any block except the smallest can be divided into two smaller blocks of permitted sizes.
Advantages: coalescence is cheap because buddy of any free block can be calculates from its address.
See eg 6.2.2
Buddy System
A suballocator obtains large blocks of memory from the system memory manager and allocates the memory to the application in smaller pieces.
Function suballocator: To avoid general inefficiency in the system
memory manager. To take advantage of special knowledge of the
application’s memory requirements To provide memory management services that
the system memory manager does not supply.
Suballocators
1. Swapping To optimize system performance by removing a
process from primary memory. RAM has limited capacity. Virtual memory as
additional of RAM. The area hard disk used for virtual memory is
called swap file. When more RAM space is needed, the OS swaps
out from RAM and moved to virtual memory. The technique of swapping items between
memory and storage called paging.
Memory Manager Strategies
Allow a process to use the CPU when only part of its address space is loaded in the primary memory.
2. Virtual Memory
Dynamic Relocation
Refers to address transformations being done during execution of a program.
Define memory allocation Give three way to allocate memory in our
system. Explain 3 functions that must be performed
by memory manager in order to execute program and data.
List three basic requirements of designing primary memory.
List 2 strategies to manage memory Define what is best fit, worst fit and first fit.
TutorialQuest of Nov2011
Quest of Nov2010
Quest of Nov2011
Quest of Nov2011
