Lecture 14 Memory Management III

1
Lecture 14 Memory Management III

• Variable Partition Allocation

2
Variable Partitions

• The number, location, and size of the partitions
  vary dynamically as processes come and go
• Improves memory utilization. Process is allocated
  exactly as much memory as required
• The number and size of the processes in memory
  vary dynamically throughout the day
• Complicates allocating and deallocating memory

3
Example
4
External fragmentation problem
OS
320 K
Process 1
224 K
Process 3
288 K
64 K
5
Solution Compaction
OS
320 K
Process 1
128 K
Process 4
288 K
Process 3
100K
Process 5
60K
6
Placement algorithms revisited

• Best-fit
• Chooses the block that is closest in size to the
  request
• worst performer overall
• since smallest block is found for process, the
  smallest amount of fragmentation is left memory
  compaction must be done more often

7
Placement algorithms revisited

• First-fit
• Begins to scan memory from the beginning and
  chooses the first available block that is large
  enough
• fastest
• may have many process loaded in the front end of
  memory that must be searched over when trying to
  find a free block

8
Placement algorithms revisited

• Next-fit
• Begins to scan memory from the location of the
  last placement and chooses the next available
  block that is large enough.
• more often allocate a block of memory at the end
  of memory where the largest block is found
• the largest block of memory is broken up into
  smaller blocks
• compaction is required to obtain a large block at
  the end of memory

9
Placement algorithmsExample
10
Memory Managementwith Bit Maps

• Memory is divided up into allocation units,
  perhaps as small as few words and perhaps as
  large as several kilobytes
• Corresponding to each allocation unit is a bit in
  the bit map, which is 0 if the unit is free and 1
  if it is occupied
• A bit map provides a simple way to keep track of
  memory words in a fixed amount of memory because
  the size of the bit map depends only on the size
  of memory and the size of the allocation unit

11
Memory Managementwith Bit Maps
12
Memory Managementwith Linked Lists

• Another way of keeping track of memory is
  maintaining a linked list of allocated and free
  memory segments.
• A segment is either a process or a hole between
  two processes

13
Memory Managementwith Linked Lists
14
Memory Managementwith the Buddy System

• Available blocks of size 2k, L k U
• 2L is the smallest block that is allocated
• 2U is the largest size block that is allocated
• To begin, the entire space available for
  allocation is treated as a single block of size
  2U
• if a request of size s such as 2U-1 lt s 2U is
  made, then the entire block is allocated
• Otherwise, the block is split into two equal
  buddies of size 2U-1.
• (The process is repeated)

15
Memory Managementwith the Buddy System